Ultrafast Laser Spectroscopy

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Ultrafast Laser Spectroscopy
How and why ultrafast laser spectroscopy? Generic
ultrafast spectroscopy experiment The
excite-probe experiment Transient-grating
spectroscopy Ultrafast polarization
spectroscopy Optical heterodyne detection
(OHD) Spectrally resolved excite-probe
spectroscopy
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Ultrafast laser spectroscopy Why?
Most events that occur in atoms and molecules
occur on fs and ps time scales because the length
scales are very small. Fluorescence occurs on a
ns time scale, but competing non-radiative
processes only speed things up because relaxation
rates add 1/tex 1/tfl 1/tnr
Biologically important processes utilize
excitation energy for purposes other than
fluorescence and hence must be very
fast. Collisions in room-temperature liquids
occur on a few-fs time scale, so nearly all
processes in liquids are ultrafast. Semiconductor
processes of technological interest are
necessarily ultrafast or we wouldnt be
interested.
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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.
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Whats going on in spectroscopy measurements?
The excite pulse(s) excite(s) molecules into
excited states, which changes the mediums
absorption coefficient and refractive index.
Unexcited medium
Excited medium
Unexcited medium absorbs heavily at wavelengths
corresponding to transitions from ground state.
Excited medium absorbs weakly at wavelengths
corresponding to transitions from ground state.
The excited states only live for a finite time
(this is the quantity wed like to find!), so the
absorption and refractive index recover.
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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.
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Modeling excite-probe measurements
Let the unexcited medium have an absorption
coefficient, a0. Immediately after excitation,
the absorption decreases by Da0. Excited states
usually decay exponentially Da(t) Da0
exp(t /tex) for t 0 where t is
the delay after excitation, and tex is the
excited-state lifetime. So the transmitted
probe-beam intensityand hence pulse energy and
average powerwill depend on the delay, t, and
the lifetime, tex Itransmitted(t) Iincident
expa0 Da0exp(t /tex)L where L
sample length Iincident expa0L
expDa0exp(t /tex)L ? Iincident
expa0L 1Da0exp(t /tex)L assuming Da0 L
/tex)L
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Modeling excite-probe measurements (contd)
Itransmitted(t) ? Itransmitted(?-) 1Da0exp(t
/tex)L
The relative change in transmitted intensity vs.
delay, t, is
DT(t) /T0 Itransmitted(t) ?
Itransmitted(??) /Itransmitted(??)
DT(t) /T0 ???Da0 exp(t /tex)L
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Modeling excite-probe measurements (contd)
More complex decays can be seen 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
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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
The excite pulse periodically changes the sample
absorption seen by the probe pulse.
Chopper
Chopped excite pulse train
Slow detector
Lock-in detector
Sample
Lens
The lock-in detects only one frequency component
of the detector voltagechosen to be that of the
chopper.
Probe pulse train
Delay
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!
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Excite-probe measurements in DNA
DNA bases undergo photo-oxidative damage, which
can yield mutations. Understanding the
photo-physics of these important molecules may
help to understand this process.
Pecourt, et al., Ultrafast Phenomena XII, p.566
(2000).
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Excite-probe measurements of bacterio-rhodopsin
Rhodopsin is the main molecule involved in
vision. After absorbing a photon, rhodopsin
undergoes a many-step process, whose first three
steps occur on fs or ps time scales and are
poorly understood.
Zhong, et al., Ultrafast Phenomena X, p. 355
(1996).
Excitation populates a new state, which absorbs
at 460nm and emits at 860nm. It was thought that
this state involved motion of the carbon atoms
(12, 13, 14). But an artificial version of
rhodopsin, with those atoms held in place, also
reveals this change on the same time scale (the
rise time is the same)!
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Excite-probe measurements of Hypericin, an
anti-viral substance
When excited by light, Hypericin deactivates HIV.
So it would be nice to understand how it works.
These curves (for two different solvents) show
the rise time for a proton-transfer process
important in its biological activity.
Relative change in absorbance
M.J. Fehr, et al., Ultrafast Phenomena IX, pg.
462 (1994).
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Excite-probe measurements of Terawatt femtosecond
UV pulses in water
High-intensity UV ultrashort pulses may someday
be used in surgery. So understanding what these
pulses do to water is important. Hydrated
electrons are formed in very high concentrations
(0.01 molar).
The induced absorption seen here is very high!
Pommeret, et al., Ultrafast Phenomena XII, p. 536
(2000).
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Excite-probe reflection spectroscopy
Exciting a surface and probing its reflectivity
later reveals surface physics. Here, a quantum
wire is studied using ultrashort pulses in a
near-field scanning optical microscope to yield
200-nm spatial resolution, too!
Emiliani, et al., Ultrafast Phenomena XII, p. 256
(2000).
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Excite-probe measurements can reveal quantum
beats Theory
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.
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Excite-probe measurements can reveal quantum
beats Experiment
Here, 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).
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Quantum beats in polymers using 5-fs pulses
Excite-probe measurements in polydiacetylene show
several different frequencies, implying several
(vibrational) states were excited.
Kobayashi, Ultrafast Phenomena XII, p. 575 (2000).
lpr
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The Coherence Spike in ultrafast spectroscopy
When the delay is zero, other nonlinear-optical
processes occur, a involving coherent 4WM between
the beams and generatingaadditional signal not
described by the simple Da model. As in
autocorrelation, its called the coherence
spike or coherent artifact. Sometimes you see
it sometimes you dont.
Alternate picture when the two input pulses
arrive simultaneously, they induce a grating,
which diffracts light from each beam into the
other.
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A background-free ultrafast spectroscopy method
is The Transient-Grating Technique.
This involves exciting the sample with two
simultaneous excitation pulses, inducing a weak
diffraction grating, probing it with another
pulse a variable delay later, and measuring the
diffracted pulse average power vs. delay
Intensity fringes in sample due to excitation
pulses
Excite pulse 1
Sample
Excite pulse 2
Slow detector
Diffracted pulse
Probe pulse
Delay
The excite pulses have a spatially sinusoidal
energy deposition in the sample. The sample
absorption and refractive index will now vary
sinusoidally in space.
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A transient-grating measurement may still have a
coherence spike!
When all the pulses overlap in time, whos to say
which are the excitation pulses and which is the
probe pulse?
Intensity fringes in sample due to an excitation
pulse and the probe acting as an excitation pulse
Excite pulse 1 (acting as the probe)
Excite pulse 2
Probe pulse (acting as an excite pulse)
Delay
A transient-grating experiment with a coherence
spike
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What the transient-grating technique measures
It 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.
If the absorption grating dominates and the
excite-probe decay is exp(-???ex), then the TG
decay will be exp(-2???ex)
H. Eichler, Laser-Induced Dynamic Gratings,
Springer-Verlag, 1986.
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Transient orientation gratings
You might think that a grating can be induced
only by a sinusoidal intensity pattern (caused by
the interference of two parallel-polarized
beams). But orthogonally polarized beams, which
have a constant intensity vs. position, also
induce a grating! An orientation grating.
Variation of the electric field vs.
position Orientation gratings can
also decay due to orientational relaxation.
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Induced gratings can also decay by diffusion.
Diffusion can wash out an induced grating.
Sometimes diffusion is faster than excited-state
decay.
Diffusion occurs on a time scale that depends on
the grating fringe spacing. If the fringes are
closely spaced, diffusion is very fast if the
fringes are far apart, then its much slower.
Varying the grating fringe spacing can determine
the time scales for both decay mechanisms.
where D diffusion coeff
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Transient-grating measurements in multiple
quantum wells
Both concentration (amplitude) and orientation
(spin) gratings induced by excite beams with
parallel and perpendicular polarizations. The
orientation grating decays much faster.
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Time-resolved fluorescence is also useful.
Exciting a sample with an ultrashort pulse and
then observing the fluoresccence vs. time also
yields sample dynamics. This can be done by
directly observing the fluroescence or, if its
too fast, by time-gating it with a probe pulse in
a SFG crystal
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Time-resolved fluorescence decay
Normal tissue
When different tissues look alike (i.e., have
similar absorption spectra), looking at the
time-resolved fluorescence can help distinguish
them.
Malignant tumor
Here, a malignant tumor can be distinguished from
normal tissue due to its longer decay time.
Svanberg, Ultrafast Phenomena IX, p. 34 (1994).
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Ultrafast Polarization Spectroscopy
A 45-polarized excite pulse will induce
birefringence in an ordinarily isotropic sample.
A variably delayed probe pulse between crossed
polarizers can watch the birefringence decay,
revealing the sample orientational relaxation.
Light changing the refractive index of a medium
is called the Kerr effect.
Its also possible to change the absorption
coefficient differently for the two
polarizations. This is called an induced
dichroism. It also rotates the probe
polarization and can also be used to study
orientational relaxation.
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Nice features of ultrafast polarization
spectroscopy
Its as easy to set up as excite-probe (just
cross two beams in space and time). Its almost
background-free (crossed polarizers transmit as
little as 10-6 of the incident light). Unlike
excite-probe, it measures both absorption and
phase effects. It can use lock-in
detection. And simultaneously, it can use
optical heterodyne detection, which optimizes
the signal-to-noise ratio.
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Heterodyned Ultrafast Polarization Spectroscopy
Optical Heterodyne Detection (OHD) polarization
spectroscopy involves slightly uncrossing the
polarizers. This allows some of the probe pulse
to leak into the detector and combine coherently
with the signal pulse.
This trivial (seemingly inappropriate!) change
can actually improve the sensitivity by many
orders of magnitude!
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Heterodyned ultrafast polarization spectroscopy
Heterodyned polarization spectroscopy adds a
small amount of the probe pulse, d?Eprobe(t), to
the (even smaller) signal pulse. As a result, we
now detect the squared magnitude of the sum of
these two fields
usual PS signal
As long as the leaked probe intensity the
signal intensity, we can neglect the latter
This yields a signal term proportional to
Esig(t), which is much larger than its squared
magnitude. And it also yields its phase.
As long as the probe intensity is stable, this
yields a huge improvement in sensitivity.
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Heterodyned ultrafast polarization spectroscopy
of liquids
Using different colors for the excite and probe
pulses, this technique is called the Optical
Heterodyne Detection-Raman-Induced Kerr Effect
Spectroscopy (OHD-RIKES).
The excite pulse can induce a change in the
refractive index seen by the probe pulse, which
is enhanced when wex wpr w10
Sample media are various amides.
Notice how very clean the data are.
Castner and Chang, Ultrafast Phenomena X, p. 296
(1996).
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Heterodyned ultrafast polarization spectroscopy
of CS2
OHD-RIKES study of CS2 at different temperatures.
Loughnane, et al., Ultrafast Phenomena X, p. 304
(1996).
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Anti-resonant Ring Transient Spectroscopy (ARTS)
ARTS is another method that subtracts off the
background and can heterodyne. The Anti-resonant
Ring (Sagnac interferometer) generates two probe
pulses that counter-propagate around a ring. The
clockwise pulse passes through the sample early.
The counter-clockwise pulse passes through the
sample later, shortly after the excite pulse (and
is modified by it).
Sample is off-center in a Sagnac
interferometer. Without the excite pulse, the two
probe pulses cancel out at the output. With the
excite pulse, output pulse indicates sample
change.
If the beam splitter is other than 50, this
method heterodynes.
Trebino and Hayden, Opt. Lett., 16, 493 (1991).
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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.
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Time-frequency-domain absorption spectroscopy of
Buckminster-fullerene
Electron transfer from a polymer to the buckyball
is very fast. It has applications to
photo-voltaics, nonlinear optics, and artificial
photosynthesis.
Brabec, et al., Ultrafast Phenomena XII, p. 589
(2000).
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Ultrafast Spectroscopy of Photosynthesis
The initial events in photosynthesis occur on a
ps time scale.
Arizona State University
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Multi-dimensional nonlinear spectroscopy
Measure signal out vs. various input delays and
wavelengths
Cresyl violet
Hannaford and coworkers, UP 2004
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Higher-order processes can be useful.
Here, six-wave mixing helps to delineate
vibrational motion in liquids.
Steffen and Duppen, Ultrafast Phenomena X, p. 213
(1996).
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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!