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Laser Pulse Generation and Ultrafast Pump-Probe Experiments

Laser Pulse Generation and Ultrafast Pump-Probe Experiments. By Brian Alberding. Goals. Basic Laser Principles Techniques for generating pulses Pulse Lengthening Pulse Shortening Ultrafast Experiments Transient Absorption Spectroscopy. L.A.S.E.R.

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Laser Pulse Generation and Ultrafast Pump-Probe Experiments

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  1. Laser Pulse Generation and Ultrafast Pump-Probe Experiments By Brian Alberding

  2. Goals • Basic Laser Principles • Techniques for generating pulses • Pulse Lengthening • Pulse Shortening • Ultrafast Experiments • Transient Absorption Spectroscopy

  3. L.A.S.E.R Light Amplification by Stimulated Emission of Radiation

  4. I I0 I1 I3 I2 Laser medium R = 100% R < 100% Basic Laser • Light Sources • Gain medium • Mirrors R. Trebino

  5. Laser Cavity

  6. Gain Medium Einstein Coefficients E2 AN2 = rate of Spontaneous emission E1 E2 BN2I = rate of Stimulated emission E1 E = hν E2 BN1I = rate of Stimulated absorption E1

  7. To achieve lasing: • Stimulated emission must occur at a maximum (Gain > Loss) • Loss: • Stimulated Absorption • Scattering, Reflections • Energy level structure must allow for Population Inversion E2 E1

  8. 3 3 2 N2 Fast decay Fast decay 2 2 Laser Pump Transition Laser Transition Pump Transition Laser Transition N1 1 1 1 Fast decay 0 Obtaining Population Inversion 2-level system 3-level system 4-level system Population Inversion is obtained for ΔN < 0 (ΔN = N1 – N2)

  9. 3 Fast decay 2 Pump Transition Laser Transition 1 Fast decay 0 Summary – Basic Laser • Source light • Reflective Mirrors (cavity) • Gain Media • Energy Level Structure • Population Inversion • Pumping Rate ≥ Upper laser State Lifetime • Upper laser State Lifetime > Cavity Buildup time

  10. Types of Lasers Solid-state lasers have lasing material distributed in a solid matrix (such as ruby or neodymium:yttrium-aluminum garnet "YAG"). Flash lamps are the most common power source. The Nd:YAG laser emits infrared light at 1.064 nm. Semiconductor lasers, sometimes called diode lasers, are pn junctions. Current is the pump source. Applications: laser printers or CD players. Dye lasers use complex organic dyes, such as rhodamine 6G, in liquid solution or suspension as lasing media. They are tunable over a broad range of wavelengths. Gas lasers are pumped by current. Helium-Neon lases in the visible and IR. Argon lases in the visible and UV. CO2 lasers emit light in the far-infrared (10.6 mm), and are used for cutting hard materials. Excimer lasers (from the terms excited and dimers) use reactive gases, such as chlorine and fluorine, mixed with inert gases such as argon, krypton, or xenon. When electrically stimulated, a pseudo molecule (dimer) is produced. Excimers lase in the UV. R. Trebino

  11. Quality of laser beams Uncertainty Principle: Δt Δν ≥ 1/4π Irradiance vs. time Spectrum Long pulse time frequency Short pulse time frequency

  12. Generating Pulses • Q-switching • Mode-Locking • Passive • Active • Pulse Shortening • Group Velocity Dispersion • Pulse Lengthening - Chirp

  13. Output intensity 100% Cavity Gain Cavity Loss 0% Time Q-Switching • Alternate presence of oscillating laser beam within the cavity • Methods • Rotating mirror • Saturable Absorber • Electro-optic shutter • Pockels Cell • Kerr Cell • Nanosecond timescales R. Trebino

  14. Mode-Locking • Technique • Shutter between mirror and gain medium • Shutter open: All modes gain at same time • Types • Active • Passive R. Trebino

  15. Active mode locking 1000 Passive mode locking Shortest Pulse Duration (fs) Colliding pulse mode locking 100 Intra-cavity pulse compression 10 Ti-Sapphire '65 '70 '75 '80 '85 '90 '95 Year Mode-Locking Methods • Active – Mechanical Shutters • Acousto-Optic Switches (low gain lasers) • Synchronous Pumping • Passive • Colliding Pulse • Additive Pulse • Kerr Lens

  16. The longer wavelengths traverse more glass. Pulse Lengthening and Shortening Group Velocity Dispersion – The velocity of different frequencies of light is different within a medium. Pulse Lengthening: Ultrashort Pulse Any Medium Chirped Pulse Pulse Shortening:

  17. The excite pulse changes the sample absorption seen by the probe pulse. Excite pulse Slow detector Sample Probe pulse Lens Change in probe pulse energy Delay Delay Pump-Probe Experiment R. Trebino

  18. Generally, small-scale self-focusing occurs, causing the beam to breakup into filaments. White-Light Generation n(ν) = n0(ν) + n2(ν)I(ν) R. Trebino

  19. Types of Experiments • Transient Absorption • Fluorescence Upconversion • Time Resolved IR • Transient Coherent Raman and Anti-Stokes Raman • Transient photo-electron spectroscopy

  20. Transient Absorption – Model System • Vibrational Relaxation (VR), Intersystem Crossing (ISC), and Internal Conversion (IC) • Aspects of VR • Pump wavelength dependence • Density of states • Probe wavelength dependence • Franck-Condon Factors • Full-spectrum, Kinetic trace • Needed Information • Steady State absorption and emission • geometry • Electron configuration

  21. James McCusker (MSU): Transition Metal Complexes • Cr(acac)3: ~Oh, d3 complex • Ligand field and charge transfer states Ligand Field Emission MLCT Ground State: 4A2 Molar Absorptivity (M-1cm-1 x 103) Photoluminescence Intensity (au) Ligand Field Abs Excited States: 2E, 4T2 2LMCT, 4LMCT Wavelength (nm)

  22. Cr(acac)3 Ligand Field Transient Absorption 100 fs excitation at 625 nm Kinetic Data Full Spectrum Data 480 nm probe τ = 1.09 ± 0.06 ps Red is single wavelength data at Δt = 5 ps Blue is nanosecond data at 90 K Long Lived = 2E state

  23. Cr(acac)3 Ligand Field Transient Absorption 100 fs excitation at 625 nm Characteristic of Vibrational Relaxation Pump Wavelength Dependence C1 = initial Abs amplitude a0 = Long time offset

  24. Cr(acac)3 Jablonski Diagram

  25. FeII polypyridyl complexes • Time scale of ΔS ≠ 0 transitions • [Fe(tren(6-R-py)3)]2+ • d6 complex, ~ Oh geometry • R = H: Low Spin, 1A1 ground state • R = CH3: High Spin, 5T2 ground state tren(py) = tris(2-pyridylmethyliminoethyl)amine

  26. [Fe(tren(6-R-py)3)]2+ Complexes – Steady State Absorption R = H R = CH3: similar to [Fe(tren(6-H-py)3)]2+ ground state Calculated Difference = Middle – Top ( ) Nanosecond Data (dotted line) Provides template for 5T2 excitedstate in low spin complex

  27. [Fe(tren(6-H-py)3)]2+ ~100 fs excitation at 400 nm LMCT excitation fs timescale decay Bleach at long times R = CH3 (5T2): No Abs at 620 nm R = H (1A1): Abs at 620 nm 620 nm Probe τ1 = 80 ± 20 fs, τ2 = 8 ± 3 ps ps timescale decay is Vibrational Relaxation

  28. [Fe(tren(6-H-py)3)]2+ ~100 fs excitation at 400 nm 5T2 state is populated in 700 fs Other excited states decay faster than time resolution Vibrational Relaxation occurs on ps timescale ΔT = 700 fs (black line) ΔT = 6 ps (blue line) Calculated difference of R = CH3/R = H (red line)

  29. Dynamics in Transition Metal Complexes • Relative Rates of VR, ISC, and IC can vary depending on the system • kISC > kVR • Fast spin forbidden transitions • ΔS = 1, ΔS = 2; Spin Orbit Coupling

  30. Other Work and Applications • Transition Metal Complexes • Ligand Field States contribute to photosubstitution and photoisomerization processes • Electron transfer processes and photovoltaics • Dr. Bern Kohler: DNA photodamage, skin cancer

  31. References • Stimulated Emission: http://hyperphysics.phy-astr.gsu.edu/hbase/mod5.html • Laser Cavity: http://micro.magnet.fsu.edu/primer/java/lasers/heliumneonlaser/index.html • Silvfast, Laser Fundamentals, 2nd ed., Cambridge University Press, pg. 439-467 • J. Am. Chem. Soc., 2005, 127, 6857-6865. • J. Am. Chem. Soc., 2000, 122, 4092-4097. • Coordination Chemistry Reviews, 250 (2006), 1783-1791 • Nature, 436, 25, 2006, 1141-1144. • Rick Trebino, Georgia Tech University, http://www.physics.gatech.edu/gcuo/lectures/index.html, Optics 1 “Lasers”, Ultrafast Optics “Introduction”, Ultrafast Optics “Pulse Generation”, Ultrafast Optics “Ultrafast Spectroscopy”

  32. A dye’s energy levels • Dyes are big molecules, and they have complex energy level structure. S2: 2nd excited electronic state Lowest vibrational and rotational level of this electronic “manifold” Energy S1: 1st excited electronic state Excited vibrational and rotational level Pump Transition Laser Transition Dyes can lase into any (or all!) of the vibrational/rotational levels of the S0 state, and so can lase very broadband. S0: Ground electronic state

  33. k = 3 Short time (fs) k = 1 k = 2 k = 7 Saturable Absorber Intensity Round trips (k) Notice that the weak pulses are suppressed, and the strong pulse shortens and is amplified. After many round trips, even a slightly saturable absorber can yield a very short pulse. R. Trebino

  34. Absorption spectra following oxidation and reduction Oxidation Reduction

  35. Jablonski Diagram [Fe(tren(6-H-py)3)]2+

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