Ultrafast Spectroscopy
Gabriela Schlau-Cohen
Fleming Group
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
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)
How can we measure things this
fast?
–6
Timescale (seconds)
10
–9
10
Electronics
–12
10
Optics
–15
10
1960
1970
1980
Year
1990
2000
Laser Basics
Four-level
system
•Population inversion
•Pump energy source
Fast decay
Pump
Transition
•Lasing transition
Laser
Transition
Fast decay
Level
empties
fast!
What we need for ultrashort pulse
generation:
• Method of creating pulsed output
• Compressed output
• Broadband laser pulse
Ultrafast Laser Overview
pump
Laser
oscillator
Amplifier
medium
3 pieces of ultrafast laser system:
• Oscillator
• Tunable Parametric
Amplifier
• Regenerative Amplifier
Oscillator generates short pulses
with mode-locking
Prisms
Cavity with
partially
reflective
mirror
Ti:Sapphire
laser crystal
Pump laser
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)
Al2O3 lattice
oxygen
aluminum
Intensity (au)
Ti:Sapphire
spectral
properties
FLUORESCENCE (au)
(nm)
Mode-locking
Mechanism of Mode-locking:
Kerr Lens Effect
n  n0  n2  I ( x )
Compression
• Prism compression
t
• Gratings, chirped mirrors
t
Chirped Pulse Amplification
Short
pulse
oscillator
t
• Stretch
Dispersive delay line
t
Solid state amplifiers
• Amplify
t
Pulse compressor
• Recompress
t
Regenerative Amplifier
• Pulsed seed
• Ti: Sapph crystal
s-polarized light
Pockels cell
• Pulsed pump laser
• Pockels cell
Faraday rotator
thin-film polarizer
p-polarized light
OPA/NOPA
• Parametric amplification
• Non-linear process
• Energy, momentum conserved
“seed"
“pump"
w1
w3
w1
w2
Optical Parametric
Amplification (OPA)
"signal"
"idler"
Non-linear processes
(1)
(2)
P   0   E   E
2

(3)
P  0
wsig
E
(5)
3
 ...
*
E1 E 2 E 3 E 4 E 5
Emitted-light frequency
“Signal pulse”
Medium under study
“Excitation pulses”
Variably delayed
“Probe pulse”
Signal pulse energy
Time Resolution for P(3)
De
Two-Dimensional Electronic
Spectroscopy can study:
• Electronic structure
• Energy transfer dynamics
• Coupling
• Coherence
• Correlation functions
2D Spectroscopy
• Diagonal peaks are
linear absorption
• Cross peaks are
coupling and energy
transfer
ωt (“emission”)
• Excitation at one
wavelength
influences emission
at other wavelengths
Dimer Model (Theory)
Excited State
Absorption
Homogeneous
Linewidth
Cross
Peak
Inhomogeneous
Linewidth
ωτ (“absorption”)
Electronic Coupling
E
E
JJ
e1
1
e2
D
2
g1
g2
1
Dimer 2
Principles of 2D Spectroscopy
τ
T
t
SIGNAL
Time
 t   g    e
 i ω3 t
e
 t  e
|  (t ) | g     e
iw 3 t
| e
e
g
ρt
ABSORPTION
FREQUENCY
w1
S
EMISSION
FREQUENCY
w3
3
( , T , t )
Recovered
from Experiment
2D Heterodyne Spectroscopy
coh. pop. echo
time time time

T
t
4=LO
1
2
3 sig
spectrometer
1
2
3
4
sample
OD3
spherical
mirror
4
3
1&2
3&4
2
1
diffractive
optic (DO)
Opt. Lett. 29 (8)
884 (2004)
2f
delay 2
delay 1
Experimental Set-up
Fourier Transform
Future directions of ultrafast
• Faster: further compression into the
attosecond regime
• More Powerful: higher energy transitions
with coherent light in the x-ray regime
2D spectrum with cross-peaks
A measurement at the amplitude level
Positively
Correlated
Spectral
Motion
w j w k  0
Negatively
Correlated
Spectral
Motion
w j w k  0