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
Absorption SpontaneousEmission
StimulatedEmission
• 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…