1. Ultrafast Spectroscopy
Gabriela Schlau-Cohen
Fleming Group

2. Why femtoseconds? timescale = distance/velocity ~~~~~~
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) ≈ 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. How can we measure things this fast?
1960
1970
1980
1990
2000 10 –6 –9
–12
–15
Timescale (seconds)
Year
Electronics
Optics

5. Laser Basics Population inversion Pump energy source Lasing transition
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:
Oscillator
Regenerative Amplifier
Tunable Parametric Amplifier

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

10. 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)
oxygen
aluminum
Al2O3 lattice

11. Ti:Sapphire spectral properties
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. Chirped Pulse Amplification
Pulse compressor t
Solid state amplifiers
Dispersive delay line
Short pulse oscillator
Stretch
Amplify
Recompress

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

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

18. Non-linear processes
wsig
Emitted-light frequency

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

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

21. 2D Spectroscopy
Dimer Model (Theory)
Excitation at one wavelength influences emission at other wavelengths
Diagonal peaks are linear absorption
Cross peaks are
coupling and energy
transfer
Excited State
Absorption
Homogeneous
Linewidth
ωt (“emission”)
Cross
Peak
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
meter
coh.
time
pop.
time
echo
time 1 2 3 4 t T t
spherical
mirror
4=LO 1 2 3
sig
sample
OD3 4
1&2 3
3&4 2 1
diffractive
optic (DO)
delay 2
Opt. Lett. 29 (8) 884 (2004)
2 f
delay 1

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