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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

10–15Ti

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