Lasers! What are they good for?

Applications in Biology and Chemistry

Kevin SchultzDept. of Physics and Astronomy

APSU

My Background

• Trained in atomic physics.

• Atomic hydrogen good.• All else Bad!• Moving on… $500k to

build a lab to study the optical properties of photovoltaics

• Many of the instruments can be used for chemistry and biology

• So… make a user facility!

a few, uh, provisos… A couple of quid pro quo.

• I am not a chemist

• I am not a biologist

• I have never performed any of these experiments

• I would like to though...

Lasers

AbsorptionSpontaneous

EmissionStimulatedEmission

• Monochromatic… Usually• Line-widths of ~30GHz can be obtained (<1cm-1) .Less than

many atomic transitions.• Wavelength stability to better than 1 part in 108

• Coherent• Spectrally bright. Lots of photons can be imaged on a sample

(some lasers are 1020 times brighter than the sun!)• Directional

• Makes interferometric techniques easier

Spectroscopy• Absorption Spectrum given by• ΔI(λ)=Io(λ)-IT(λ)

Example: a 1m spectrograph has ~0.01nm resolution corresponding to Δν=12GHz at λ=500nm

Doppler Width of a molecule at room temperature ~1GHz

Single mode lasers have resolutions sub-kHz->ΔI/I 12 times larger than conventional absorption spectroscopy!

•Advantages of Laser Spectroscopy• No need for a monochromator• Resolution limited only be the

absorbing molecular transitions• Detection sensitivity increases

with increasing spectral resolution

• Long absorption cells can be used

Image from: Demtroder. Laser Spectroscopy

Lasers in Spectroscopy

Image from: Demtroder. Laser Spectroscopy

Cavity Ring Down Spectroscopy

• Assume a cavity with two mirrors with reflectivity R

• The transmission of the mirror is then• T=1-R-A<<1, where A includes

other losses

• After a time t the detected power will be• P(t)=P1exp(-t/τ1)

• The decay times with and without the sample are measured and it can be shown:• αL=(1-R)Δτ/τ1

• Let R=99.99%, Δτ/τ1=10-4 αL=10-8

• For L=1m α=10-10cm-1

Image from: Demtroder. Laser Spectroscopy

Fluorescence I

Images from Lakowicz, Principles of Fluorescence Spectroscopy

Laser Induced Fluorescence

• Advantages:• Single excitation makes a

simpler spectrum and thus easier level identification

• Stronger excitation levels, allows for detection of transitions with small Franck-Condon Factors

• High sensitivity allows reconstruction of potential curves

• Can determine population distributions

• Level spacings of polyatomic molecules show signatures of chaos.

Images from: Demtroder. Laser Spectroscopy

Non-linear Spectroscopy

Saturation Spectroscopy

Laser Raman Spectroscopy

• Lasers are an improvement for Raman Spectroscopy

• Fluorescence minimized • Overcomes weak Raman

effect

•Used in Analytic Chemistry• Surface studies• FT-Raman similar to FTIR• Raman microscopy possibleImages: McCreery, Raman Spectroscopy for Chemical Analysis

Time Resolved Methods

Time-resolved FluorescenceTime Domain

Images from Lakowicz, Principles of Fluorescence Spectroscopy

Time-resolved FluorescenceFrequency Domain

Images from Lakowicz, Principles of Fluorescence Spectroscopy

Time-resolved Fluorescence

Time Domain Lifetime Measurement

Frequency Domain Lifetime Measurement

Images from Lakowicz, Principles of Fluorescence Spectroscopy

Pump-Probe Spectroscopy

Images: Lutz, et.al PNAS, vol. 98, 962—967, 2001

Multiphoton Microscopy

Multi-photon Microscopy II

Handbook of biomedical nonlinear optical microscopy By Barry R. Masters, Peter T. C. So

Confocal Microscopy

Wikimedia: Diaspro, Bianchini, Vicidomini, Faretta, Ramoino and Usai

Optical Coherence Spectroscopy

medOCT group, Center of biomedical Engineering and Physics, Medical University Vienna

Stuff I Left Out• Ionization Spectroscopy• Two-Photon Ionization Spectroscopy• Optogalvanic Spectroscopy• Photoacoustic Spectroscopy• Velocity Modulation Spectroscopy• Laser Magnetic Resonance and Stark Spectroscopy• Polarization Spectroscopy• Saturated Interference Spectroscopy• Doppler Free Multiphoton Spectoscopy• Doppler Free Laser-Inuced Dichroism and Birefringence• Hetrodyne Polarization Spectroscopy• Stimulated Raman Scattering• Coherent Anti Stokes Raman Spectoscopy (CARS)• Resonant CARS• Hi Anna• BOX-CARS• Hyper-Raman Effect• Firckin’ lasers• Resonance Raman Effect• Time-Resolved Raman Spectroscopy• Laser Spectroscopy of Molecular Beams• Cluster Spectroscopy• Nonlinear spectroscopy in Molecular beamsLaser Spectroscopy of Fast Ion

Beams• Spectroscopy of Radioactive Elements• Photofragmentation Spectroscopy• Laser Photodetachment Spectroscopy• Mass Spectrometry• Optical Pumping• XKCD• Laser Spectroscopy of Fast Ion Beams• Spectroscopy of Radioactive Elements• Photofragmentation Spectroscopy• Laser Spectroscopy of Fast Ion Beams• Spectroscopy of Radioactive Elements• Photofragmentation Spectroscopy• Laser Photodetachment Spectroscopy• Mass Spectrometry• Laser Spectroscopy of Fast Ion Beams• Spectroscopy of Radioactive Elements

• Photofragmentation Spectroscopy• Laser Photodetachment Spectroscopy• Mass Spectrometry• Optical Pumping• Laser-RF Double Resonance Spectroscopy• Laser-microwave Double Resonance Spectroscopy• Optical-Optical Double Resonance Spectroscopy• Bored yet• Triple Resonance Spectroscopy• OODR Polarization Spectroscopy• Hole-burning and Ion-Dip Double-Resonance Spectroscopy• Level-crossing Spectroscopy• Quantum Beat Spectroscopy• Excitation and Detection of wave-packets• Photon Echoes• Optical Free-induction Decay• Heterodyne Spectroscopy• Correlation Spectroscopy• Single-molecule detection• Spectroscopy of collision processes• Spectroscopy of Reactive collisions• Photon Assisted Collision Energy Transfer• Optical Cooling and Trapping• Single Ion Spectroscopy• Bose-Einstein Condensation• Optical Ramsey Fringes• Hi Gracie• Atom Interferometry• Spectral resolution within atomic line-widths• Absolute Optical Frequency Measurement and Standards• Optical Squeezing• Laser Induced Chemical Reactions• Laser Femtosecond Chemistry• Isotope Separation with lasers• LIDAR• Spectroscopy of Combustion Processes• Energy Transfer in DNA complexes• Laser Diagnosis and Therapy• AND SO ON…