Applications of LASERs

Jeremy Allam

Optoelectronic Devices and Materials Research Group

Tel +44 (0)1483 876799Fax +44 (0)1483 876781

Applications of lasers

Applications of lasers

Longitudinal Coherence of Laser Lightphase noise or drift(spontaneous emission, temperature drift, microphonics, etc)leads to finite spectral width

phasor at t=0phasor at t=t1leads to finite coherence time tcoh. (or length lcoh.)tcoh. (or lcoh.)

Measuring Longitudinal Coherenceuse interferometer e.g. Michelson interferometerD(path length) = 2L1-2L2

Applications of interferometersLINEAR TRANSLATION: interferometric translation stageFLATNESS/UNIFORMITY: e.g. Twyman-Green interferometerLINEAR VELOCITY OF LIGHT: famous Michelson-Morley experiment c is independent of motion of reference frameDETECTING GRAVITATIONAL WAVES: minute movement of end mirrorsROTATION (e.g. of earth): Sagnac interferometer as an optical gyroscope:Measurement of length:Measurement of optical properties:REFRACTIVE INDEX: Rayleigh refractometerLIGHT SCATTERING: heterodyne spectrometryULTRAFAST DYNAMICS: pump-probe / coherent spectroscopy{see Smith and King ch. 11}Numerous other applications...For N loops of area A and rotation rate W, phase difference is:

Holography{see Smith and King ch. 19}Photography - record electric field intensity of light scattered by objectHolography - record electric field intensity and phaseRECORDINGREADING / RECONSTRUCTING

Applications of lasers

http://www-cms.llnl.gov/wfo/laserfab_folder/index.htmla high-speed, low-cost method of cutting beryllium materialsNo dust problem (Be dust is poisonous) autogenous welding is possibleAchieved using a 400-W pulsed Nd-YAG laser and a 1000-W CW CO2 laserNarrow cut width yields less Be waste for disposalNo machining damageLaser cutting is easily and precisely controlled by computerLaser fabrication of Be components

1kW Nd:YAG cutting metal sheet

Photograph of the laser delivery handpiece with a hollow fiber for sensing temperature. The surgeon is repairing a 1 cm-long arteriotomy.http://lasers.llnl.gov/mtp/tissue.htmlLaser Tissue WeldingLaser tissue welding uses laser energy to activate photothermal bonds and/or photochemical bonds. Lasers are used because they provide the ability to accurately control the volume of tissue that is exposed to the activating energy.

http://www.llnl.gov/str/Powell.htmlNuclear Fusion: National Ignition Facility

ultrashort pulses (5fs) broadband gain(700-1000nm) high power(TW) THz pulsegeneration pulse shaping coherent control parametric conversion Why femtosecond lasers? timing physical processes time-of-flight resolutiongenerate: UV X-rays, relativistic electrons123(Titanium-sapphire properties)

What is ultrashort?

Mode-locked Ultrafast Lasers'65'70'75'80'85'90'95101001000Shortest Pulse Duration (femtoseconds)YearUltrafast Ti:sapphire laserA 4.5-fs pulseReports of attosec pulses, too!

19601970198019902000Speed (seconds)YearElectronicsUltrafast Optics vs. ElectronicsOpticsNo one expects electronics to ever catch up.

Ultrafast Laser Spectroscopy: Why?Most events that occur in atoms and molecules occur on fs and ps time scales. The length scales are very small, so very little time is required for the relevant motion.Fluorescence occurs on a ns time scale, but competing non-radiative processes only speed things up because relaxation rates add:

1/tex = 1/tfl + 1/tnr

Biologically important processes utilize excitation energy for purposes other than fluorescence and hence must be very fast.Collisions in room-temperature liquids occur on a few-fs time scale, so nearly all processes in liquids are ultrafast.Semiconductor processes of technological interest are necessarily ultrafast or we wouldnt be interested.

Ultrafast Spectroscopy of PhotosynthesisArizona State UniversityThe initial events in photosynthesis occur on a ps time scale.

The 1999 Nobel Prize in Chemistry went to Professor Ahmed Zewail of Cal Tech for ultrafast spectroscopy.Zewail used ultrafast-laser techniques to study how atoms in a molecule move during chemical reactions.

Selective photochemistryA chemists dream: control of chemical reaction pathway by selective optical excitation of chemical bondThe difficulty with using CW light or long pulses is intramolecular vibrational redistribution: excite one bond, and a few fs later, the whole molecule is vibrating and the weakest bond breaks.

Coherent control with shaped fs pulsesSOLUTION:(1) Use fs pulse to break bond before IVR occurs(2) shape the pulse to optimise the desired yield

Termed coherent control of chemical reactions

Pulse shaping in time and frequency domainsIntensity and phase of an optical pulse may be specified in either the time or frequency domain:Similarly, modulation can be performed in time or frequency domain:difficult - modulators too slow!easy!

The Fourier-Synthesis Pulse-shaperAmplitude maskTransmission = T(x) = T(l)Phase maskPhase delay = j(x) = j(l)Fourier Transform Planeffffffgratinggrating

Laser ablation with CW and long pulse (ns) :High average powerDominant process: thermalmaterial heated and vaporisedexpansion and expulsion of target material Possible problemscrater formation heat affected zone (HAZ) surface contamination (dross)shock wave damage to underlying material limiting precision / resolutioncollateral damageabsorption within illuminated regionpoor vertical controlMicromachining with CW lasers

Extreme conditions* at focus of ultrashort pulse:

1J pulse focussed to (1 m)3 gives:T~1MKp~10Mbar

*Eric Mazur, Harvard UniversityFemtosecond pulses in micromachiningUltrashort high peak intensity (ps or fs) pulses:High peak power, low mean powerDominant process: creation of plasma direct and rapid generation by multi-photon ionisationincident energy absorbed in plasma negligible cratering, HAZ, shock-wave damage or drossstrong NL effects only at focus -> sub-surface machining

Femtosecond vs. picosecond laser ablationablation with fs pulses appears to be more deterministicdue to (?) statistics of photoionisation (by light field or by multi-photon absorption) and subsequent avalanche ionisation

http://tops.phys.strath.ac.uk/machining.htmApplications of femtosecond micromachininghigh-precision ablationencoding information on micron scaleengineering dielectrics for e.g. optical waveguidessurgery...

pig myocardium drilled by an USPL showing a smooth-sided hole free of thermal damage to surrounding tissue. pig myocardium drilled by excimer laser, illustrating extensive thermal damage surrounding the hole. small, high precision cuts without kerfno thermal or mechanical damage to surrounding areas i.e. no burning or coagulationsub-surface surgeryhttp://lasers.llnl.gov/mtp/ultra.htmlSurgery with femtosecond laser pulses - 1

thermal damage and cracking to tooth enamel caused by 1-ns laser ablation. smooth hole with no thermal damage after drilling with a USPL.http://lasers.llnl.gov/mtp/ultra.htmlSurgery with femtosecond laser pulses - 2

Femtosecond interstromaFemtosecond LASIKFemtosecond laser surgery of cornea - 1

Femtosecond laser surgery of corneaLenticle removal using Femtosecond LASIK

(Biomedical) imaging using ultrashort laser pulsesProblems with conventional microscopytransparent objects require staining (toxic, fading) 3D imaging by sectioninginternal structures (e.g. retina) not always accessibleopaque objects cannot be viewed in transmissionlow contrast due to background transmission Ultrashort pulse imaging methods address some of these problems :Multi-photon imagingballistic photon imagingoptical coherence tomographyT-rays

Nonlinear microscopy for 3D imagingThird harmonic generationThree photon fluorescenceTwo photon fluorescenceLinear processes do not favour the focus signal~intensity x area~z-2 x z2 ~constantNonlinear (multi-photon) processes favour the focussignal~(intensity)2 x area~z-4 x z2 ~ z-2 (2-photon)signal~(intensity)3 x area~z-6 x z2 ~ z-4 (3-photon)femtosecond pulsedetection of nonlinear signalregion of NL interactionfilterz

Two-Photon Fluorescence* ImagingPollen grain(Clivia Miniata)1.5 m axial resolution200 mW in 16 beamlets46 sections separated by 0.5 min the axial dimension.2 seconds/image~14 mConventional image(using fluorescence)*requires fluorescent dye

Imaging by Third Harmonic Generation (THG) THG occurs at focus of intense ultrashort pulse Uniform material: THG light from either side of focus interferes destructively Discontinous material: allows some constructive interference and THG emission. THG imaging depends on Dc(3) THG is sensitive to interfacesDemonstration using an optical fiber in index-matching fluid(~100 fs pulses at 1.2 m, 1 kHz repetition rate.)Barad et al, Appl. Phys. Lett. 70, 922 (1997)

Sectional THG images of spiral algae formationSquier et al, Optics Express 3, p. 315 (1998)

More Real-Time THG ImagesArtificial blood vessel (two cover slips) with real red blood cells flowing in it. Scanning scheme used a Lissajou pattern.

scattering mediumballistic photons (early arrival): small lateral scattering, low intensitysnake photonsdiffusive photons (late arrival):large lateral scattering, high intensity)Time-resolved imaging for opaque media Scattering is a major problem in e.g. mammography The problem is weak signals:mean free path for photons = Ls ~ 0.5 mm for breast tissuesample length = L=25mm

fraction of ballistic photons is exp(L / Ls) = exp(50) = 1022

but for a pulsed laser with 1 Watt average power, there are only 1019 photons per second ...

Optical Coherence Ranging and TomographyThis work has been pioneered by Jim Fujimoto and coworkers of MIT.Huang, et al., Science, 254 (1991)cross-sectional micron-scale imaging

real-time, in-situ, in-vivo

optical fibre coupling for internal organs

commercial device available for ophthalmologists

OCT can see otherwise invisible micro-tears in the retinaPhotographs cant see the tears

Inside a blood vessel (in vitro)Brezinski, et al., Am. J. Cardiology 77 (1996)The OCT images have significantly higher resolution than intravascular ultrasound (IVUS).

THz imaging for biomedical applicationsfills THz gap between microwave and optical frequenciesmixed time / frequency domain spectroscopychemical fingerprints at THz frequencies (e.g. rotational transitions)strong sensitivity to water content

coherent method (like OCT)imaging on 100 micron scalemany variation of imaging method:intensitytime-of-flightabsorption at key frequencies (f1)relative absorption (f1/f2)

THz imaging of biomedical samplesCentre of Medical Imaging Research University of LeedsTeraVision project (EU-IST)

Principles:System:Surrey Femtosecond high-power broadband source

High rep rate near-infrared system (Spectra)high rep rate (80MHz) for good signal-to-noiseworkhorse system for communications wavelengths

Ti-sapphire oscillator and regenerative amplifierhigh pulse energies for THz beam generation, material processing, and upconversion of weak luminesencedual parametric amplifiers for non-degenerate pump-probe, and difference frequency generator for mid-infraredwavelength range 550nm to >10multrashort pulse version: < 60fs pulsesDual colour / mid-infrared system (Coherent)

Broadband sources for spectroscopyUVvisibleNIRMIRFIRmmWRFTHzFELUltrafast electronicsOPATi-S laserTi-S SHGTi-S THGDFMSFMHG-OPA

Laboratory Layout

Photosynthesis: An Ultrafast Biochemical Reaction. At ASU we study another ultrafast, light-driven biological process, photosynthesis. What happens when light is absorbed by the leaf of a plant? Ordinarily when molecules absorb light, the light energy is converted into heat and lost within a few nanoseconds. In order for the photosynthetic energy-capturing reactions to compete with this nonproductive process of energy loss, they must be much faster than a nanosecond. In fact, the initial photosynthetic events occur on the picosecond time scale. The light energy is collected by an array of antenna complexes and transferred to a special complex called the reaction center where the light energy is used to power a series of electron transfer reactions (movement of electrons from one molecule to another). These reactions result in a separation of charge across a biological membrane.Larry Orr Arizona State (Prof or Administrative Assistant?) Jim Allen--Chemistry and Biochemistry Robert E. Blankenship--Chemistry and Biochemistry Wayne Frasch--Plant Biology Ian Gould--Chemistry and Biochemistry Devens Gust--Chemistry and Biochemistry Michael N. Kozicki--Electrical Engineering/Nanostructures Research Group Ana Moore--Chemistry and Biochemistry Tom Moore--Chemistry and Biochemistry Vincent Pizziconi--Chemical, Bio and Materials Engineering Seth Rose--Chemistry and Biochemistry Wim Vermaas--Plant Biology Andrew Webber--Plant Biology Neal Woodbury--Chemistry and Biochemistry Research Scientists: Dan Brune Paul Liddell Su Lin--Laser Facility Ronald A. Nieman--NMR Facility Aileen Taguchi JoAnn Williams

The Academy's citation:For his studies of the transition states of chemical reactions using femtosecond spectroscopy.

1999s laureate in Chemistry is being rewarded for his pioneering investigation of fundamental chemical reactions, using ultra-short laser flashes, on the time scale on which the reactions actually occur. Professor Zewail's contributions have brought about a revolution in chemistry and adjacent sciences, since this type of investigation allows us to understand and predict important reactions.Articial blood vessel (two cover slips) with real red blood cells flowing in it. Scanning scheme (Lissajou pattern).