Steering ultrafast processes in artificial photosynthesis Dr. ir. Annemarie Huijser.
Ultrafast Spectroscopy - University of California, Berkeley 7... · Ultrafast examples: •...
Transcript of Ultrafast Spectroscopy - University of California, Berkeley 7... · Ultrafast examples: •...
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
10–15Ti
mes
cale
(sec
onds
)
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 oscillator
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
Al2 O3 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 (a
u)
Inte
nsity
(au)
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 conservedω1
ω3ω2
Optical Parametric Amplification (OPA)
ω1 "signal"
"idler"
“seed"
“pump"
Non-linear processes
Emitted-light frequency
(1) (2) 2 (3) 30 ...ε χ χ χ⎡ ⎤= + + +⎣ ⎦c X X X
(5) *0 1 2 3 4 5E E E E Eε χ=c
ωsig
Time Resolution for P(3)
“Excitation pulses”Variably delayed “Probe pulse”
“Signal pulse”Medium under study
Sig
nal p
ulse
ene
rgy
De
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
issi
on”)
Dimer Model (Theory)
Electronic Coupling
1 2Dimer
E
g1
e1
g2
e2Δ
ε1
ε2
J
E
J
Principles of 2D Spectroscopy
τ T t
( )tψ = e( ) i tt eψ β −= + ⋅ 3ωg e
g
e
( )ρ t ABSORPTIONFREQUENCY
EMISSIONFREQUENCY
1ω 3ω
SIGNAL
Recoveredfrom Experiment
( )3 ( , , )S T tτ
Time
⟩⋅+⟩=⟩ eegt ti ||)(| 3ωβψ
12
34
delay 1delay 2
1 23 4
1&2
3&4
diffractiveoptic (DO)
sample
2 f
sphericalmirror
spectro-meter
1 2 3 sig4=LO
coh.time
pop.time
echotime
τ 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 kδω δω <
0j kδω δω >
NegativelyCorrelated Spectral Motion
PositivelyCorrelated Spectral Motion
2D spectrum with cross-peaksA measurement at the amplitude level