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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 - PowerPoint PPT Presentation
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Ultrafast Spectroscopy
Gabriela Schlau-CohenFleming Group
Why femtoseconds?
timescale = distance/velocity~~~~~~
distance ≈ 10 ÅE ≈ 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?
1960 1970 1980 1990 2000
10–6
10–9
10–12
10–15T
imes
cale
(se
cond
s)
Year
Electronics
Optics
Laser Basics
Level empties
fast!
Four-level system
Laser Transition
Pump Transition
Fast decay
Fast decay
•Population inversion
•Pump energy source
•Lasing transition
• Method of creating pulsed output
• Compressed output
• Broadband laser pulse
What we need for ultrashort pulse generation:
Ultrafast Laser Overview
Laser oscillato
r
Amplifier medium
pump
3 pieces of ultrafast laser system:
• Oscillator• Regenerative
Amplifier
• Tunable Parametric Amplifier
Oscillator generates short pulses with mode-locking
Ti:Sapphirelaser crystal
Cavity with partially reflective mirror
Pump laser
Prisms
Titanium: Sapphire
oxygenaluminum
Al2O3 lattice
• 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)
Ti:Sapphire spectral
properties(nm)
FLU
OR
ES
CE
NC
E
(au)
Inte
nsity
(a
u)
Mode-locking
Mechanism of Mode-locking: Kerr Lens Effect
)(20 xInnn
Compression
• Prism compression
• Gratings, chirped mirrors
t t
Chirped Pulse Amplification
Pulse compressor
t
t
Solid state amplifiers
t
Dispersive delay linet
Short pulse
oscillator
• Stretch
• Amplify
• Recompress
Regenerative Amplifier
• Pulsed seed• Ti: Sapph crystal
Faraday rotator
thin-film polarizerPockels cell
• Pulsed pump laser• Pockels cell
p-polarized light
s-polarized light
OPA/NOPA
• Parametric amplification• Non-linear process• Energy, momentum conserved
1
32
Optical Parametric Amplification (OPA)
1 "signal"
"idler"
“seed"
“pump"
Non-linear processes
Emitted-light frequency
(1) (2) 2 (3) 30 ... P E E E
(5) *0 1 2 3 4 5E E E E E P
sig
Time Resolution for P(3)
“Excitation pulses”
Variably delayed “Probe pulse”
“Signal pulse”Medium under study
Sig
nal
pul
se e
nerg
y
Delay
Two-Dimensional Electronic Spectroscopy can study:
• Electronic structure
• Energy transfer dynamics
• Coupling
• Coherence
• Correlation functions
2D Spectroscopy
• Excitation at one wavelength influences emission at other wavelengths
• Diagonal peaks are linear absorption
• Cross peaks are coupling and
energy transfer
Excited StateAbsorption
Inhomogeneous Linewidth
HomogeneousLinewidth
CrossPeak
ωτ (“absorption”)
ωt
(“em
iss
ion
”)
Dimer Model (Theory)
Electronic Coupling
1 2Dimer
E
g1
e1
g2
e2
1
2
J
E
J
Principles of 2D Spectroscopy
τ T t
t e i tt e 3ωg e
g
e
ρ t ABSORPTIONFREQUENCY
EMISSIONFREQUENCY
1 3
SIGNAL
Recoveredfrom Experiment
3 ( , , )S T t
Time
eegt ti ||)(| 3
1
2
3
4
delay 1delay 2
1 2
3 4
1&2
3&4
diffractiveoptic (DO)
sample
2 f
sphericalmirror
spectro-meter
1 2 3 sig4=LO
coh.time
pop.time
echotime
T t
OD3
2D Heterodyne Spectroscopy
Opt. Lett. 29 (8) 884 (2004)
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
0j k
0j k
NegativelyCorrelated Spectral Motion
PositivelyCorrelated Spectral Motion
2D spectrum with cross-peaksA measurement at the amplitude level
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