April 2015 Special Issue … · und erleben Siemens-Technologie hautnah. ... Florian Otto and...

www.physik-journal.deApril 2015 Special Issue

BestPhysics’Large Area Scan MacroscopeInvestigation of the Microstructure of Polar Ice Cores

CARS Microscopy with Spectral Focusing

Nanoscale Imaging Gone Dry

Scan of an ice corefrom Antarctica.

The light granular structure and dark

gas bubbles areclearly discernable.

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© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Physics’ Best April 2015 1

Cover:The rapid analysis (scan time 3 s) provided by the Large Area Scan Macroscope by Schäfter+Kirchhoff with a resolution of 5 µm has proven to be an essen-tial tool for analyzing the microstructures of ice cores, both in the field and in the labora-tory. (Photo of train: Lars Berg Larsen, NEEM ice core project).

See page 20

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2 Mode-Hop-Free Quantum ExplorationsRudolf Neuhaus and Tim Paasch-Colberg

6 Tuning for ResonancesLuis F. Gomez and Jaroslaw Sperling

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1 1 Precision Positioning for X-Ray AnalyticsBirgit Schulze

1 4 Nanoscale Imaging Gone DryFlorian Otto and Christoph Bödefeld

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O P T I C S A N D P H O T O N I C S

1 9 Products

2 0 Microstructure Analysis of Polar Ice CoresAnja Krischke, Ulrich Oechsner and Sepp Kipfstuhl


24 CARS Microscopy with Spectral FocusingHans-Erik Swoboda and Tissa Gunaratne

2 7 What Can Be Done About Noise?Gerhard Holst

S O F T W A R E

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V A K U U M T E C H N O L O G Y

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U 3 Company Index U 3 Masthead

www.physik-journal.deApril 2015 Special Issue

BestPhysics’Large Area Scan MacroscopeInvestigation of the Microstructure of Polar Ice Cores

CARS Microscopy with Spectral Focusing

Nanoscale Imaging Gone Dry

Scan of an ice corefrom Antarctica.

The light granular structure and dark

gas bubbles areclearly discernable.

LaserMode-Hop-Free Quantum ExplorationsContinuously tunable diode lasers explore the micro, nano and quantum world.� page 2

InstrumentationPrecision Positioning for X-Ray AnalyticsTailor-made positioning solution for ana-lytical methods performed on a beamline:Mechanical system approaches limit of technical feasibility.� page 11

Optics and PhotonicsWhat Can Be Done About Noise?Why image noise occurs and what you can do about it.� page 27

10 µm

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Physics’ Best April 2015 © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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The continuously tunable laser CTL enables mode-hop-free wave-length tuning up to 100 nm with narrow linewidth and highest accuracy. It will improve measure-ments on small structures and in the quantum regime that rely on mode-hop-free laser operation.

C ontinuously tunable, single fre-quency lasers are now available

with ultrawide mode-hop-free tuning, unprecedented low noise levels, as well as convenient digital control electronics. Active control with the new digital electronics is a key element that enables such wide mode-hop-free tuning and hands-off operation. The new lasers facili-tate sophisticated measurements on the quantum level, reaching from observation and manipulation of quantum oscillations in microca-vities or detecting single molecules in solution towards QED studies with single quantum dots. Other important applications are optical component testing and molecular spectroscopy.

Quantum dots as well as micro-cavities have become increasingly important for numerous applica-tions, for example single photon sources, qubits for quantum com-puters, telecommunication devices, frequency combs or nonlinear optics. Many of these applications approach quantum limits and de-pend on light sources that fulfill de-manding requirements in terms of linewidth, noise, high resolution tu-ning, flexibility and control. Usual-ly, narrow-bandwidth lasers that are tunable without mode-hopping are utilized to detect, study and use resonance frequencies of quantum dots and microcavities.

Obviously, spectroscopic appli-cations are limited when the mode-hop-free tuning range of the laser

is smaller than the spectral width of the studied feature(s). Patching numerous independent narrow mode-hop-free scans is tedious and involves the risk of missing impor-tant details. Only wide and mode-hop-free tuning safely enables applications that study individual broad spectral features, or several lines that are widely separated. Searching for resonance frequen-cies over a large wavelength range and following lines when they shift with out unpredictable mode-hopping is possible only with such tunable lasers.

In general, mode-hopping can be caused by unsteady external conditions or instable laser para-meters. For example, a temperature variation of the gain medium shifts the wavelength of highest gain, whereas the frequencies of the resonator modes are not affected in exactly the same manner. There-

fore the previous lasing mode may no longer be the mode of highest gain and the system may start la-sing at an other mode. Likewise, a drift of the resonator length shifts its frequency modes and causes mode-hopping if the change is not compensated in the gain medium. Additionally, other influences like moving or vibrating optical elements can disturb the smooth single-mode operation.

As a consequence, a sophisti-cated laser design is required to prevent mode-hopping of single-frequency lasers. Mode-hop-free tuning of a few tens of nanometers can be achieved using a good me-chanical and well isolated laser setup, i. e. perfect synchronization of tuning elements, in combina-tion with low drift and noise of the diode current. For guaranteeing even wider tuning, TOPTICA has integrated active control into their

Mode-Hop-Free Quantum ExplorationsContinuously tunable diode lasers explore the micro, nano and quantum world.

Rudolf Neuhaus and Tim Paasch-Colberg

Dr. Rudolf Neuhaus and Dr. Tim Paasch-Colberg, TOPTICA Photonics AG, Loch-hamer Schlag 1, 21 Gräfelfing

Fig. The widely tunable laser CTL en-ables mode-hop-free wavelength tuning of 100 nm with a narrow linewidth and supreme accuracy. Operated with the

digital controller DLC pro it provides high flexibility, low noise and drift values, as well as fantastic convenience.

© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Physics’ Best April 2015

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CTLs (Fig. ). It enables mode-hop-free scans of more than 100 nm across the full gain spectrum of the integrated laser diode.

Active Stabilization

The active control in TOPTICA’s CTL is called SMILE (Single Mode Intelligent Loop Engine). It ensures mode-hop-free operation by ana-lyzing signals in the laser head and synchronizing the involved tuning elements by active feedback while the laser is running – even during wavelength scans (Fig. a).

The wavelength can be tuned in several ways: Motorized coarse tuning with a step size of approx. 1 pm and a speed of up to 10 nm/s, as well as piezo-based fine tuning. The piezo can scan the wavelength across more than 30 motor steps with an extremely high resolution of less than 5 – 10 kHz. In addition, extremely fast wavelength changes and therefore fast laser stabilization are enabled by controlling the laser diode current.

The CTL is now available with two specified wavelength ranges: 915 nm to 95 nm or 1530 nm to 1620 nm with up to 0 mW output power. Other wavelengths will follow soon. Despite the incredibly large tuning range, the CTL has a notably narrow linewidth, which has been measured to be ~5 kHz on short timescales (5 µs) in a self-he-terodyne beat measurement (Fig. c) and ~100 kHz in a beat experiment with two individual identical lasers

and a sweep time of 50 ms. The narrow linewidth and low drift va-lues make it ideal for measurements involving quantum dots or micro-cavities.

Light for Quantum Measurements

Quantum properties are usually not observable in macroscopic objects due to environmental decoherence unless specific sample geometries and cooling are utilized. Employing microcavities, for example, is one possibility to observe quantum ef-fects in a relatively large, microme-ter-scaled structure. Fig. 3a illustrates an isolated, donut-shaped glass microcavity of about 30 microns in diameter. With this geometry, it is a macroscopic mechanical oscillator and a ring-shaped high Q optical cavity at the same time: light of certain wavelengths can be cou-pled into the cavity via evanescent wave coupling. The oscillating light bounces off the walls of the donut by total internal reflection and thus transfers a small force on the struc-ture by radiation pressure.

In this way, the coupled light can influence the vibrational behavior of the structure and vice versa. This turns microcavities into interesting objects for quantum research. For example, in [1] such parametric coupling between light and mecha-nical oscillations was observed. In [] a sensor that is based on opto-mechanical coupling was used for active feedback cooling of such a microcavity.

Because of their small size, the free spectral range of microcavities is relatively large, and tiny size de-viations cause large spectral shifts. Hence, a widely mode-hop-free tunable laser is an invaluable tool to find and study the resonance frequencies of microcavities.

The spectral dependence on size and other environmental parame-ters of microcavities can be exploi-ted for a promising application: label-free detection of single biolo-gical molecules in solution. This is enabled using a microtoroid optical resonator in combination with a widely tunable mode-hop-free la-ser like CTL. In [3] it is described how such a laser is stabilized to a microtoroid optical resonator and how shifts of the optical resonance frequency caused by molecules bin-ding to the resonator are observed. The laser follows the frequency change and by examining the shift in laser frequency, particles of ra-dius between 2 nm and 100 nm can be detected and distinguished.

This application can strongly benefit from DLC pro’s optional locking functions: Feeding a signal from the experiment back into the controller, the scanning of the laser across a cavity resonance can be optimized by simple pinch, swipe and spread gestures on the touch display. After selecting a cavity re-sonance frequency on the display, the laser can then easily be stabi-lized to this resonance by the push of a button. The results were further extended towards creating a non-invasive tumor biopsy assay, and

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Fig. a) Mode-hop-free wavelength tuning over 100 nm is enabled with the active SMILE stabilization of the CTL 1550. b) Motorized wavelength tuning can be performed with step sizes of

approx. 1 pm, higher accuracy is achie-ved by a piezo driver. c) A self-heterody-ne beat measurement with the CTL 50 using a fiber of 1 km length (according to 5 µs delay) reveals a linewidth of appro-

ximately 5 kHz. The linewidth is deter-mined by reproducing the interference pattern of the beat measurement with a parameterized model.

4 Physics’ Best April 2015 © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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provide a basis for an “optical mass spectrometer” in solution.

Coupled Double Quantum Dot

Semiconductor quantum dots are exciting nanostructures that show atom-like behavior becau-se of their small size and three-dimensional confinement. Due to the confinement, electronic states in quantum dots are quantized and such structures are often referred to as artificial atoms. Atom-like properties have been verified by demonstrating strong photon antibunching and near lifetime-limited linewidths. Quantum dots in general are interesting systems to realize qubits, and especially semiconductor quantum dots are promising candidates for scalable

quantum computers since semicon-ductor processing is well studied and understood: Unlike real atoms, semiconductor quantum dots can be grown and placed in a control-led fashion. By integrating quantum dots into other semiconductor structures like waveguides or pho-tonic crystal structures (e. g. cavi-ties), even cavity QED experiments are possible without the need for trapping atoms.

En route to scalable qubit ar-rays, coupled quantum dots have recently raised a lot of interest. Electron transport measurements on coupled quantum dots have de-monstrated spin-sensitive coup ling and manipulation of electron and nuclear spins, and optical spectra of coupled excitons have been measured and calculated in self-assembled coupled quantum dots.

Fig. 4 a) Schematic band-edge diagram of a coupled double quantum dot. The wavelength-dependent differential trans-mission signal ΔT/T of such a system is shown in b). It was recorded by a single

mode-hop-free scan over a wavelength-range of more than 30 nm. c) shows a zoom into the resonances of both quantum dots.

Resonant optical excitation of quantum dot states is crucial in par-ticular for coherent state manipu-lation and detection. However, due to the intrinsically random growth process, all quantum dots are slight-ly different in size and hence feature different optical resonance frequen-cies. To find and resonantly excite the optical transitions of a single quantum dot, a widely mode-hop-free tunable, narrow band laser is an ideal tool.

A first series of quantum dot measurements with CTL was car-ried out at ETH Zürich [4]. A single GaAs quantum dot was resonantly excited using a DLC CTL 950 and its resonance fluorescence signal was detected. By changing the gate voltage, the quantum dot was charged with a single electron and transferred from the neutral state to a negatively charged state. This change resulted in a spectral shift of the excitonic transition. The new resonance frequency could be easily found by tuning the laser mode-hop-free by approximately 4 nm.

In another experiment, the differential transmission of both quantum dots in a coupled double quantum dot system was measured. In this structure, two quantum dots are grown on top of each other (Fig. 4a). Both quantum dots can be brought into resonance by changing their common gate voltage which is a means to control their coupling strength. The variable coupling strength makes these double quan-tum dots interesting for qubits and quantum computing applications.

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Fig. 3 a) Electron microscopy image of a microcavity that was used to demon-strate parametric coupling between light and this mechanical oscillator [1].

b) Microtoroid optical resonator for de-tecting molecules in solution. By stabi-lizing a widely mode-hop-free tunable laser to the resonator, molecules bin-

ding to the resonator can be detected by analyzing shifts of its optical reso-nance frequency [3].

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Their optical resonances differ by several tens of nanometers (appro-ximately 10 THz).

The differential transmission signal ΔT/T of the double quantum dot system for one common gate voltage is shown in Fig. 4b: The reso-nance frequencies of both quantum dots are more than 30 nm apart. The presented spectrum was achie-ved in one single mode-hop-free scan across the resonance frequen-cies of both quantum dots. Because such wide scans are tedious without a mode-hop-free laser, the blue quantum dot was not studied with this type of measurement before.

Improved Light Sources

These examples show how well quantum dot and microcavity pro-perties can be studied with power-ful optical tools like the CTL. Fur-ther investigations with even more sophisticated structures of different geometry, size and material are necessary to reveal deeper insight into the interaction between light and matter on the quantum level. These experiments require reliable light sources and will benefit from

the unique stability, accuracy, nar-row linewidth and mode-hop-free tuning range that is provided by the widely tunable laser CTL.

CTL’s versatile digital controller DLC pro enables easy access to all relevant laser parameters via touch-screen and additional knobs. Since the controller is digital, it ensures highest flexibility and future com-patibility, as well as lowest noise and drift values. Intelligent features like SMILE and FLOW (see below) are enabled with the DLC pro only. Furthermore, remote control of the laser is possible via USB or TCP/IP using a preconfigured PC GUI or a powerful command language.

In the unlikely event of a laser cavity misalignment, for example after mechanical shock or large temperature changes, the inte-grated FLOW (Feedback Light Optimization Wizard) of the CTL re-optimizes the cavity upon the push of a button. This re-establishes stable and mode-hop-free opera-tion. Since this optimization can be performed “in the field”, shipping of the laser system back to the manu-facturer is not required.

The outstanding parameters of the CTL in combination with its

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[1] E. Verhagen et al., Nature 482,63 (2012) [2] D. J. Wilson et al., arXiv:1410.6191v2

(2015) [3] Tsu-Te Judith Su, Label-Free Detection

of Single Biological Molecules Using Microtoroid Optical Resonators. Disser-tation (Ph.D.), California Institute of Technology, USA (2014)

[4] Measurements carried out by Dr. Mar-tin Kroner and Yves Delley, Quantum Photonics Group of Prof. Imamoglu, ETH Zürich.


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Due to a very distinct spectrum of resonant frequencies, the characte-rization of optical microcavities calls for widely tunable sources of continuous wave laser light, of-fering suitably narrow linewidths and single-mode operation over the entire tuning range. This is why external cavity diode lasers, which allow for mode-hop free wavelength tuning, often over more than a hundred nanometers, have matured to a fundamental tool of the optical microresonator research apparatus.

D iode lasers are ubiquitous in their use as cost-effective

sources of laser light in a broad va-riety of commercial and industrial applications. However, low cost “off-the-shelf ” semiconductor laser diodes typically operate with sever-al longitudinal modes lasing simul-taneously, leading to low coherence and large linewidths. Owing to their relatively poor optical output characteristics, such devices are typically not suitable when more demanding linewidth specifications and single-mode operation are required. Furthermore, a variety of applications, such as the characte-rization of optical microresonators that is touched on in an illustrative manner in the present article, de-mand that the laser wavelength be continuously tunable over as large a range as possible.

The development of tunable single frequency diode lasers has been driven by the evolution of co-herent optical telecommunication systems. Nowadays, so-called “mo-nolithic” semiconductor lasers can be roughly categorized into three configuration designs [1]. Most of the tunable telecom lasers are based on distributed feedback (DFB) technology (i. e., on a resonator

medium with a periodic structure) and wavelength tuning that is ac-complished by varying the tempe-rature and/or the operating current. Distributed Bragg reflector (DBR) lasers, in turn, use a gain medium sandwiched between Bragg grating sections. Vertical-cavity surface-emitting lasers (VCSELs) with micro-electro-mechanical system (MEMS) based tuning elements are a relatively recent development.

All three of these configurations, however, are plagued by limita-tions on the available wavelength range, tunability, and/or achieva-ble linewidth. Also, these types of single-mode tunable laser diodes may exhibit a phenomenon called “mode-hopping”, in which the laser output frequency discontinuously hops from one value to another du-ring tuning of the laser wavelength.

External Cavity Diode Lasers

As an alternative to monolithic designs, external cavity diode la-sers (ECDLs) are another practical approach to achieve wavelength tunability of semiconductor diode

lasers. Basically, an ECDL can be defined as a device that consists of a diode with an anti-reflection coating on (at least) one side, a collimator, and an external mode selection filter [1]. Though com-mercial ECDLs are based on off-the-shelf laser diodes, the output characteristics of the diode can be greatly enhanced by integration into the external cavity. By doing so, the diode acts purely as a gain ele-ment while the wavelength selective optics helps to ensure that only a single mode lases at any given time. While a variety of particular cavity designs has been demonstrated [2], the so-called Littrow and the Litt-man-Metcalf configurations are the two most prevalent, both of which employ gratings to select single-mode operation (Fig. 1).

In the Littrow design (Fig. 1, left) the grating is positioned at the Littrow angle (where the angle of incidence equals the angle of diffraction). Simply speaking, the lasing wavelength of the cavity is determined by a combination of the standing-wave condition (cavity-length) and the center wavelength of the grating feedback. For mode-

Rendition of a toroid microresonator being resonantly excited by light propa-

gating through an optical fiber.

Luis F. Gomez and Jaroslaw Sperling, Photonic Instru-ments, Newport Spectra-Physics GmbH, Guerickeweg 7, 64291 Darmstadt, E-mail: [email protected]

Tuning for ResonancesWidely tunable diode lasers advance microresonator research

Luis F. Gomez and Jaroslaw Sperling


L A S E R

hop free tuning, the cavity length has to be continually readjusted to match the grating retro-reflection condition. This can be accomplis-hed by rotating the grating about a carefully chosen pivot point, however, the tolerance of the pivot point location error is very restric-tive, which imposes limitations on practical tunability.

The tunability limitations of the Littrow geometry are overcome in the Littman-Metcalf configuration (Fig. 1, right), which uses a double-pass through the grating under a steep angle of incidence (typi-cally ≈ 85° versus ≈ 30°). Since at grazing-incidence the grating dis-persion is very high (due to a large number of grooves encountered by the incident beam), the Littman-Metcalf design intrinsically offers very high mode selectivity, frequen-cy stability, and narrow linewidth.

The principal advantage of the Littman-Metcalf design, however, is a distinctly wider mode-hop free wavelength tuning range, which can be achieved without any beam-walk of the laser output beam, through rotation of the retro-reflecting ca-vity end-mirror. This rotation can be accomplished with a variety of actuators, ranging from microme-ter screws, piezoelectric actuators, servo motors, or voice-coil type designs, depending on the tuning range and the tuning speed re-quired by the specific application. For example, the commercial plat-form shown in Fig. 2, employing a combination of a servo motor and a piezoelectric transducer, offers mo-de-hop free tuning ranges of up to

more than 100 nm, while maintai-ning a linewidth of < 200 kHz.

Optical Microresonators

Broadly speaking, microresonators are micro-scale photonic devices that confine and store light within small volumes and with very small losses, by resonant recirculation [3]. Fig. 3 illustrates size and geometry of just a couple of microresonator geometry variants that have been developed for various applications. While microresonators can be fabricated “on-chip” from glasses, polymers, or III-V binary semi-conductors, it is worth noting that SiO2 based devices, in particular, can be patterned onto silicon wafers through techniques standard in the integrated circuit industry [4].

Coupling of light into a microre-sonator of toroid type (Fig. 4) can be accomplished by carefully aligning the device closely enough to the ta-pered region of a tapered optical fi-ber waveguide. By doing so, some of the light carried through the fiber is evanescently coupled into the toroid, initiating a second longitudinally propagating wave within its ring. Confinement and storage of light within the toroid ring, however, occurs only at certain wavelengths (respectively resonance frequen-cies). Notably, these resonances or modes are called “whispering gallery modes”, named after the legendary whispering gallery under the dome of St. Paul’s cathedral in London, UK, in which a whisper at one point along the circular wall of the dome can be heard at the opposite side of the gallery along the wall.

Lens

Output Output

Lens

Mirror

Diffraction GratingDiffraction Grating Laser DiodeLaser Diode

Fig. 1 Comparison of the Littrow (left) and the Littman-Metcalf (right) configu-ration in tunable external cavity diode lasers (ECDLs). While the Littrow design typically offers less than a tenth of a na-

nometer mode-hop free tuning range, the Littman-Metcalf design achieves mode-hop free tuning in the range of tens to hundreds of nanometers.

Fig. 2 Commercial turn-key widely tuna-ble diode laser system from New Focus, consisting of a laser head and a single easy-to-use control electronics unit. The laser platform permits access to wave-length ranges from 400 nm to 2175 nm.

Fig. 3 Scannning electron microscope images of different geometry microreso-nators: a) toroid, b) ring, c) disk, and

d) sphere. Black bars indicate size com-parison.

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wavelength with sub-angstrom re-solution. Obviously, this procedure will only be effective if the laser line width is by far narrower than the width of the resonance peak (the latter typically in the MHz range). To close, representative data from a transmission spectrum measurement of a SiO2 toroid is shown in Fig. 5.

Outlook

With the toroid being just one of numerous examples, optical micro-resonators are an emerging oppor-tunity of scientific research, driven towards commercialization by the high interest in versatile micro-scale photonic devices. Intrinsically linked to the fundamental optical properties of microresonators, their experimental characterization requires widely tunable sources of continuous wave laser light with sufficiently narrow linewidth. Suitably designed External Cavity Diode Lasers are proven to be an indispensable tool in this context.

AcknowledgementsThe authors gratefully acknowledge the laboratory of Prof. Andrea M. Armani, University of Southern California, for providing images and traces used in the figures for this article.

[1] C. Ye, Tunable External Cavity Diode Lasers, World Scientific Publishing (2004)

[2] B. Mroziewicz, Opto-Electronics Review 16, 347 (2008)

[3] K. J. Vahala, Nature 424, 839 (2003) [4] A. J. Maker and A. M. Armani, Journal

of Visualized Experiment 65, e4164 (2012)

Measuring Essential Characteristics of a Toroid Microresonator

In the absence of any losses, an op-tical microresonator would confine light for an infinite period of time. Real world resonators can confine light for only a finite period of time, known as the photon lifetime. The photon lifetime, in turn, is intima-tely linked to the so-called quality factor (Q-factor), defined as the ratio between the center frequency (of a resonance) to the width of the resonance peak. Since devices with a higher quality factor will have a longer photon lifetime, the Q-factor is an essential value characterizing the quality of microresonators, and of high interest for most applications

(in some of the highest-Q toroids re-ported to date photon lifetimes can reach hundreds of nanoseconds).

Experimentally, the characteri-zation of toroid microresonators is typically accomplished by passing light from a tunable source (such as an ECDL) through a tapered op-tical fiber waveguide (as described above), while recording the amount of light that is transmitted through the fiber. When scanning the laser frequency through a resonance, a large fraction of the power through the tapered fiber will couple into the toroid, resulting in a drop in the transmitted power. The spacing between successive resonances, commonly referred to as the free spectral range (FSR), is typically on the order of nanometers, which underscores the need for wide tuna-bility and makes Littman-Metcalf ECDLs particularly well-suited for the experiment.

After surveying the resonances by performing a broad scan of the wavelength (typically over a cou-ple of the microresonator’s FSRs), a high-resolution scan is carried out to eventually determine the Q-factor. This can be accomplished by piezo-dithering the wavelength about a resonance peak, which allows for fine-scanning the laser

Fig. 4 Side view of a toroid microresona-tor aligned with an optical fiber (percep-tible as horizontal blur). Light at 410 nm is being coupled into the toroid.

Fig. 5 Representative data from a trans-mission spectrum measurement of a SiO2 toroid. The oscilloscope screenshot shows a triangular waveform voltage used to tune the laser wavelength

(black line) along with the recorded transmission intensity (red line). Lorentz ian fits (zoom) yield quality factors of Q ≈ 107 for the device.

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L A S E R


Single-frequency Laser with Narrow Linewidth for Scientific ApplicationsManufacturer: Coherent.Product: “Mephisto” family of single-frequency lasers that offer ultra-narrow linewidth (~1 kHz over 100 ms). The lasers are ideal for demanding applica-tions such as atom trapping and cooling, optical heterodyning, injection locking and seeding, and other applications that require extremely narrow linewidth. In addition, the kilometer range coherence length of these lasers also brings superior resolution and accuracy to research on gravitational waves and interference-based metrology applications.

Features: The basic “Mephisto” laser delivers up to 2 W of CW output at a wavelength of 1064 nm (1319 nm versions are also available). Output powers up to 55 W at 1064 nm with substantially similar beam properties are achieved by using a Master Oscillator Power Amplifier (MOPA) flexible architecture incorporating one or more amplification stages. The “Prometheus” model delivers over 100 mW of single-frequency CW green (532 nm) output by combining the basic oscillator with a simple but highly efficient extra-cavity doubler based on a

periodically poled non-linear crystal. The “Mephisto Q” provides time bandwidth-limited nanosecond pulses using an in-ternal passive Q-switch integrated in the NPRO resonator. n

Coherent (Deutschland) GmbH Dieselstr. 5b 64807 Dieburg, Germany Phone: +49 (0)6071 968-0 Fax: +49 (0)6071 968-499 E-mail: [email protected] Website: www.coherent.de

www.coherent.com

Micro Laser with External CavityManufacturer: Sacher Lasertechnik.Product: Innovative miniaturized tuna-ble external cavity diode laser “Micron S1” for the high requirements in Bose-Einstein Condensation, optical trapping, quantum computing and spectroscopy applications. Features: The laser in TO-39 housing comes in a very compact design without any movable parts; this allows new ap-plications in aerospace and medicine and operation in critical environments. The new laser is based on a range of the manufacturer’s patent families which represent a basis for consequent and continuous further development of their products. Offering the same characteri-stics like the established series of exter-

nal cavity diode lasers, the new model features a wavelength range of 650 to 180 nm, tunability of typically 2 nm, high sidemode suppression of > 40 dB, high mode purity and stability with small linewidth of < 20 kHz. Interfaces, such as GPIB and USB enable for easy integration in complex systems as well as hassle-free data communication.n

Sacher Lasertechnik GmbHRudolf-Breitscheid-Str. 1–535037 Marburg/Lahn, GermanyPhone +49 (0)6421 305-0Fax: +49 (0)6421 305-299E-mail: [email protected]: www.sacher-laser.com

Offset-free Frequency CombManufacturer: TOPTICA.Product: Difference frequency comb “DFC” operating with “CERO” technolo-gy (“zero-vCEO”) which obtains a phase-stable laser output with an inherently vanishing frequency offset. Features: Femtosecond laser pulses cen-tered at 1560 nm are generated with a mode-locked Er3+-doped fiber-oscillator. Subsequent amplification and coherent spectral broadening of the pulses in a nonlinear fiber creates a broad supercon-tinuum. Difference frequency generation (DFG) between the lower and upper spectral parts of the supercontinuum produces a frequency comb output cen-tered at 1560 nm. Since the two spectral parts have identical carrier-envelope frequencies, the resulting comb is offset-free due to common-mode suppression

in the DFG process. Thus the carrier-envelope frequency is automatically fixed at zero and no active stabilization of the phase is necessary. This passive approach results in improved phase-stability and higher reliability compared to conventio-nal active stabilization techniques. The absence of an electronic feedback-loop for phase stabilization leads to a reduced complexity of the system and circumvents electronic noise limitations. The system comes with an integrated radio frequency reference and locking electronics to syn-chronize the repetition rate. Optionally it can be locked to an optical reference.

n

TOPTICA Photonics AG Lochhamer Schlag 19 82166 Graefelfing, Germany Phone: +49 (0)89 85837-0 Fax: +49 (0)89 85837-200 E-mail: [email protected] Website: www.toptica.com



Universal X-Ray Diffractometer Manufacturer: EFG.Product: Vertical three-axis diffractometer “Omega/Theta” for standard measurements of single crystals (sizes above a few millimeters) using the Omega-scan as well as the Theta-scan geometry. Features: By means of the Omega-scan method, the crystal orientation can be quickly determined. The X-ray measure-ment may be combined with the simulta-neous measurement of a laser reflection signal to relate the orientation to the crystal surface .The Theta-scan mode enables, e. g., the determination of lattice

parameters, of lattice-plane angles and the measurement of rocking curves and diffraction line profiles. It is possible to change simply between arrangements with and without channel-cut crystal col-limator in the primary beam (optionally also in the reflected beam). Additional-ly, an x-y table for the mapping of the corresponding crystal parameters can be delivered. Special pre-programmed measuring variants are: the orientation determination of given kinds of crystal in defined orientation ranges (measuring time a few sec.); the search for given re-flections; rocking-curve measurement;

line-profile measurement; combined rocking-curve and line-profile measure-ment (“reciprocal-space mapping”); local mapping in a given raster; and, in special cases, the determination of the unknown orientation.n

EFG GmbH Beeskowdamm 6 14167 Berlin, Germany Phone: +49 (0)30 809740-0 Fax: +49 (0)30 8022355 E-mail: [email protected] Website: www.efg-berlin.de

Hybrid Photon Counting DetectorsManufacturer: Dectris.Product: Hybrid Photon Counting (HPC) detectors that measure X-rays from Ti to Ag sources and beyond directly, with highest spatial resolution, suppression of fluorescence, free of read-out noise, and dark current. They are available in main-tenance-free, compact and robust instru-ments, suitable for all X-ray applications, especially in laboratory environments with weak sources.Background: Traditional X-ray analysis relied on detectors that transformed X-ray photons into visible light. Their performance suffered from low efficien-cy, dark current, blooming, long readout and poor resolution. HPC technology, once obtainable at synchrotron sources only, is now available for laboratory instruments. Counting each individual photon, this technology overcomes many of the limitations of traditional detectors and is perfectly suited to ex-ploiting the potential of even weak laboratory sources. Features: The HPC detectors measure

X-rays from Ti to Ag sources and beyond directly, with highest spatial resolution, suppression of fluorescence, free of read-out noise, and dark current. The “PILATUS3 R” family features two-dimensional pixel detectors with a pixel size of 172 × 172 μm2 and readout time of 7 ms. In combination with frames rates of up to 20 Hz, it enables high-throughput data acquisition in the laboratory. In small-molecule crystallography, the dynamic range of 20 bit yields excellent data for absolute structure determination and charge density as well as in surface diffraction studies. Thanks to noise-free detection, these detectors are the best choice for macromolecular crystallogra-phy, SAXS/WAXS and GISAS. “MYTHEN2 R” detectors are one-dimensional microstrip detectors. The smallest strip size (50 μm), highest dyna-mic range (24 bit), readout of 0.9 ms and optimal signal-to-noise ratio from Ti to Ag radiation make the detectors ideal for applications ranging from residual stress to PDF analysis.

“EIGER R” two-dimensional pixel detectors fulfill the needs of the most demanding user. Smallest pixel size (75 × 75 μm2) combined with continuous readout results in highest temporal and spatial resolution in all diffraction and imaging experiments.n

DECTRIS Ltd. Neuenhoferstr. 107 5400 Baden, Switzerland Phone: +41 56 500 2100 Fax: +41 56 500 2101 E-mail: [email protected] Website: www.dectris.com

Helium liquefier Manufacturer: Quantum Design. Distribution: LOT-QuantumDesign. Product: Novel helium liquefiers “ATL” series (Advanced Technology Liquefier) which use a Gifford-McMahon (GM) cryo-cooler to liquefy helium gas. Features: The GM cryo-cooler has a closed helium gas loop that is pressu-rized by a compressor. The compressed helium gas is then expanded in the cold head of the liquefier which generates low temperatures that cool down the dewar and the helium gas inside. The helium li-quefier has no sophisticated installation requirements and can easily be operated in any laboratory. A touch-screen panel allows easy control of the automated

system. The “ATL” can use both helium gas from a high-pressure cylinder and recycled gas from a cryogenic experi-ment. Thanks to the liquefier’s outstan-ding design, the helium is automatically liquefied with very high efficiency.

The systems feature an average rate of 10 l/d (ATL-80) and 22 l/d (ATL-160), respectively. Depending on the mode of operation, these rates can even be high-er. As an unit can simply be rolled to the cryogenic experiment, the liquid helium can be directly transferred. This saves time and reduces helium losses.n

LOT-QuantumDesign GmbH Im Tiefen See 58 64293 Darmstadt, Germany Phone: +49 (0)6151 8806-0 Fax: +49 (0)6151 8806-64 E-mail: [email protected] Website: www.lot-qd.com/de



N owadays X-ray and tomogra-phic methods help to detect

very fine structures inside objects. For spatial resolutions down to 100 nanometers, diffractive X-ray optics are available today. However, various challenges have to be over-come to obtain a three-dimensional image with volume resolution in this range. It is a major challenge to achieve the extremely high mechanical accuracy and stability required for the alignment of the optics and samples in the X-ray beam and for the entire experimen-tal setup. Even minute changes in temperature or vibrations could de-grade the desired resolution. This is why the improvement of the X-ray optics must always go hand in hand with the mechanical perfection of the entire setup. If specialists work closely together, considerable pro-gress can be made as shown by the example described below.

At the X-ray light source PETRA III at the DESY research center (German Electron Syn-chrotron) in Hamburg, Germany, the Helmholtz-Zentrum Geest-hacht – Center for Materials and Coastal Research (HZG) operates the Imaging Beamline P05, which

includes two experimental hutches, one for nanotomography (Fig. 1) and one for microtomography. Each name designates the attainable (spatial) resolution. In the nano-tomography hutch, X-ray optics for three-dimensional micrographs

with resolutions around 100 nm are used, that consist of up to ma-ny hundreds of diffractive lenses, which were developed at the Karlsruhe Institute of Technology (KIT) at Karlsruhe, Germany. The setup also includes microscopy optics for visible light, used for further magnification of the X-ray micrographs and their transfer to a camera.

Meeting High Standards

With the aim to carry out as many different experiments as possible, the HZG provides two different X-ray optics configurations: An imaging setup, in which the sample is positioned in front of the objec-tive optics (Fig. 2), and a cone-beam setup, in which the sample is placed in the diverging beam behind the

Precision Positioning for X-Ray AnalyticsTailor-made positioning solution for analytical methods performed on a beamline: Mechanical system approaches limit of technical feasibility.

Birgit Schulze

Dipl.-Phys. Birgit Schulze, Marketing & Products, Physik Instrumente (PI) GmbH & Co. KG, Auf der Römerstr. 1, 76228 Karlsruhe, Germany, www.pi.ws, E-mail: [email protected]

Fig. 1 The Imaging Beamline P05 at the X-ray light source PETRA III includes this experimental setup of nanotomo graphy.

PI /

HZG

PI’s parallel-kinematic positioning sys-tems offer a series of advantages over serially stacked assemblies, such as a lower moving mass, resulting in im-proved dynamics, less space required in combination with higher stiffness. Thus, for motions with six degrees of freedom, either the strut length of the Hexapods can be changed – or in the SpaceFABs mentioned later in the text – the angle can be varied if the strut length is constant.

The SpaceFAB principle is based on three XY-stages that jointly position a platform using three struts of constant

length and a suitable joint configura-tion (Fig. i). It is the principle of choice in particular when long distances have to be covered in the X and Y directions or a low-profile design is required.

P A R A L L E L- K I N E M A T I C P O S I T I O N I N G S Y S T E M S

Fig. i Assembly principle of a SpaceFAB

PI



optics. In both cases, high mecha-nical stability and precision posi-tioning are essential in order to obtain micrographs of high quality. This is why the instruments used for the experiments at the P05 beamline must meet very high standards.

However, thanks to the close cooperation of the clients with the engineers and developers from PI (Physik Instrumente), this complex task could be solved in a practice-oriented manner. After all, PI has valuable know-how and many years of experience especially in the “Beamline Instrumentation” field of application. The aim of the team of specialists, coordinated by PI miCos, is to develop application-specific solutions that go beyond offering individual components and include system integration as well as the complete instrumentation. On the Beamline P05, they once again demonstrated these capabili-ties. A particular challenge was to configure the control, which was based on an industrial controller. The challenge consisted of control-

ling almost 50 axes independently of one another while ensuring collision protection. The entire system was finally integrated into the TANGO interface customary for beamlines.

The Platform

To minimize the effect of vibrations and securely fasten the individual components and stabilize them, relative to one another, a granite base 6.8 m in length forms the basis of the instrument. Another four moving granite platforms driven by linear motors are arranged on this base on air bearings. This makes it possible to position all compo nents with high speed and precision: The sample stage, the X-ray optics, and the detector. The substructure its-elf, which weighs several tons, is al-so mounted on air bearings (Fig. 3). This allows the entire assembly to be moved out of the X-ray beam with minimal effort when the se-cond experimental station is to be used, while maintaining a stable

position as soon as the air flow is switched off.

A particular challenge was the construction of the sample stage, since it had to be mechanically sta-ble in the range below 100 nm, in order to achieve the required spa-tial resolution. To this end, several positioning systems have to work hand in hand with maximum preci-sion, to ensure that always the same volume element is investigated when the sample is rotating.

Complex Sequences

The basis of sample positioning is a horizontal positioning unit which moves the sample stage into the beam. It has a travel range of 20 mm, can be subjected to a load of 300 kg and works with a repea-tability of 30 nm. What drives this high-precision positioning unit are stepper motors combined with high-resolution optical linear en-coders. When driven accordingly, this allows closed-loop step sizes of a few nanometers. The preci-sion crossed roller guides and ball screws used also contribute to the high positioning accuracy.

This displacement unit is equipped with three lifting ele-ments which perform the height adjustment, tilt correction, and orthogonal alignment, relative to the beam (Fig. 4). Mounted on this Z-stage is an air-bearing supported rotation stage (Fig. 5). In developing this stage, the designers had to push the limits of technical feasibility: What was required was a really

Fig. 3 The picture shows the assembly of the substructure, which weighs several tons and is mounted on air bearings, in the parking position outside the beam.

Fig. 4 The Z lifting stage is based on three identical, symme-trically arranged, and position-controlled stepper motors, combined with worm gears and spindle drives.

PI /

HZG

PI /

HZG

Fig. 2 This X-ray imaging setup has the sample positioned in front of the optics.

X-ray optics

Sample

Detector

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“pure” rotary motion of the sample with minimal wobble, radial runout or eccentricity. Only then can sharp pictures over 360 degrees be made which all refer to the same volu-me element and can all be clearly assigned when reconstructing the picture. This is why the rotation stage, which rotates at a velocity of 36 °/s, works with flatness de-viations of less than 100 nm at a resolution of 0.5 µrad. The air bea-ring does not produce any fric tion, which over time would lead to a deterioration of these values.

Parallel Kinematics

The actual sample holder is loca-ted in the aperture of the rotation stage on the moving platform of a six-axis parallel kinematic machine (Fig. 6). The SpaceFAB (see Box „Parallel-Kinematic Positioning Systems“) clearly makes work ea-sier for the researchers, since the small samples – only a few 10 to 100 micrometers in size – plus the holder can initially be inserted into the stage with low precision. They can then be aligned automatically using software commands. Thus, no additional mechanical com-ponents are required for correct alignment. The samples are posi-tioned with six degrees of freedom. Essential features are the freely se-lectable pivot point of the parallel-kinematic system and its high stiff-ness. A six-axis parallel-kinematic machine of this type is also used for the positioning of the optics. In

nanotomography, which allows three-dimensional micrographs with resolutions below 100 nm, this machine is used to align compound refractive lenses (CRL) in the beam with high precision.

A wide range of areas, from industrial research to materials science and examination of bones in biology, can benefit from the

investigation results obtained by means of these high-resolution tomographic methods on the Imaging Beamline P05. The posi-tioning solutions tailor-made by the “Beam line Instrumentation” specia lists, used to align the small samples and optical components with high precision, make an im-portant contribution.

Fig. 5 Mounted on the Z stage the air-bearing supported rota-tion stage, which provides rotary motion of the sample with minimal wobble.

Fig. 6 Close-up view of the actual sample holder that is loca-ted in the aperture of the rotation stage on the moving plat-form of a six-axis parallel kinematics.

PI /

HZG

PI /

HZG

DE (+49) 36 41 66 88 0 US (+1) 508 634 - [email protected]

Piezo StagesPositioning Technology

www.piezosystem.com

Piezoslit Solution for Precise Beam ControlPureEdges for Beam Collimation

Beam collimation control

Flexible beam adaption

High resolution and variable aperture alignment

Fast acting piezo shutter



Scanning probe microscopy (SPM) at cryogenic temperatures and in high magnetic fields allows study-ing many quantum phenomena at the nanoscale. Due to the ongoing helium crisis, closed-cycle cryostats are becoming more and more po-pular. However, almost all available dry coolers suffer from severe vi-brations, which prohibits sensitive measurement techniques. In this article, we present state-of-the-art SPM measurements performed in a new type of dry magnet system with extremely low vibrations. This cryostat system provides a unique solution for sensitive techniques that require a calm, yet cold envi-ronment.

M any quantum phenomena studied in physics as well

as materials science today require variable or cryogenic tempera-tures, often in conjunction with high magnetic fields. In order to reach low temperatures and to cool super conducting solenoids, which provide high magnetic fields, he-lium is the only available cooling agent. For more than a hundred years now, scientists have made use of liquefied helium and successful-ly employed its low boiling point of 4.2 K to cool their samples. Even lower temperatures can be achieved by pumping on liquid he-lium to reduce its vapor pressure. However, liquid helium has never been cheap, and in recent years, prices have gone up considerably. At the same time, supplies have become increasingly unreliable even in high-tech locations such as some of Europe’s capitals. This in-security has started to threaten the scientific progress of many scien-tists, and hence the move towards the so-called “dry” technologies is a natural evolution.

Such cryostats still have no other choice than to cool with helium, but they keep a fixed amount of the precious gas contained in a closed-cycle, which maintains low tem-peratures by a cyclic compression and expansion of the working gas. Hence, the running costs are kept at a fraction of those compared to liquid helium based coolers. The invention of the Gifford-McMahon and pulse-tube coolers seems to have solved most problems at first sight. However, one very significant problem has remained with almost all commercially available setups: The cold heads generate relatively strong vibrations on the order of many microns, which is prohibitive for most sensitive measurement techniques – in particular scanning probe microscopy, which extends down to (sub-)atomic resolution routinely.

In this article, we present a new cooling system, which by design minimizes the vibrations dramati-

cally, and hence enables even deli-cate measurements without further modifications. In particular, the microscopes do not require spring-mounting or active dampening inside to further reduce vibrations. This even enables the combination of scanning force microscopy with high-resolution confocal imaging and spectroscopy using free optical beams, as well as combinations with magnetic fields. We show some recent examples of state-of-the-art applications of scanning probe microscopy based research performed in an exceptionally quiet dry cryostat.

The Cryostat

The design of attocube’s dry cryo-cooling system (Fig. ) is based on a pulse tube cooler, with two separate cooling stages at T = 40 K (top stage) and T = 4 K (lower stage). The 2-stage pulse tube cryocooler

Nanoscale Imaging Gone DrySensitive scanning force microscopy in closed-cycle cryostats allows studying quantum phenomena.

Florian Otto and Christoph Bödefeld

Dr. Florian Otto and Dr. Christoph Böde-feld, attocube systems AG, Munich, Germany

Fig. The closedcycle cryocooling system is based on pulse tube technology and consists of three main parts: a pulse tube, a rotary valve, and a compressor.

A T superconducting magnet, fixed to the lower temperature stage of the pulse tube, surrounds the sample space coaxially and is cooled by thermal contact.

40 K stageSample space with SPM insert

4 K stage

Pulse tube cooler

Superconducting magnet

Top table CompressorRotary valve

Dampers

Top-loading insert

Vacuum shield

Exchange gas chamber

Microscope



is driven by a remote compressor and connected via a rotary valve that controls the frequency of the pulsing. Often, such systems are also equipped with superconduc-ting magnets, conductively cooled by the pulse tube through thermal contacts.

This particular design ensures mechanical decoupling between the pulse tube and the sample space in such a way that the vibrations from the pulse tube cryocooler do not influence vibration-sensitive experiments, while still ensuring a good thermal contact for sufficient cooling power. Initial cooling of the whole system requires – 10 hours with no magnet and 10 – 1 hours including a T magnet. Usually, the cryostat and magnet are always kept cold between measurements, so that cooling down a microscope insert from T = 300 K to base temperature (4 K) is normally achieved within 1 – 2 hours, depending on the mass of the microscope. Warming up the microscope system also takes only about 1 hour. Thanks to the top-loading architecture, the sample space is immediately accessible after warming up. The configuration of the system allows for a turnover time of about three hours, including sample or tip exchange, which typi-cally requires only a few minutes.

Low Vibration Sample Space

For the initial characterization, a calibrated vibration detector was used to measure vibrations in-situ directly at the sample location [1]. Careful measurements of the am-plitude spectral density show a red-

uction of up to three orders of ma-gnitude at the pulse tube frequency: We measure absolute vibration amplitudes below 20 nm (vertical axis) and nm (horizontal axis) respectively compared to the dis-placements measured on the cold head itself of approximately µm [2]. The measured absolute vibrati-on amplitude along the vertical axis in the nm range is low enough to enable scanning probe micros copy measurements such as atomic and magnetic force microscopy (AFM, MFM) as described below.

Contact Mode AFM Measurements

One estimate for the overall vibra-tions and hence the noise encoun-tered in a real AFM measurement is given by the relative displacement amplitude between the AFM tip and the sample without scanning.

In the simplest configuration, this quantity (z-noise) is acquired in contact mode. In contrast to most other AFMs, our instrument does not use a 4-quadrant diode detection system, but a fiber-based

interferometer (Fig. a) [3]. We acquire z-height data over a few minutes (10,000 points @ .12 ms sample time), while keeping the tip in contact with the sample surface at a fixed lateral position. The data is line-by-line flattened to remove slow drifts below 1 Hz (e. g. piezo creep), so that the ef-fective bandwidth is approximately 1 to 200 Hz. Typically the z-noise data has a Gaussian distribution characterized by its standard de-viation (Fig. b), which in this case was σ = pm rms. This value is measured with the feedback loop enabled (with the same parame-ters as used for regular imaging), and is almost as low as in typical liquid helium cryostats. When the feedback is turned off, the noise amplitude increases approximately by a factor of 4. To further demon-strate the low vibration noise we imaged atomically flat terraces of height corresponding to the lattice parameter a = 0.3 nm in our dry cryostat at T = 3.2 K on a strontium titanate (SrTiO3) commercial wafer (Fig. c, d). This demonstrates the extremely low noise in the cryostat

Fig. a) Interferometric head for AFM measurements with builtin fiberbased interferometer. b) Contact mode noise scan histogram of the zheight values measured with a bandwidth of

1 ... 200 Hz at 3.2 K. The measured amplitude of the relative displacement standard deviation is 5 pm. c) Contactmode AFM image of atomically flat terraces on SrTiO (200 scan lines) at 3.2 K.

The step height is 0.3 nm, corresponding to the lattice parameter of the crystal. d) Line profile showing the height of the atomically flat terraces on SrTiO.

Fig. 3 MFM measurements of the vortex lattice in superconducting BSSCO at different magnetic fields: a) –1 mT, b) –4 mT. The images taken at T = 4 K show con

stant height measurements ~30 nm above the sample, with an active phaselocked loop (PLL).

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on a real sample, hence enabling the investigation of other interesting physical phenomena further dis-cussed below.

Magnetic Force Microscopy

Physics at low temperatures offers the challenging and rewarding opportunity of investigating fun-damental properties of matter. In particular, the scanning probe fa-mily of magnetic imaging methods includes powerful techniques to probe fundamental interactions on the nanometer scale.

One interesting example of quantum objects that are intensely studied with these methods are vortices in type-II superconductors, motivated mostly by the desire to pin down the nature of high-Tc su-perconductivity. The magnetic pro-perties of a superconducting vortex originate in a circular supercurrent, which allows one magnetic flux quantum (Φ0 = 2.07 . 10–15 Tm2) to penetrate the superconductor. The density of vortices can be tuned and is directly proportional to the external applied magnetic field in the intermediate region between the lower critical magnetic field Hc1 and the critical field Hc2 above which superconductivity vanishes. Through the mutual repulsion of neighboring circular currents, a vortex lattice forms in order to minimize the free energy density, which in the easiest case is hexa-gonal as theoretically predicted by Abrikosov [4]. However, due to na-tural or artificial defects present in most materials, there are additional pinning forces, which immobilize and trap vortex cores at preferred locations, distorting the ideal lat-tice. While vortices can move upon application of an external current and induce electrical resistance, controlled tailored pinning is desi-rable to reduce this adverse effect and minimize electrical losses.

Magnetic force microscopy (MFM) is a viable tool to observe such vortex lattices, e.g. on a freshly cleaved Bi2Sr2CaCu2O8+x (Bi-2212) cuprate superconductor (Fig. 3). The orientation of the vortices with

respect to the moment of the tip is indicated by the colour of the vortices: bright colours indicate repulsive forces. Here, the tip was scanned at the constant height of about 30 nm above the surface of a freshly cleaved piece of Bi-2212 (sample courtesy of A. Erb, TU Munich, Germany). Note that the applied field was always much lower than the coercitivity of the hard-magnetic tip (~40 mT), hence the orientation of the tip moment was kept unchanged. While at low vortex densities (Fig. 3a) pinning effects dominate and the lattice is disor dered, higher fields and hence higher vortex densities result in a more ordered hexagonal Abrikosov pattern (Fig. 3b). Statistical evalua-tions of such measurements are of-ten used in material science to find out about the pinning forces in the-se materials (see for example [5]).

As an indicator for the perfor-mance of the system, we measured the signal-to-noise ratio (SNR) from the peak heights of the isola-ted vortices for example at B = 1 mT to be larger than 20:1, with a 10 ms bandwidth, matching the SNR measured in low noise liquid heli-um cryostats.

Skyrmions in A Crystal

Magnetic skyrmions are nanoscale spin textures in chiral magnets. The name skyrmions allude to a non-linear field theory proposed in the context of nuclear physics by T. H. R. Skyrme [6]. In magnetic materials corresponding topologi-cally nontrivial configurations were recently discovered within a narrow region of temperature and magne-tic field [7]. The skyrmion-lattice phase can be observed in materials without inversion symmetry. The topological stability of magnetic skyrmions makes them excellent candidates for future high density magnetic storage materials [8, 9]. Skyrmions as little as 1 nm were recently reported [10]. Furthermore, the demonstration of the creation and annihilation of a single skyr-mion [11, 12] shows their potential for the application in information technology.

Using a high-quality single-crystal of Fe0.5Co0.5Si (sample courtesy of A. Bauer and C. Pflei-derer, TU Munich, Germany, [13]), both helimagnetic structures at T = 3.2 K in zero magnetic field as well as a skyrmion-lattice texture at

Fig. 4 MFM images of a polished surface of bulk samples of Fe0.5Co0.5Si. a) Helimagnetic phase of the sample at T = 3.2 K after zerofield cooling (B = 0 T). b) Meta

stable skyrmionlattice phase measured at T = 3.4 K in an external magnetic field B = 15 mT after fieldcooling.

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T = 3.4 K in an externally applied field B = 1 mT (Fig. 4) could be observed during measurements in our labs. In both cases, the magne-tic tip was kept at a constant height of 20 – 30 nm over the sample surface, with a phase-locked loop activated to track the cantilever resonance frequency. For field coo-ling to observe the skyrmion phase, the sample was first heated to 0 K, then the mag netic field was incre-ased to 1 mT, and subsequently the sample was field-cooled to base temperature again. With the persistent switc h heater of the super conducting magnet enabled, the temperature stabilized at 3.4 K at the sample position and 3. K at the magnet.

In summary, we have shown above that our cryostat design en-ables very sensitive scanning probe microscopy experiments at low temperatures and in high magnetic fields. This is possible even without the need for additional spring sus-pension or internal damping in the microscope itself. The low noise in AFM measurements was demons-trated via imaging of atomic steps, with further applications ranging from vortex imaging in supercon-ductors to the observation of exotic new phases in materials potentially interesting for magnetic storage at nanometer length scales. [1] F. P. Quacquarelli et al.,

arXiv:1404.204v1 (2014) [2] T. Tomaru et al., Cryogenics 44, 30

(2004) [3] D. Rugar, H. J. Mamin and P. Guethner,

Appl. Phys. Lett. 55, 2 (1) [4] A. A. Abrikosov, Soviet Phys. J. Exptl.

Theoret. Phys. 5, 1174 (17) [] H. Yang et al., Phys. Rev. B 85, 01424

(2012) [] T. H. Skyrme, Proc. R. Soc. Lond. 260,

127 (11) [7] X. Z. Yu et al., Nature 465, 01 (2010) [] C. Pfleiderer and A. Ros, Nature 465,

0 (2010) [] N. Nagaosa and Y. Tokura, Nature

Nanotech. 8, (2013) [10] S. Heinze et al., Nature Physics 7, 713

(2011) [11] P. Milde et al., Science 340, 107 (2013) [12] N. Romming et al., Science 341, 3

(2013) [13] A. Neubauer et al., Rev. Sci. Instrum. 82,

01302 (2011)

Confocal Microscope with Optical Cryostat

Manufacturer: Montana Instruments. Distribution: LOT-QuantumDesign.Product: “Cryostation” (an optical cryostat with closed helium cycle) that is perfectly suited for optical experiments thanks to its high mechanical stability. With the confocal microscope option, the sample can easily be focused to an unsurpassed accuracy, even at temperatures as low as 3. K. Features: The system is based on a vacuum-compatible Zeiss “EC Epiplan-Neofluar 100 x” objective with an infinity color-corrected image distance, a numerical aperture (NA) of 0.0 and 0.31 mm working distance. Drift of both the sample and the optic, even when cooling down the sample, is eliminated by a patented design. Sample translation and focus is accomplished with built-in nano positioners. Temperatures of the high-resolution objectives and sample are controlled to an accuracy of 0.01 degrees for undetectable drift levels. The provided re-placement power supply ensures performance. Applications: The “Cryostation” microscopy option is the ideal tool to run confocal microscopy on single molecules with a

cryostat with a closed helium circuit in an almost drift-free setup with a high numerical aperture. �

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Dual Rotating Compensator Ellipsometer

Manufacturer: Woollam. Distribution: LOT-QuantumDesign.Product: Spectroscopic ellipsometer “RC2”, the first device of this kind with two rotating compensators. Features: The ellipsometer combines the best features of pre-vious instruments with innovative new technology: dual rota-ting compensator, achromatic compensator design, advanced light source and next-generation spectrometer design. The device is the first commercial spectroscopic ellipsometer to collect all 1 elements of the Mueller matrix. Mueller matrix SE allows characterization of the most advanced samples and nanostructures. Synchronous operation of both compensators allows highly accurate data without waiting to “zone-average” over optical elements. The system collects the entire spectrum (over 1000 wavelengths) simultaneously in a fraction of a sec-ond. Thus the device is a near-universal solution for the diverse applications of spectroscopic ellipsometry.�

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Terahertz SpectrometerManufacturer: Laser Quantum.Product: High-resolution Terahertz-spectrometer “HASSP-THz” that per-forms measurements of solid samples, thin films, liquids, powders and gas cells in the range 0.05–6.5 THz . Typical signal-to-noise ratios (SNR) of more than 65 dB with up to 1 GHz frequency resolu-tion are achieved at acquisition times of less than one minute.Features: The device is a time-domain THz spectroscopy system with up to 1 GHz spectral resolution and a spectral coverage of more than 6.5 THz for scien-tific applications. It employs high-speed asynchronous optical sampling (ASOPS) including two separate femtosecond Ti:Sa lasers at a repetition rate of 1 GHz which are slightly detuned. The ASOPS principle makes a mechanical delay stage redundant and consequently allows ultra-sensitive time-domain THz spectroscopy at video-rates with 30 dB peak dynamic

range. In less than one minute acquisi-tion time a SNR of more than 65 dB at a frequency resolution of 1 GHz is possible. A frequency precision on the order of 150 MHz has been demonstrated, a value that out-performs any other published time-domain spectroscopy instrument. The housing can be evacuated and may be purged with gas or evacuated. The spectroscopic geometry is easily inter-changeable between transmission mode, reflection mode, using an optional 25 cm long collimated beam-line for gas cells

or Attenuated-Total-Reflection (ATR) configuration.Application: THz spectroscopy in scien-tific applications where ease of use, fast acquisition times, high precision and high spectral resolution are required. The rapid data acquisition capability (up to 10,000 single-scan traces per second at 1 GHz frequency resolution) permits the investigation of dynamic processes and studies under rapidly varying environ-mental conditions. It also enables very short pixel dwell times in spectroscopic imaging applications, allowing images to be acquired in a matter of a few seconds.�

Laser Quantum GmbHMax-Stromeyer-Str. 11678467 Constance, GermanyPhone: +49 (0)7531 368371Fax: +49 (0)7531 368372E-mail: [email protected]: www.laserquantum.de

Collimators for X-Ray TechniquesManufacturer: piezosystem jena.Product: New piezo based collimator system for X-Ray techniques, developed in cooperation with Lawrence Berkeley National Lab.Background: Current techniques in X-Ray microscopy, crystallography and small angle X-ray scattering require ex-tremely well-collimated beams. In order to achieve the best signal to noise ratio in an X-ray system, the aperture size should be as closely matched as possible to the sample size. Most X-ray systems use a pinhole aperture that has been drilled or laser ablated from a tungsten or tantalum

disk of material. In practice this means that for every new sample size, the pin-hole should be removed and a new size inserted. Features: The new collimator was en-gineered based on two piezo driven slit systems of the series “PZS 4”. The “PZS” stages are mounted of 90° in respect to each other. Because of the high resolution piezo element, the slit edges can be adju-sted precisely to the required size. With a resolution of <8 nm, the system is the most accurate variable aperture system commercially available. The open archi-tecture of the “PZS” slit system design

allows for easy integration into a variety of X-Ray systems. A V-clamp holder allows the customer to add his own or our accessories. The entire system can be made vacuum compatible.�

piezosystem jena GmbHStockholmer Straße 1207747 Jena, GermanyPhone: +49 (0)3641 6688-0Fax: +49 (0)3641 6688-66E-mail: [email protected]: www.piezosystem.de

Nanoindenter for High-temperature ApplicationsManufacturer: Micro Materials. Distribution: LOT-QuantumDesign.Product: High-temperature nanoindenter “NanoTest Vantage” for nanomechanical and tribological examinations on materi-als and coatings at elevated temperatures.Background: Subjecting materials to high temperatures often leads to signifi-cant changes in their mechanic proper-ties. Especially in aeronautics there is a wide array of applications in the array above 400 °C and for wear protective coatings. In more than 10 years, Micro Materials has gained substantial know-how, by the close cooperation with lea-ding scientists, to develop marketleading nano and micro mechanical test plat-forms for high-temperature measure-ments up to 750 °C.

Features: All these measurement systems including the nanoindenter “NanoTest Vantage” show inherent thermal stability and excel by a unrivaled design. A listing of all publications on high temperature nanoindentation at the end of 2013 sho-wed that all relevant data was measured in a temperature range above 400 °C with “NanoTest” systems.�

LOT-QuantumDesign GmbH Im Tiefen See 5864293 Darmstadt, GermanyPhone: +49 (0)6151 8806-0Fax: +49 (0)6151 8806-64E-mail: [email protected]: www.lot-qd.com/de



Closed-Cycle Cryo-Optical Table Manufacturer: attocube.Product: Closed-cycle cryostat system “attoDRY800”, integrated into an optical table. Special care was taken during the development of the system to keep the displacements to a maximum of 1 nm (RMS) by a special patented vibration iso-lation technology. Hence, even extremely sensitive measurements are possible.Background: Quantum optics experi-ments often require cryogenic tempera-tures combined with optical access. Most experimental setups contain numerous optical elements delicately arranged on an optical table to shape and prepare the incident light, as well as to efficiently col-lect and convert the emitted light from the sample. The available space on the optical table is in such cases paramount to many complex setups.Features: The ultra-low vibration cold breadboard platform is integrated into an optical table, hence making use of the before often unused space underneath it.

This unique design ensures unobstructed optical access to the cold sample from all directions on the optical table via 4 side and 1 top optical window. Apochromatic objectives with high numerical aperture (NA = 0.81–0.95) can either be integrated into the cryostat, into the vacuum shroud, or put in close working distance next to the optical windows from the outside. This ensures extremely low drifts and optimal collection efficiency. The closed-cycle cryostat can replace all flow cryostat setups, with the huge advantage that it requires no liquid cryogens, thus mini-mizing running costs. In addition, a fully automated temperature control between 4 and 350 K conveniently enables unatten-ded long measurement cycles. The cold breadboard sample space is predestined to host several of attocube’s patented nanopositioners, as well as complete microscope or photonic probe station solutions.

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attocube systems AG Königinstr. 11a 80539 Munich, Germany Phone: +49 (0)89 2877809-0 Fax: +49 (0)89 2877809-19 E-mail: [email protected] Website: www.attocube.com

Next-generation Miniature Spectrometer Manufacturer: Ocean Optics.Product: Modular miniature spectrome-ter of the “Flame” series that combines decades of miniature spectrometer design expertise with industry-leading manu-facturing techniques. It offers a thermal stability of 0.05 nm/°C from 200–850 nm and highly consistent unit-to-unit per-formance that meets the needs of OEMs and other high-volume customers. With its small footprint (89 × 63 × 34 mm) and low weight (265 g), the device is ideal for OEM integration.Features: The spectrometers deliver high thermal stability and low unit to unit va-riation without compromising the flexibi-lity and configurability. Features such as interchangeable slits, indicator LEDs and simpler device connectors provide great flexibility for a wide range of UV-Vis ap-

plications including OEM integration and lab, industrial and field use. The spectro-meters are fully configurable across the 190–1100 nm wavelength range for use in absorbance, transmission, reflectance, irradiance and colour applications. They

come preconfigured or custom confi-gured, with interchangeable slits that enable users to adjust resolution and throughput on demand. For example, the user can reconfigure the same spectrome-ter from high resolution for absorbance to high throughput for fluorescence in seconds. To further increase measure-ment power, the device works seamlessly with Ocean Optics’ range of light sources, optical fibres, sampling accessories and software.n

Ocean Optics Geograaf 24 6921 EW Duiven, Netherlands Phone: +31 26 3190500 Fax: +31 26 3190505 E-mail: [email protected] Website: www.oceanoptics.com

AHF analysentechnik AG · +49 (0)7071 970 901-0 · [email protected]

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Optical FiltersFor Fluorescence Spectroscopy



W ith climate change and its implications for society and

the Earth being a frequent topic in both politics and science, measure-able data on the influence of man-kind on current and past climate has become essential for making predictions and decisions about future climate.

The polar ice sheet provides information about temperature, precipitation as well as gas and aerosol concentration as a unique depiction of climate throughout hundreds of thousands of years. The informa tion obtained from ice cores enables future climatic events as well as general material proper-ties of ice to be better understood. The longest ice core drilled in An-tarctica has a length of 3270 meters and contains climate information dating back more than 800 000 years.

The rapid analysis (min. scan time 3 s) provided by the Large Area Scan Macroscope (Fig. 1) with a resolution of 5 µm has proven to be an essential tool for analyzing the microstructures of ice cores, both in the field and in the laboratory. A stratigraphic image that supports dating the ice cores can be obtained using the Intermediate Layer Core Scanner.

The densification of snow to ice at the glacier surface is a complex process influenced by temperature, the amount of precipitation, wind and the presence of impurities on trace levels. Snow is compressed throughout the years to firn, which is still permeable to air, and finally to ice. At the transition of firn to ice, 50 to 100 m below the surface, atmospheric air is enclosed in air bubbles, which at greater depths transform into air clathrates and make the ice transparent like plexi-glas (Fig. 2).

By analyzing the microstructure of the ice core for example, glacio-logists learn about deformation and recrystallization of ice and the en trapped gases in order to reconstruct climatic events in the past.

Ice Coring Projects – EPICA and NEEM

The challenge of retrieving the ice cores necessary for these eva-luations was taken on by several ice drilling projects, such as the EPICA (European Project for Ice Coring in the Antarctica) that managed to drill two cores through the Antarctic ice sheet, one on Dome C (3270 m) in the Pacific sector and a second one at Kohnen Station (2774 m) in the Atlantic sector.

One drilling camp was Kohnen-Station, about 760 km inbound from the Neumayer Station, the German over-wintering base in the Antarctica. All equipment, in-cluding the measuring equipment

to analyze the ice cores, was either flown into the camp or transported by overland traverses using cargo sledges, that take an average of ele-ven days to get there [1].

Drilling operations took place during the austral summer (December until the beginning of February) from 2001 throughout 2006. The temperature in the drill trench during the austral summer season was about –30 °C. The annual average air temperature at Kohnen Station is –45 °C (at Dome C: –55 °C).

Another ice coring project is NEEM (North Greenland Eemian Ice Drilling Project) from 2007 – 2011. The 2.5 km long ice core retrieved in Greenland provides data characteristic for the Northern hemisphere dating back to the last interglacial period, also called Eem warm period (115 000 – 130 000 years ago) when the Earth was about 3 – 5 °C warmer than today, and thus comparable with the climate we might be facing soon.

Microstructure Analysis of Polar Ice CoresAnalyzing past climates using the Large Area Scan Macroscope

Anja Krischke, Ulrich Oechsner and Sepp Kipfstuhl

Fig. 1 Large Area Scan Microscope for the microstructure analysis of polar ice cores.

Dipl.-Phys. Anja Krischke, Dr. Ulrich Oechsner, Schaefter+Kirchhoff GmbH, Optics, Me-trology, and Photo-nics, www.sukham-burg.de; Dr. Sepp Kipfstuhl, Alfred Wegener Institute, Am Alten Hafen 26, 27568 Bremerhaven, Germany, www.awi.de

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Standard analysis using a microscope

One method used to analyze the ice cores is microstructure mapping. It maps the microstructure of firn and ice at microscopic resolution, visualizing air inclusions, grain and subgrain boundaries, thus also grain size and shape.

The ice cores are first cut into pieces of 1 m, and then the single cores are cut according to a prede-termined cutting scheme for indi-vidual analyses. Defined sections of about 45 × 90 mm2 and 6 mm thickness are cut and microtomed. The polished surface is exposed to the free atmosphere for subli-mation: The scratches produced by the microtome blade disappear and grooves, lines and pits start to develop at the sites where grain or subgrain boundaries or inclusions meet the surface.

The most common analysis method for ice cores uses an opti-cal microscope, a CCD area scan camera, a frame grabber as well as an xy-stage [2]. Images are taken in transmission.

An overall area of 45 × 90 mm2 is mapped by acquiring single microscopic images about 2.5 by

1.8 mm in size. After image capture, the ice core sample is moved using the translation stage so that an image is taken about every 2 mm. The overlap of about 0.5 mm bet-ween the images is helpful for the later reconstruction. A series of 1500 images with an acquisition time of about an hour to 90 mi-nutes is necessary to reconstruct a sample section with a resolution of 3 – 4 µm.

Fig. 3 shows a single (b) and a fully reconstructed picture (a) of a 20 × 45 mm2 wide ice core section. The dark lines are grain bounda-ries, the dark spots air hydrates. The few gas inclusions indicate that this sample was taken from a grea-ter depth (1291 m).

Rapid ice core analysis – the Large Area Scan Macroscope

The time-consuming ice core analysis using a microscope was replaced in 2007 by using the spe-cially developed Large Area Scan Macroscope (LASM) with a line scan camera (Fig. 1).

Line scan cameras are a popular choice whenever a high resolution image of a large area is necessary.

In order to achieve a 2D image, the object is moved with defined velo-city against the sensor and the indi-vidual line signals are put together to form the complete image.

Fig. 3a – c show the scans of three ice cores samples obtained from different depths. The ice core image from 60 m depth (a) shows well de-fined grain boundaries (dark lines) and pores. The air inclusions be-come rarer and smaller with depth (b; 615 m depth). Finally, at 1035 m almost all gas inclusions have trans-formed into air hydrates.

The Large Area Scan Macros-cope is depicted in Fig. 4a and con-sists of a line scan camera with 8192 pixels and Gigabit Ethernet interface, a high resolution lens as well as an illumination unit. The ice is imaged in reflection with a resolution of 5 µm. The measuring width is 41 mm with an unlimited measuring length and a scan speed of up to 36 mm/s. Total scan time for a sample with 40 × 90 mm2 is less than 1 minute (min. actual scan time 3 s). In order to capture the relevant microstructures, bright-field illu mination is used. The principal scheme of this technique is depicted in Fig. 4b.

The light directed at the sample is reflected by surfaces parallel to the sensor. Light reflected from structured areas and edges is re-flected away from the sensor and appears dark. Thus, also in the images obtained with this method, the grain boundaries appear as dark lines and gas inclusions appear as dark bubbles or spots.

Fig. 2 Ice core sample images from diffe-rent depths. All scans are obtained using the Large Area Scan Macroscope. The ice

core image from 60 m depth (a) shows grain boundaries and pores. These in-clusions become rarer and smaller with

depth (b; 615 m depth). Finally, at a depth of 1035 m almost all gas inclusions have transformed into air hydrates.

a b c

Fig. 3 Image of a 20 × 45 mm2 wide ice core section mapped by an optical microscope. A series of 300 images is necessary for the fully reconstructed picture (a). A single picture, 2.5 × 1.8 mm2

in size, is marked with a white rectangle and shown in b. The dark lines show grain boundaries, the inclusions are air hydrates [2].

ba



Undisturbed, High Quality Images in Much Less Time

While for the image acquisition technique using a conventional microscope, thousands of images have to be stitched to form a com-plete picture, only two or three scans are necessary using the Large Area Scan Macroscope depending on sample dimensions. This redu-ces the imaging time considerably and obviates the alignment and matching of the many individual images of these sections, which re-quires significant computing time. Since the microscope method takes a long time, all images are addi-tionally taken with slightly different contrast due to the ongoing subli-mation process, which also needs to be corrected for. In order to stitch the complete picture, the images also have to be corrected for vignet-ting and distortion.

Using the Large Area Scan Microscope, a shading correction done prior to scanning allows for evenly illuminated images that also do not show significant signs of distortion due to an excellent correction of the field of curvature. Since only two or three images are necessary to cover the whole sam-

ple the time required for stitching is severely reduced.

The short time necessary to acquire a complete picture (from > 1 h to about 1 – 2 minutes) of an ice core allows for many more samples to be taken during the limited time available in the field, providing a much more detailed picture of the microstructure wi-thin the whole ice cores. While with the microstructure mapping approach a 10 cm sample can only be mapped every 10 m, using the LASM 10 images can be obtained over 1 m in the same time. Since the image acquisition is so fast, the ice core samples can even be scan-ned several times to document the sublimation process (for example right after microtoming, and some time later) which is not possible using the microscope technique.

Dating the Ice Cores – The Stratigraphy Scanner

The annual variations in the amount of precipitation and the de-position of mineral dust and other particles lead to a layered structure of the ice.

Visual stratigraphy visualizes these climate induced annual vari-ations [3] and helps to date the ice cores by counting the layers. Global climatic events, such as the erup-tions of volcanoes are sometimes

visible when the layers contain ash particles.

The specially developed Inter-mediate Layer Core Scanner (ILCS, Fig. 5a) is used to examine samples of up to 1.7 m in length. After micro-toming the sample on both sides, the layered structure is captured using a line scan camera based scanner. The camera (SK2048GPD-4L with 2048 pixels and Gigabit Ethernet interface) located above the sample is moved synchronously to an indirect light source, that is mounted below the sample. As can be seen in Fig. 5b depicting this dark-field illumination technique, the light from two sources is focused into the sample from two sides. On-ly light scattered from the sample is directed back into the camera, di-rect light from the illumination unit does not reach the sensor.

With a resolution of 51 µm and an imaging width of 105 mm, ice core samples of up to 1200 mm (1700 mm with two overlapping scans) can be scanned with a speed of up to 22.7 mm/s. Total scanning time is ~10 s for a core of 1100 mm in length.

Fig. 6 shows a stratigraphic image. Transparent ice appears dark while bubbles or dust parti-cles form bright visible layers. The number density of layers in a core section characterizes the climate, colder periods show more and brighter layers, whereas transpa-

Fig. 5 Intermediate Layer Core Scanner (ILCS, a) and optical scheme (b). The camera located above the ice sample is moved synchronously to an indirect light source that is mounted below the sample. Relevant structures are visua-lized using the dark-field illumination

technique. The light from the two sour-ces is focused into the sample from two sides. Only light scattered in the sample is directed back to the camera, direct light from the illumination unit does not reach the sensor.

a b Line Scan Camera

Focused Illumination

Ice Sample

a

b

Fig. 4 Large Area Scan Macroscope (a) and optical scheme of bright-field illumi-nation (b): The light directed at the sam-ple is reflected by surfaces parallel to the sensor. Light reflected from structured areas and edges is reflected away from the sensor and appear dark.

rent thus dark ice indicates that the ice was formed during a milder climate period. Colored layers in-dicate volcanic ash layers.

Acquiring High Resolution Images in Harsh Environments

As the analysis of both the micro-structure as well as the visual stra-tigraphic need to be done in the field during drilling as well as in the lab in Bremerhaven or elsewhere, both line scanners need to be ro-bust and insensitive to the harsh environment.

The components used in both setups (mechanical, optical as well as electrical) are designed to work properly at temperatures down to –45 °C and are stable and robust enough to endure the long and bumpy ride to and from the drilling site (11 days each way for Kohnen Station).

Both scanners described here have been used in the field in Ant-arctica as well as in Greenland mul-tiple times. Whenever drilling is not ongoing they are used in the lab at AWI in Bremerhaven.

Conclusion

The polar ice sheet (in itself one of the purest natural materials on Earth) provides unique information about temperature, precipitation as well as gas and aerosol con-centration depicting the climate throughout hundreds of thousands of years.

The microstructure of the ice cores can be imaged rapidly and with high contrast and resolution by using the Large Area Scan Ma-croscope with integrated bright-field illumination. While the stan-dard microscopic method takes hours for image capture (about 1500 images are necessary for a sample size of 45 × 90 mm2) and requires extensive computing time, only two or three scans taking a total time of 1 minute (min. scan time 3 s) are necessary to acquire high resolution images of much larger sections with the Large Area Scan Macroscope. Accordingly, more scans can be taken during the limited time available in the field, thus depicting a much more detailed picture of the micro structure within the moving ice sheets.

The Intermediate Layer Core Scanner that uses dark field-illu-mination to visualize the layers in the ice helps to date ice cores. The robustness of both scanner systems allows their transport as well as their use in the field at tempera-tures down to –45 °C.

*More information on glaciology

on www. awi.de.

[1] H. Oerter et al., Polarforschung 78, 1 (2008)

[2] S. Kipfstuhl et al., Journal of Glaciology 52, 398

[3] A. Svensson et al., Journal of Geophysi-cal Research 110, D02108 (2005)

Fig. 6 Stratigraphic scan showing the layered structure of the ice core. Trans-parent ice appears dark while micro-inclusions like mineral dust or salt parti-cles form brighter visible layers. Colored layers indicate volcanic ash horizons, grey layers mineral dust and salts inclu-sions.

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Schäfter+Kirchhoff develop and manufacture laser sources, line scan camera systems and fiber optic products for worldwide distribution and use.

[email protected] www.SuKHamburg.de

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M ultiphoton/nonlinear laser scanning microscopy has be-

come an important tool to investi-gate biological phenomena, where high resolution, 3-dimensional imaging with depth resolution is essential to uncover underlying bi-ological functions. Two-photon or multiphoton imaging is preferred to single photon imagining techniques due to several intrinsic advantages of multiphoton techniques. One of the advantages is having a nonline-ar intensity dependent absorption. For example, two-photon absorpti-on is restricted to the focal volume so there is virtually no out-of-focus bleaching or signal contribution. As opposed to single photon imaging where the excitation wavelength is in the visible or near-UV range, multiphoton imaging with near-IR wavelengths is associated with reduced scattering. This helps with extended depth resolution from a few microns to several hundreds of microns. Additionally there is a physiological benefit from using near-IR wavelengths in multiphoton microscopy, as these wavelengths are safe in preserving cell viability due to reduced out-of-focus absorption restricting linear absorption in the surrounding area.

Coherent anti-Stokes Raman scattering (CARS) is a powerful imaging technique that continues

to grow and draw attention of bio-logists due to its ability to provide label-free molecule-specific or functional group specific images of live cells and tissues. The different energies involved in the CARS set up are shown in Figure .

CARS microscopy and micro-spectroscopy is generally combined with other multiphoton microscopy techniques such as two-photon flu-orescence (TPF), second harmonic generation (SHG), third harmonic generation (THG) and other non-linear techniques to generate a true multimodal microscopy. The ver-satility of CARS is to scan through the Raman resonance, providing molecule-specific information of the specimen in question.

For a true multimodal imaging system, the TPF, SHG, THG etc. needs shorter pulses as short pulses pack maximum energy within the pulse. This is desirable for these nonlinear microscopic methods but not for CARS microscopy as the non-resonant (background) signal can overpower the CARS signal. CARS imaging is traditionally done with a pair of picosecond (narrow bandwidth) pulses either electroni-cally synchronized or generated wi-

th synchronously-pumped optical parametric amplifiers to match the vibrational resonance to enhance resonant contribution and suppress the non-resonance (background) CARS signal.

CARS with a spectral bandwidth of several to several hundred nano-meters is becoming commonplace in order to achieve true multimo-dal imaging. On one hand, broad bandwidths allow the simultaneous probing of a large number of vibra-tions while on the other hand short pulses with large bandwidth may lead to a non-resonant signal con-tribution and poor degree of spatial resolution. One way to eliminate the large non-resonant background is to use a chirped pump or Stokes beam by purposely dispersing one pulse with a grating compressor, a pair of chirp mirrors or a highly dispersive glass block while the other beam remains near-transform limited. One drawback of this ap-proach is that the interaction time between two pulses is restricted as one pulse is much shorter than the other so that the Stokes power is out of phase with most part of the pump and does not contribute to the CARS signal. As a result CARS

CARS Microscopy with Spectral FocusingWith the help of spectral focusing, vibrational imaging modalities using multimodal CARS have become invaluable tools in neuroscience.

Hans-Erik Swoboda and Tissa Gunaratne

Dr. Hans-Erik Swo-boda, Horiba Jobin Yvon GmbH, Neu-hofstr. , 25 Bens-heim, and Dr. Tissa Gunaratne, Clark-MXR, 300 West Huron River Drive,Dexter, Michigan 130, USA

Fig. In this CARS diagram both pump and probe wavelengths are the same. The CARS signal appears at the wave-length ωCARS = 2ωp – ωS.

a

ωpump

ωStokes

ωprobe

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Fig. 1 The Model cOPA has been used for multimodal microscopy in the past years.



signal levels are lower even though the background is minimized.

To overcome the poor spectral resolution with broad bandwidth pulses and increase the overlap of pulses, there are alternative methods that can be utilized yielding spectral resolution comparable to CARS imaging with picosecond pulses [1].

In order to overcome the difficu-lty of using short pulses for CARS imaging, spectral focusing, an in-novative method of using spectrally dispersed or chirped pulses, was introduced by Zumbusch and co-workers in 2004 [2]. Figure 3 shows the principle of this approach, in which both pump and Stokes pulses are intentionally chirped, with the result, that the frequency content of this pulse is spread out in time. In spectral focusing a broad-bandwidth pulse with controlled chirp can achieve Raman spectral resolution comparable to what can be achieved with picosecond pulses. The concept rests on the recognition that what is important in CARS is that the instantaneous difference between the pump and stokes pulse matches the Raman mode of interest. By far the simplest approach is to control linear (qua-dratic) chirp, a linear variation of frequency com ponents within the pulse. When using matched linear dispersion (chirp), the interaction is maintained over a wide range of delays. Temporal delay with equally chirped beams can select different vibrations. Another important pa-rameter is the repetition rate of the laser as photo damage and photo bleaching are crucial for biological

samples.Even though CARS is a third

order nonlinear process, in most cases photo bleaching has a lower nonlinearity than the CARS process and can vary between 1.1 and 2.5 [3]. Even though the exact mechanism is unknown, linear absorption by surrounding molecules leading to heating plays a significant role. In this case high peak power and low rep rate lasers are better than high-rep rate lasers such as Ti:Sapphire oscillator (~80 MHz) as high rep rate and linear absorption can lead to excessive local heating resulting in photo damage [4]. Therefore it is advantageous to use a laser with lower repetition rate in the few MHz range.

A laser system which has been used in the last years in the field of multimodal microscopy as descri-bed above is the Model cOPA from Clark-MXR (Fig. 1 and 4). The whole system consists of an all-fibre based

amplifier system (model Impulse), plus a set of optimized OPA-sys-tems (optical parametric amplifier).

Impulse is a one-box system consisting of a Yb-doped femtose-cond fibre oscillator seeding a fibre amplifier. The system delivers sub 250 fs pulses at adjustable repetition rates from 200 kHz up to 25 MHz at a wavelength of 1030 nm. In this particular set-up, the output of Im-pulse is used to pump two indepen-dently tunable NOPA (non-colinear optical parametric amplifiers). In the cOPA system two parallel OPAs are arranged to achieve two parallel synchronized output arms of wave-length tunable beams. Typical tu-ning ranges of the NOPA are given in Figure 4. With additional options it is also possible to get into the UV-range. Another very interesting capability of the NOPA is a substan-tial pulse shortening, potentially into the range of 20 fs or even lower [5]. This is a pulse length reduction of more than one order of magnitu-de as compared to the fundamental pump beam from Impulse. The Model cOPA, especially designed for microscopy applications, comes with a pulsewidth control mecha-nism that allows to adjust pulse duration from tens to hundreds of femtoseconds.

Beside the two arms from the NOPAs, Impulse delivers one ad-ditional output with a pulse energy of around 1 µJ at the fundamental wavelength of 1030 nm (Fig. 4).

One outstanding application for this system as a multimodal

Fig. 3 Chirped CARS (spectral focusing) process. Constant slope gives narrow and constant frequen-cy difference (ωp – ωs) and center tuning aids selec-tion of vibrational modes, which can also be achieved by temporal delay.

a 0,1

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Fig. 4 Schematics of Model cOPA. Model IMPULSE pumps two NOPAs/OPAs giving two independently tunable, synchro-nized outputs. A synchronized, residual

fundamental at 1030 nm with excess of 1 μJ is available from a third port that can be used to generate e.g. white light continuum.

Model IMPULSEYb-doped FiberOscillator/Amplifier

1030 nm > 1 μJ

Tuning Range (each arm)700 – 900 nm > 1 nJ 1130 – 1300 nm > 80 nJ at peak1125 – 1950 nm (optional)

Adjustable reprate to 25 MHz

Noise < 1%

Bandwidth > 150 cm-1

IndependentlyTunable NOPA #1

IndependentlyTunable NOPA #1 }



microscopy tool has been realized by the group of Peter K. Stys at the Hotchkiss Brain Institute at the University of Calgary, Canada. In this group spectral CARS and two-photon microscopy were recorded simultaneously to visualize mouse

ventral root (transgenic YFP).Figure shows an image of mouse

ventral root, transgenic YFP, taken with spectral CARS (panel a), an ex-panded section of the same sample showing central axon (b), and the overlay of the spectral CARS image

and the two-photon image taken si-multaneously (c) []. These pictures were generated by a unique non-linear data analysis package develo-ped at the Hotchkiss Brain Institute.

In summary, with the help of spectral focusing, vibrational ima-ging modalities on tissue using Coherent anti-Stokes Raman Scat-tering (CARS) and TPF have been recognized as invaluable tools in neuroscience to probe intrinsic mo-lecular biochemistry of neurological disease. It has been proven in the last few years that the Model cOPA from Clark-MXR is the ideal tool for this particular application.

References [1] C. Brideau, K. Poon and P. K. Stys, Proc.

SPIE 8588, (2013), doi: 10.111/12.2005512

[2] T. Hellerer, A. M. Enejder and A. Zum-busch, Appl. Phys. Lett. 85, 25 (2004)

[3] B. Chen and Sang-Hyun Lim, J. Phys. Chem. B. 112, 353 (2008)

[4] P. G. Antal and R. Szipőcs, Appl. Phys. B. 107, 1 (2012)

[5] C. Homann, C. Schriever, P. Baum and E. Riedle, Optics Express 16, 54 (2008)

[] K. W. Poon et al., Proc. SPIE (2015)

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S O F T W A R E

Fig. Transgenic YFP mouse ventral root. Left: spectral CARS. Middle: Zoom on cen-tral axon. Right: Overlay of CARS and two-photon image.

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An undesirable phenomenon that occurs in photos and videos is an unavoidable aspect of the nature of light: image noise. With the choice of the right camera for the particular application, however, such interference can be con-trolled. It is not only a matter of noise level, but also of the desired signal-to-noise ratio of the came-ra and the overall photographic situation.

T he sound of the sea, trees in the wind or a passing train: not all

noise is the same, and this also ap-plies to image processing. Different types of noise occur during photo-graphic recording – and not all of them can be controlled. The noise signal consists essentially of two components: the entire read-out noise and the photon noise. Photon noise is the result of natural fluctu-ations in light intensity. Because the behavior of the photons according to the laws of quantum physics cannot be calculated exactly, light

intensity measured by the image sensor in the camera also fluctuates, even if the illumination remains consistent. This can easily be obser-ved in a pixel-by-pixel comparison of two consecutive images taken in the same photographic situation.

As opposed to natural photon noise, the undesired readout noise is generated by the camera itself with its image sensor. The most im-portant source is the image sensor itself, which converts the incident light into electric-charge carriers, whose number varies depending on the intensity. As a result of heat and imperfections in the semiconductor material, additional charge carriers are generated that are unrelated to the incident light and have an ad-verse effect on the image. Another component of the readout noise is the transfer noise from charge carriers within the chip. Additional noise components are caused when signal voltages are amplified and converted to digital signals in an analog-digital converter. Whether

the readout noise or the photon noise dominates in a particular photographic situation depends on the light conditions: photon noise dominates if there is much light and readout noise is stronger in darker situations.

The Bottom Line

More important than the absolute noise level, however, is the signal-to-noise ratio, or the ratio of the useful signal to the noise signal. Before the digital era, it did not help to turn up the volume, if there was random noise on a car radio. That not only made the music louder, but also the static noise. The loud-ness of the signal in relation to the noise level was more important to understand the speaker or enjoy the music. It is similar with image noise: the higher the signal-to-noise ratio – i. e. the stronger the image signal compared to the noise signal – the better the image quality (Fig. ).

What Can Be Done About Noise?Why image noise occurs and what you can do about it.

Gerhard Holst

Dr. Gerhard Holst, Science & Research, PCO AG, Donaupark 11, 330 Kelheim, E-Mail: [email protected]

Fig. Two pictures of a coffee cup photographed with diffe-rent exposure times: The picture at the left has noise (standard deviation in this case) in the area of the rectangle of 1.3 counts and the picture at the right has noise of 2. counts, i. e. the noise in the right picture is much higher than in the picture at

the left; nevertheless, the image quality is actually better, due to the better signal-to-noise ratio, because the SNR in the left picture is .0 and in the right picture 21., which can be seen clearly in the selected enlargement.

Two pictures of a coffee cup photographed with diffe- the left; nevertheless, the image quality is actually better, due the left; nevertheless, the image quality is actually better, due



But that is only a partial explana-tion. The situation is complicated because the two factors mutually influence each other.

The More Light, the Better

Zero noise and one hundred per-cent quantum efficiency: Such an image sensor would be the dream of every camera user. The quan-tum efficiency describes the ability of the image sensor to convert as many photons to charge carriers as possible. One hundred percent efficiency would therefore mean that all photons that fall onto a pixel of the sensor would be converted to an electric signal. Unfortunately, such an ideal image sensor does not exist. For one thing, it is impossible to boost the quantum efficiency to one hundred percent. Because depending on the image sensor architecture, only about every se-cond photon generates an electric-charge carrier, which corresponds to 50 percent quantum efficiency. The best image sensors currently available reach 70 to 90 percent, de-pending on the technology used. In addition, neither the readout noise nor the photon noise can be avoi-ded completely. The natural physi-cal properties of light prevent this.

The maximum possible signal-to-noise ratio in the measurement of light corresponds to the number

of photons divided by the root of the number of photons. The photon noise therefore increases with the number of photons. Since the image signal also increases with higher light intensity – and at a faster rate compared to the noise signal, the following rule of thumb applies: the more light is available for the photo or video shooting, the better. This is also confirmed by experience from the era of analog film, when the technically best image quality was achieved with the least sensi-tive films, because they needed the largest amount of light for a good picture. In the case of image sen-sors, this is true until the maximum capacity of the sensor pixels for charge carriers is reached, i.e. until the full-well capacity of the image sensor is exhausted.

Approach the Optimum

To control image noise it is advis-able to choose a camera based on the light conditions of the pho-tographic situation. If the user is fortunate enough to be able to use long exposure times and/or to ha-ve a well-illuminated subject, the signal-to-noise ratio is high and the readout noise of the camera is ne-gligible. Only the photon noise af-fects the image in this case. To take advantage of the light situation it is worthwhile to choose a camera

with a high full-well capacity in or-der not to be limited by the capacity for charge carriers. Image sensors with large pixels generally have a higher full-well capacity, while sen-sors with small pixels are characte-rized by low readout noise.

However, consistently well-illuminated subjects and/or applica-tions that allow long exposure times are rare. More often light is the limiting factor: in quality assurance, for example, large quantities or mo-ving objects generally necessitate short exposure times. Surveillance cameras must be specially designed for use at night. And dark glass bottles place challenges on bottle inspection systems. In research, applications with high throughput in DNA analysis are critical, becau-se the samples, which are usually labeled with fluorescent dyes, emit very little light (Fig. 2, 3).

Since there is very little light available in all these applications, the full-well capacity of the sensor is more or less irrelevant. On the other hand the readout noise of the camera is a crucial factor in situations with low light levels. To achieve a good signal-to-noise ratio here requires the strongest possible image signal. High quantum effi-ciency is therefore an advantage when choosing the camera, since it determines how effectively the camera chip converts light signals into electric signals. On the other

Fig. 2 Cut-outs of a scanned cell sample (Panoramic 250 Flash scanner for digital pathology) that were labeled with three different fluorescent dyes that link to

different cell elements (nucleus, mem-brane, …). These fluorescence images are normally only faintly luminous, ne-cessitating a good signal-to-noise ratio

in the low light range dominated by the camera + image sensor readout noise. The picture is a false-color image con-sisting of three single images.

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hand, the camera should feature low background noise, which can vary widely among manufacturers and camera models.

The best image sensors in came-ras for such low-light applications offer an average readout noise of 0.9 charge carriers. That means that for each exposure they add only about one interfering charge carrier per pixel to the useful signal. This allows contrast to background noise even in very weak light situations with only a few charge carriers per pixel.

No matter how much light: It is a good idea to be familiar with the specific application and the techni-cal circumstances. Finding the right camera also requires a careful and critical study of the manufacturer’s data sheets. In the past the technical data provided by many camera ma-nufacturers were incomplete or not suitable for comparison purposes. That is why the European Machi-ne Vision Association introduced the EMVA Standard 1288 in 2005. This standard recommends how to measure camera parameters such as sensitivity, readout noise or quantum efficiency. At present, not all manufacturers have adopted this standard. Customers would therefore do well to explicitly ask about EMVA-1288 data. Parame-ters that allow a comparison are the only way to find the right camera for a specific situation – to ensure that the relevant image information

stands out from the background noise.

What Can Be Done About Noise?

More important than the noise level itself is the signal-to-noise ratio of the camera in the specific photo-graphic situation. If there is enough light, the signal-to-noise ratio is high and the readout noise as a characte-ristic of the camera and image sensor is negligible. To take advantage of this light situation it is worthwhile to choose a camera with a high full-well capacity, to avoid the limitations of the capacity for charge carriers or low sensitivity. If light is the limiting factor, on the other hand, the full-well capacity of the sensor is irrele-vant, since in this case the readout noise of the camera dominates. To achieve a good signal-to-noise ratio here requires the strongest possible image signal. High quantum effici-ency is therefore advantageous when choosing the camera. On the other hand, the lowest possible camera background noise is desirable. In the search for the right camera model the user should ask the manufacturer about the EMVA-1288 data, the stan-dard recommended by the European Machine Vision Association for the measurement of camera parameters. This is the only practical way to compare different models in order to find the right camera for the specific application.

Fig. 3 Another example of fluorescent labeling of chicken ganglion cells, F-actin, linked with Atto 488 to phallo-din. The image corresponds to a maxi-mum projection of a Z-stack of 35 images taken with structured illumination (SIM) using a hexagonal grid and a rolling

shutter with synchronous laser line illu-mination (false color image). These fluo-rescence images also are normally only faintly luminous, necessitating a good signal-to-noise ratio in the low light ran-ge dominated by the camera + image sensor readout noise.

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Non-diffracting WavesEdited by Hugo E. Hernandez-Figueroa, Michel Zamboni-Rached and Erasmo RecamiISBN: 978-3-527-41195-5 | 512 pages | 2013

Compact Semiconductor LasersEdited by Richard De La Rue, Jean-Michel Lourtioz and Siyuan Yu ISBN: 978-3-527-41093-4 | 328 pages | 2014

Introduction to Metal-Nanoparticle PlasmonicsBy Matthew Pelton and Garnett W. Bryant ISBN: 978-1-118-06040-7 | 296 pages | 2013

Optical Devices in Ophthalmology and Optometry: Technology, Design Principles and Clinical ApplicationsBy Michael Kaschke, Karl-Heinz Donnerhacke and Michael Stefan Rill ISBN: 978-3-527-41068-2 | 638 pages | 2014

Optical Engineering of DiamondEdited by Rich Mildren and James RabeauISBN: 978-3-527-41102-3 | 446 pages | 2013

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April 2015 Special Issue … · und erleben Siemens-Technologie hautnah. ... Florian Otto and...

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