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Ultrafast Spectroscopy

Ultrafast Spectroscopy. Gabriela Schlau-Cohen Fleming Group. Why femtoseconds?. timescale = distance/velocity ~~~~~~ distance ≈ 10 Å E ≈ h ν ≈ (6.626*10 -34 kg*m 2 /s)*(3*10 8 m/s /6*10 -7 m) ≈ 3*10 -19 kg*m 2 /s 2 E= ½mv 2

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Ultrafast Spectroscopy

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  1. Ultrafast Spectroscopy Gabriela Schlau-Cohen Fleming Group

  2. Why femtoseconds? timescale = distance/velocity ~~~~~~ distance ≈ 10 Å E ≈ hν ≈ (6.626*10-34kg*m2/s)*(3*108m/s /6*10-7m) ≈ 3*10-19kg*m2/s2 E= ½mv2 v=√(2*E*/m) =√(2*E*/9*10-31kg) =√(2*3*10-19/(9*10-31 )m2/s2) v=8*105 m/s ~~~~~~ timescale ≈ (10*10-10m)/(8*105m/s) ≈ 10-15 sec

  3. Ultrafast examples: • Photosynthesis: energy transfer in <200 fs (Fleming group) • Vision: isomerization of retinal in 200 fs (Mathies group) • Dynamics: ring opening reaction in ~100s fs (Leone group) • Transition states: Fe(CO)5 ligand exchange in <1 ps (Harris group) • High intensity: properties of liquid carbon (Falcone group)

  4. –6 10 –9 Electronics 10 Timescale (seconds) –12 10 Optics –15 10 1960 1970 1980 1990 2000 Year How can we measure things this fast?

  5. Laser Basics Four-level system • Population inversion • Pump energy source • Lasing transition Fast decay Pump Transition Laser Transition Fast decay Level empties fast!

  6. What we need for ultrashort pulse generation: • Method of creating pulsed output • Compressed output • Broadband laser pulse

  7. Ultrafast Laser Overview pump Laser oscillator Amplifier medium

  8. 3 pieces of ultrafast laser system: • Tunable Parametric Amplifier • Oscillator • Regenerative Amplifier

  9. Oscillator generates short pulses with mode-locking Ti:Sapphire laser crystal Prisms Cavity with partially reflective mirror Pump laser

  10. Al2O3 lattice oxygen aluminum Titanium: Sapphire • 4 state system • Upper state lifetime of 3.2 μs for population inversion • Broadband of states around lasing wavelength • Kerr-Lens effect (non-linear index of refraction)

  11. Absorption and emission spectra of Ti:Sapphire (nm) Ti:Sapphire spectral properties Intensity (au) FLUORESCENCE (au)

  12. Mode-locking

  13. Mechanism of Mode-locking: Kerr Lens Effect

  14. Compression • Prism compression t t • Gratings, chirped mirrors

  15. Short pulse oscillator t Dispersive delay line t Solid state amplifiers t Pulse compressor t Chirped Pulse Amplification • Stretch • Amplify • Recompress

  16. Faraday rotator thin-film polarizer Pockels cell Regenerative Amplifier • Pulsed pump laser • Pockels cell • Pulsed seed • Ti: Sapph crystal s-polarized light p-polarized light

  17. OPA/NOPA • Parametric amplification • Non-linear process • Energy, momentum conserved w1 w1 "signal" “seed" w2 w3 "idler" “pump" Optical Parametric Amplification (OPA)

  18. Non-linear processes wsig Emitted-light frequency

  19. “Signal pulse” Medium under study Signal pulse energy Variably delayed “Probe pulse” Delay “Excitation pulses” Time Resolution for P(3)

  20. Two-Dimensional Electronic Spectroscopy can study: • Electronic structure • Energy transfer dynamics • Coupling • Coherence • Correlation functions

  21. 2D Spectroscopy Dimer Model (Theory) Excited State Absorption Homogeneous Linewidth ωt (“emission”) Cross Peak Excitation at one wavelength influences emission at other wavelengths Diagonal peaks are linear absorption Cross peaks are coupling and energy transfer Inhomogeneous Linewidth ωτ (“absorption”)

  22. Electronic Coupling E E e2 J J D e1 e2 e1 g1 g2 1 Dimer 2

  23. Principles of 2D Spectroscopy SIGNAL Time ABSORPTION FREQUENCY EMISSION FREQUENCY Recovered from Experiment

  24. 2D Heterodyne Spectroscopy spectro- meter echo time coh. time pop. time 1 2 3 4 t T t spherical mirror 4=LO 1 2 3 sig sample 4 OD3 3 1&2 3&4 2 1 diffractive optic (DO) delay 2 delay 1 Opt. Lett. 29 (8) 884 (2004) 2 f

  25. Experimental Set-up

  26. Fourier Transform

  27. Future directions of ultrafast • Faster: further compression into the attosecond regime • More Powerful: higher energy transitions with coherent light in the x-ray regime

  28. 2D spectrum with cross-peaks A measurement at the amplitude level Positively Correlated Spectral Motion Negatively Correlated Spectral Motion

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