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Ultrafast Laser Spectroscopy
How do we do ultrafast laser spectroscopy?
Generic ultrafast spectroscopy experiment
The excite-probe experiment
Lock-in detection
Transient-grating spectroscopy
Ultrafast polarization spectroscopy
Spectrally resolved excite-probe spectroscopy
Theory of ultrafast measurements: the Liouville equation
Iterative solution – example: photon echo
Ultrafast laser spectroscopy: How?Ultrafast laser spectroscopy involves studying ultrafast events that take place in a medium using ultrashort pulses and delays for time resolution.
It usually involves exciting the medium with one (or more) ultrashort laser pulse(s) and probing it a variable delay later with another.
The signal pulse energy (or change in energy) is plotted vs. delay.
The experimental temporal resolution is the pulse length. S
igna
l pul
se e
nerg
y
DelayExcitation
pulses
Variably delayed Probe pulse
Signal pulseMedium under study
What’s going on in spectroscopy measurements?
The excite pulse(s) excite(s) molecules into excited states, which changes the medium’s absorption coefficient and refractive index.
The excited states only live for a finite time (this lifetime is often the quantity we’d like to find!), so the absorption and refractive index return to their initial (before excitation) values eventually.
Unexcited medium Excited mediumUnexcited medium absorbs heavily at wavelengths corresponding to transitions from ground state.
Excited medium absorbs
weakly at wavelengths
corresponding to transitions from ground
state.
Epr(t–)
Variable delay,
Detector
Esig(t,)
Probe pulse
The simplest ultrafast spectroscopy method is the Excite-Probe technique.
Excite the sample with one pulse; probe it with another a variable delay later; and measure
the change in the transmitted probe pulse energy or average
power vs. delay.
The excite and probe pulses can be different colors.This technique is also called the Pump-Probe technique.
Cha
nge
in p
robe
pu
lse
ener
gy
Delay, 0
The excite pulse changes the sample absorption of the sample, temporarily.
Excite pulse
Eex(t)
Samplemedium
Modeling excite-probe measurementsLet the unexcited medium have an absorption coefficient, 0.Immediately after excitation, the absorption decreases by 0. Excited states usually decay exponentially:
() = 0 exp(– /ex) for > 0
where is the delay after excitation, and ex is the excited-state lifetime.
So the transmitted probe-beam intensity—and hence pulse energy and average power—will depend on the delay, , and the lifetime, ex:
where L = sample length 0 0exL e L
transmitted incidentI I e
0 0exL e L
incidentI e e
001 exL
incidentI e e L assuming 0 L << 1
00 1 extransmittedI e L
Modeling excite-probe measurements (cont’d)
The relative change in transmitted intensity vs. delay, , is:
Cha
nge
in p
robe
-be
am in
tens
ity
Delay, 0
0
00
transmitted transmitted
transmitted
T I IT I
00 1 extransmitted transmittedI I e L
0
0
exT
e LT
Modeling excite-probe measurements (cont’d)
More complex decays occur if intermediate states are populated or if the motion is complex. Imagine probing an intermediate transition, whose states temporarily fill with molecules on their way back down to the ground state:
Excite transition
Probe transition
0
1
23
Excited molecules in state 1: absorption of probe
Cha
nge
in p
robe
-be
am tr
ansm
itted
in
tens
ity o
r pow
er
Delay, 0
0
Excited molecules in state 2: stimulated emission of probe
Lock-in Detection greatly increases the sensitivity in excite-probe experiments.This involves chopping the excite pulse at a given frequency and detecting at that frequency with a lock-in detector:
Chopped excite pulse train
Probe pulsetrain
Lock-in detection automatically subtracts off the transmitted power in the absence of the excite pulse. With high-rep-rate lasers, it increases sensitivity by several orders of magnitude!
The excite pulse periodicallychanges the sample absorption seen by the probe pulse.
Chopper
Lock-indetector
The lock-in detects only one frequency component of the detector voltage—chosen to be that of the chopper.
Excite-probe studies of bacterio-rhodopsin
Rhodopsin is the main molecule
involved in vision. After absorbing a
photon, rhodopsinundergoes a many-
step process, whose first three steps occur on fsor ps time scales
and are poorly understood.
Excitation populates a new state, which absorbs at 460nm and emits at 860nm. It is thought that this state involves motion of the carbon atoms (12, 13, 14). An artificial version of rhodopsin, with those atoms held in place, reveals this change on a much slower time scale, confirming this theory!
Native ArtificialProbe at 460nm
(increased absorption):
Probe at 860 nm (stimulated emission):
Zhong, et al., Ultrafast Phenomena X, p. 355 (1996).
Excite-probe measurements can reveal quantum beats
Excitation-pulse spectrum
0
12
Excite pulse
Probe pulse
Since ultrashort pulses have broad bandwidths, they can excite two or more nearby states simultaneously.
Probing the 1-2 superposition of states can yield quantum beats in the excite-probe data.
Excite-probe measurements can reveal quantum beats: ExperimentHere, two nearby vibrational states in molecular iodine interfere.
These beats also indicate the motion of the molecular wave packet on its potential surface. A small fraction of the I2 molecules dissociate every period.
Zadoyan, et al., Ultrafast Phenomena X, p. 194 (1996).
Time-frequency-domain absorption spectroscopy of Buckminster-fullerene
Brabec, et al., Ultrafast Phenomena XII, p. 589 (2000).
Electron transfer from a polymer
to a buckyball is very fast.
It has applications to photo-voltaics,
nonlinear optics, and artificial
photosynthesis.
The coherence spike in ultrafast spectroscopyWhen the delay is zero, other nonlinear-optical processes occur, involving coherent 4WM between the beams and generatingadditional signal not described by the simple model. As in autocorrelation, it’s called the coherence spike or coherent artifact. Sometimes you see it; sometimes you don’t.
Alternate picture: the pulses induce a grating in the absorption and/or refractive index,
which diffracts light from each beam into the other.
Intensity fringes in sample when pulses arrive simultaneously
Probe pulse
SampleExcite pulseThis spike
could be a very veryfast event that couldn’t be resolved. Or it could be a coherence spike.
Taking advantage of the induced grating: the Transient-Grating Technique.Two simultaneous excitation pulses induce a weak diffraction grating, followed, a variable delay later, by a probe pulse. Measure the diffracted pulse energy vs. delay:
This method is background-free, but the diffracted pulse energy goes as the square of the diffracted field and hence is weaker than that in excite-probe measurements.
Diff
ract
ed
puls
e en
ergy
Delay, 0
Delay
Excite pulse #1
SampleExcite pulse #2
Probe pulse
Diffracted pulse
Intensity fringes in
sample due to excitation
pulses
A transient-grating measurement may still have a coherence spike!When all the pulses overlap in time, who’s to say which are the excitation pulses and which is the probe pulse?
A transient-grating experiment with a coherence spike: D
iffra
cted
be
am e
nerg
y
Delay, 0
Delay
Excite pulse #1 (acting as the probe)
Excite pulse #2
Probe pulse (acting as an excite pulse)
Intensity fringes in
sample due to an
excitation pulse and the probe
acting as an excitation
pulse
What the transient-grating technique measuresIt measures the Pythagorean sum of the changes in the absorption and refractive index. The diffraction efficiency, , is given by:
This is in contrast to the excite-probe technique, which is only sensitive to the change in absorption and depends on it linearly.
Diff
ract
ed b
eam
in
tens
ity
Delay, 0
Tran
smitt
ed
inte
nsity
Absorption(amplitude)grating
Refractive index(phase) grating
H. Eichler, Laser-Induced Dynamic Gratings, Springer-Verlag, 1986.
If the absorption grating dominates and the excite-probe decay is exp(- /ex), then the TG decay will be exp(-2 /ex):
2 2
4 2L n kL
Time-resolved fluorescence is also useful.
Fluo
resc
ent
beam
pow
er
Delay
Exciting a sample with an ultrashort pulse and then observing the fluorescence vs. time also yields sample dynamics. This can
be done by directly observing the fluorescence or, if it’s too fast, by time-gating it
with a probe pulse in a SFG crystal.
Delay
Slow detector
Excite pulse
Sample
LensProbe pulse
SFG crystal
Fluorescence
Time-resolved fluorescence decay
When different tissues look alike (i.e., have similar absorption spectra), looking at the time-resolved fluorescence can help distinguish them.
Here, a malignant tumor can be
distinguished from normal tissue due to
its longer decay time.
Normal tissue
Malignant tumor
Svanberg, Ultrafast Phenomena IX, p. 34 (1994).
Temporally and spectrally resolving the fluorescence of an excited molecule
Exciting a molecule and watching its fluorescence reveals much about its potential surfaces. Ideally, one would measure the time-resolved spectrum, equivalent to its intensity and phase vs. time (or frequency).
Here, excitation occurs to a predissociative state, but other situations are just as interesting. Analogous studies can be performed in absorption.
Ultrafast Polarization Spectroscopy
It’s also possible to change the absorption coefficient differently for the two
polarizations. This is called induced dichroism. It also rotates the probe polarization and can also be used to study orientational relaxation.
Delay
45°polarized excite pulse Sample
Probe pulse
HWP
0° polarizer
90° polarizer
A 45º-polarized excite pulse induces birefringence in an ordinarily isotropic sample via the Kerr effect. A variably delayed probe pulse
between crossed polarizers watches the birefringence decay, revealing the sample orientational
relaxation.
Other ultrafast spectroscopic techniques
Photon Echo
Transient Coherent Raman Spectroscopy
Transient Coherent Anti-Stokes Raman Spectroscopy
Transient Surface SHG Spectroscopy
Transient Photo-electron Spectroscopy
Almost any physical effect that can be induced by ultrashort light pulses!
• Treat the medium quantum-mechanically and the light classically.
• Assume negligible transfer of population due to the light.
• Assume that collisions are very frequent, but very weak: they yield exponential decay of any coherence
• Use the density matrix to describe the system. The density matrix is defined according to:
For any operator A, the mean value is given by:
• Effects that are not included in this approach: saturation, population of other states by spontaneousemission, photon statistics.
Semiclassical Nonlinear-OpticalPerturbation Theory
ATraceA n mmn
If the state of a single two-level atom is:
The density matrix, ij(t), is defined as:
Since excited state populations always eventually decay to ground state populations, ii generally depends on time, ii(t).
And coherence between two states usually decays even faster, so the off-diagonal elements also depends on time.
The density matrix
cc
* *
**
c c c cc ccc
ρ or ρ are the population densities of states and .
ρ and ρ are the degree of coherence between states and .
For a many-atom system, the density matrix, ij(t), is defined as:
where the sums are over all atoms or molecules in the system.
The density matrix for a many-atom system
*
*
*
*
( )( ) (
( () ( ) ( ) ( )) ( )
)( ) (( ) )t c t c t c t
cc t
t c t c t c tt
t t
2
*
*
2
( ) ( ( )
( ) )
)
( ) (
c t
c t
c t c t
c c tt
Simplifying:
The diagonal elements (gratings) are always positive, while the off-diagonal elements (coherences) can be negative or even complex.
So cancellations can occur in coherences.
Why do coherences decay?
Atom #1
Atom #2
Atom #3
Sum:
A macroscopic coherence is the sum over all the atoms in the medium.
The collisions "dephase"the emission, causingcancellation of the total emitted light, typically exponentially.
Grating and coherence decay: T1 and T2
A grating or coherence decays as excited states decay back to ground.
A coherence can also cancel out if collisions have randomized the phase of each oscillating atomic dipole.
The time-scales for these decays to occur are always written as:
Grating [(t) or (t)]: T1 “relaxation time”
Coherence [(t) or (t)]: T2 “dephasing time”
The measurement of these times is often the goal of nonlinear spectroscopy!
Collisions cause dephasing but not necessarily de-excitation; therefore, it is generally true that T2 << T1.
The Liouville equation for the density matrix is:
(in the interaction picture)
which can be formally integrated:
Nonlinear-Optical Perturbation Theory
,di Vdt
0
0
( ) ( ) 1/ ( '), ( ') 't
t t i V t t dt
t
This can be solved iteratively:
Note that i.e., a “time ordering.”
( )
0
( ) ( )n
n
t t
0 -1 1 ... n nt t t t t
-11
0 0 0
( )1 2 1 2 0( ) 1/ ... ( ), ( ), ... ( ), ( ) ...
ntttnn
n n
t t t
t i dt dt dt V t V t V t t
Expand the commutators in the integrand:
Consider, for example, n = 2:
Thus, contains 2n terms.
Perturbation Theory (continued)
1 2 0 1 2 0 0 2( ), ( ), ( ) ( ), ( ) ( ) ( ) ( )V t V t t V t V t t t V t
2 0 1 0 2 1( ) ( ) ( ) ( ) ( ) ( )V t t V t t V t V t
( ) n
1 2 0( ), ( ), ... ( ), ( ) ...nV t V t V t t
1 2 0 1 0 2( ) ( ) ( ) ( ) ( ) ( )V t V t t V t t V t
• population of state j = jj
• polarization operator p is:
0
0
and so the macroscopic polarization P is:
2112
2221
1211
00
N
TrN
pTrNpNP
Hamiltonian H = H0 + Hint:
00
*int EE
EpH
For an ensemble of 2-level systems in the presence of a laser field:
1
2
Density matrix & Hamiltonian
000
E00E
H2
10
(we have defined the zero of energy as the energy of state 1)
Liouville equation for the diagonal element:
1
eq222222
Ti2,H2
ti
1
eq2222
21121221 TiHH
…and similarly for 11.
1
2222
21
12
2221
1211
2221
1211
21
12 200
2T
iH
HH
H eq
1
021121221 T
iHH2t
i
Derive from these an equation for the population difference = 22 - 11
Diagonal elements of
Liouville equation for the off-diagonal element:
2
2112
Ti2,H1
ti
212
2112
TiH
ti
Now use the known form for the perturbation:
ti*ti*2112 e)t(Ee)t(EHH
1
0titi*1221 T
EeeEi2t
212
ti*ti21
T1ieEEei
t
Off-diagonal elements of
tim
m
)m(
tin
n
)n(2121
e)t(
e)t(
Insert into the previous equations, and match terms of like frequency:
1
)n(0
)n()1n*(
21*)1n(
21
)n(
TEEi2
t
)1n()n(21
)n(21 EiAt
i
T1A
2
which can be integrated to yield:
t
't
)1n(t
)n(21 "Adtexp)'t(Ei'dt
1
)1n*(21
*)1n(21
t)n(
Tt'texp)'t(E)'t(Ei2'dt
Rotating wave approximation
We have a system of equations of the form: )1n()n(
21
)1n(21
)n(
G
F
Start with 21(0) = 0 and (0) = 0 and iterate:
)0()1(21
)2()0()1(21 GFFG
)0()2()3(21 GFGG
21(3) term looks like:
2
3
2
3
1
21
t
t
*3
*21
t
t32
*1
t
t1
12t
3
t
2
t
1
3
0)3(
21
'dtAexp)t(E)t(E)t(E'Adtexp)t(E)t(E)t(E
'AdtT
ttexpdtdtdti2
Must be a (3) process!
Iterative method for solving perturbatively
2
3
2
3
1
21
t
t
*3
*21
t
t32
*1
t
t1
12t
3
t
2
t
1
3
0)3(
21
'dtAexp)t(E)t(E)t(E'Adtexp)t(E)t(E)t(E
'AdtT
ttexpdtdtdti2
Suppose there are two pulses, both short compared to all relevant time scales:
k1
k2
E(t) = E1 (t) eik1r + E2 (t ) eik2r
A product of three of these E(t) fields gives 8 terms, each with one of these four wave vectors:
k1+k2-k2 k1+k2-k1 k1+k1-k2 k2+k2-k1
phase matching, of a sort: pick the direction you care about
Multiple pulses
k1
k2
2k1 k2
Choose the 2k1 - k2 direction.
Then only two terms (of 16 in 21(3)) contribute:
2
3
2
3
1
21
t
t
*321
t
t321
t
t1
12t
3
t
2
t
1
3
2210
)3(21
'dtAexp)t()t()t('Adtexp)t()t()t(
'AdtT
ttexpdtdtdtEEi2
First term: must have t2 ≥ 0 AND t2 =
must have t1 ≥ AND t1 = 0
≥ 0≤ 0} signal only
for = 0
"coherent spike"
Second term: must have < 0 and t > 0 (no other constraints);it gives rise to signal at values of other than merely = 0
Example: application to the two-pulse echo
k1
k2
2k1 - k2
0*
t
0
3
2210
)3(21 'dtA'AdtexpEEi2
t > 0 < 0
Homogeneous broadening
Polarization = N 21 ~ exp[(t + )/T2']
measured signal S():
otherwise
0 for
0edtPS
'2T
22)3(
where21
'2 T
1T1
T1 +=
P(3) is non-zero only at t =
otherwise
0 for
0eS
'2T
4
Example: application to the two-pulse echo
t
Inhomogeneous broadeningWe must integrate over the inhomogeneous distribution g():
gede
gdPtiTt
)3(21
)3(
2~
tedtedtP)(S 22 Tt2
0
T2
0
2)3(
Homogeneous case: phase-matched free-induction decay
pulse #1 pulse #2free-induction decay: 2k1 - k2
Inhomogeneous case: photon echo
pulse #1 pulse #2
echo
It can be difficult to distinguish between these two cases experimentally!
Echo vs. FID: can we tell?
k1
k2
2k1 - k2
One way to distinguish:
FWM upconversion using a third optical pulse
e.g., M. Mycek et al., Appl. Phys. Lett., 1992
Echo vs. FID: how to tell
Photon echo: what’s going on?The pseudo-vector: z component denote population state
xy components denote polarization statez zz
The photon echo is physically equivalent to the “spin echo” in NMR spectroscopy, except for the extra complication of wave-vector phase matching (2k2 – k1)