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Today we're going to be talking about the femtosecond pump probe transient absorption
spectrometer. So it's quite a long name, but it's pretty descriptive, and if we work our
way from the back. It is an absorption spectrometer, so it's a lot like the UV vis absorption spectrometers
that are pretty much in every chemistry or spectroscopy laboratory. In those systems
we basically look at the absorption spectra species that exist in the ground state, and
since species almost always exist in the ground state, for sources, we can use something that’s
more steady state. So something that’s like a tungsten bulb, or a fluorescent bulb, or
something like that. With this particular system, we're going to be looking at the absorption
properties of species that exist in the excited state. Since the excited states typically
are short-lived, or transient, we actually need a slightly different technique to be
able to measure them. So in addition to measuring their spectral properties, because they're
short lived and they're going to decay, we also need to be able to measure their temporal
properties, which is one of the reasons why people call this 2-dimensional spectroscopy,
as we're getting information not only in the spectral domain, but in the temporal domain
as well. So in order to do that, you need sources that are a little bit different than
typical UV-vis spectrometers. For instance, to get to the excited state, we're going to
have to optically pump the sample, so we use an optical pulse to do the pumping, we get
the species in the excited state, and then we need a probe to be able to effectively
probe that excited state. So to be able to probe something that’s very short-lived,
we need something that’s very short-lived, so we also use a probe-pulse that’s very
short in duration, so the pump pulse and the probe pulse are about 100 femtoseconds in
duration, more or less, so this allows us to get good temporal resolution and if we
make the probe broad band, we can actually get good spectral resolution. It sounds kind
of like a niche spectroscopy tool, but in effect we can actually do measurements on
a bunch of different systems. So people look at things as different as the electronic absorption
states of organics. You can look at charge species in organic photovoltaics, you can
look at carrier dynamics in semi-conductors, or you can look at ultrafast processes like
two photon absorption or raman. So because of this, this type of technique has become
ubiquitous, so much so that now these systems are no longer just developed in individual
laboratories, they are commercially sold. So this particular system that is made by
Ultrafast Systems is the one that we use in our laboratory today.
Our excitation source is an amplified femtosecond laser, and from this femtosecond laser, we
are going to pump one of these tunable femtosecond sources. This is an optical parametric amplifier,
and this allows us to generate our pump pulse which is going to be tunable anywhere from
280 nanometers, all the way into the infrared. So we have the capability of pumping just
about anywhere we need to, depending on where our system is going to absorb. In addition,
a portion of the light that comes out of here is actually going to be used to generate our
broad band probe. So if we walk down here, the tunable pump beam enters through this
mirror here, and we actually follow the path through here, we have a neutral density filter
wheel which allows us to control the amount of energy that we're actually going to use
to excite the sample, and we actually go through an optical chopper here. And the chopper actually
has a specific function in that it basically allows every other pulse to excite the sample
and that becomes critical when we're trying to do ratios between a sample that’s being
pumped and not being pumped so that we can actually look at what occurs during pumping.
The pump beam then actually goes into this focusing lens. It goes through this polarization
optic which effectively allows us to measure only population dynamics in the sample. It
hits this mirror and then goes to our sample which is actually located in this cuvette
right here. So that is the pump line. Now what we want to look at is our probe. And
our probe is actually generated from that femtosecond source, it's in the infrared,
it's at 800 nanometers and you can actually see it through the phosphorescence that’s
on this infrared card. So what we'll do now is we'll enter through this aperture here
and what happens is the probe pulse goes into this mirror which is on a delay stage. So
this stage allows us to control the optical delay between the pump and the probe pulses
and this is what’s going to allow us to get the temporal dynamics of our system. In
order to get more delay, you actually have to move this delay line further, so in order
to make the system more compact, what we do is pass the beam multiple times through this
mirror here and this allows us to get more optical delay in a more confined space. So
after it comes out of this retro-reflector, it hits this mirror, it goes through another
neutral density filter wheel so we can control the amount of power. It goes to this mirror
which actually focuses on this crystal here. Up until now we've had a relatively narrow
band probe centered around 800nm but if we wanted to do broad band spectroscopy, we need
something that has a lot of wavelengths in it. So what this crystal, which is a piece
of sapphire, allows us to do, is when we focus near infrared radiation into it, we can generate
white light. So here on the card, we can actually see both the pump that is in the red, and
we can see the broad band probe which is kind of white to the eye, and we want to spacially
overlap both the pump and the probe on our sample. And so you can see they're gradually
coming together here, they pass through these two mirrors, and eventually they're going
to overlap directly onto our sample here. So you can see the pump and the probe pass
through the sample, and if we allow the pump to go ahead and be dumped somewhere because
all we care about at this point is the probe, it hits this mirror and goes through a series
of lenses and some filters in order for it to pass into the spectrometer.
The basics of a pump probe are, the pump that you're using to optically excite your sample
actually induces species which go from the ground state to the excited state. Now it’s
the job of the probe to actually monitor the spectral characteristics of those species
in the excited state before they decay back down. So what we have is a pump pulse which
effectively instantaneously excites the sample and then we have a probe that comes after
it, after some period of time, and kind of takes a spectral snapshot of what exactly
those excited species are doing. And so by delaying that time delay between when you
excite the sample and when you probe it, it actually allows you to map out not only the
spectral characteristics, but actually how the system decays over time. So what we're
viewing here on the screen is a spectrum of the white-light continuum probe. What you
can see here, is a probe pretty much encompasses the entire visible portion of the spectrum
all the way into the near infrared. This will allow us to get spectral information over
this very broad band range. So again, what you’re seeing here is the spectrum of the
white light. So we'll actually put our sample in front of the white light and what you'll
be able to see is portions of this particular continuum will disappear because they are
being absorbed by the actual sample. You can see that portions of this spectrum have been
decreased, in some regions quite substantially, because again, the absorption of this sample
is quite high there. So what we're looking at right now is more of what you would see
on a typical steady state spectrometer, a spectrometer that you would typically find
in chemistry or spectroscopy laboratories. Now what we'll do is move from looking at
the probe here, to actually being able to visualize what happens in the excited state.
And so now what you're actually seeing is the actual spectrum of the excited state species.
Before we were seeing the spectrum of the ground state species. What you can see here
is that there is a certain spectral signature associated with this. Now, what happens when
I block the pump, meaning I block the ability of all the species to get from the ground
state to the excited state, you'll see that the spectrum goes away. So again, we'll unblock
the pump again and you'll see that the excited state absorption spectrum is visible here.
In addition to just blocking the pump, since this is a pump probe technique, what we can
do is we can make sure that the pump doesn’t get to the sample before the probe does. So
if the pump doesn’t get there before the probe does, we won’t be able to excite any
species. And so what I’m actually going to do now is I’m going to change the delay
so that the pump doesn’t get there before the probe, and what you'll see, is again,
the spectrum goes away. What we can also do is instead of changing it so that the pump
comes after the probe, we can make sure that the pump comes before the probe. And by doing
this we should be able to map out the spectral evolution of the excited state. So again,
what we're seeing here is that the pump comes before the probe, in this case about 2 picoseconds.
So now if we increase that delay about 180 picoseconds what we should see is that this
spectrum should actually change because again, what’s happening is the excited state is
decaying. So now if we take a shot here we should see that the spectrum has changed slightly,
and if we move even further away, let’s say we move almost 1 nanosecond away from
where we were before, we should see that the spectrum is actually going to change again.
And what we're mapping out here is the evolution of the excited state absorption spectrum as
a function of time. So what we can do is set up the system to be able to measure the spectra
over a series of different delays and we should be able to get a 2-dimensional graph. On one
axis is going to be time, and the other axis is spectrum. Everything here is automated
so what you do before you start your experiment is to specify where you want your pump and
your probe to start with respect to one another. Then what you're going to specify is how far
in time you want your probe to be delayed, and then you can specify a number of different
parameters here, which is allowing you to sample at different time intervals. So if
you have something that decays very fast, you'll want to make the step size quite small.
Right now we're sampling at about 60 femtoseconds per step. So if you have something that lives
a very long time period, you'll want to sample at a much longer resolution, something like
40 picoseconds. And by putting all of this information in here, and starting the program,
you can actually map out the evolution of the excited state absorption spectrum. And
that’s shown here. On the x-axis we have wavelength and on the y-axis you have time.
So again, this is a two-dimensional representation of what you see here. What’s shown in the
lower left, is at a particular delay, what does the spectrum looks like. This is what
we were looking at before. So while the lower left hand corner gives you the spectrum at
one particular delay, in the upper right hand corner at a particular wave length, what you
can see is the evolution of the signal as a function of delay time. So what this shows
you is how the kinetics at this particular wavelength evolve. This is the analysis software
that was provided to us and this shows what we had seen before which is what you get out
of a pump probe experiment. It’s a two-dimensional array where you have, again, time on one axis
and wavelength on the other axis, so you have thousands of data points that are available
for you to use for analysis on here. It’s a little bit overwhelming in terms of the
amount of information available, so what this software allows you to do is to go to a particular
wavelength and what you can see is the kinetics or the time evolution of the excited state
changes pretty dramatically as we go from one wavelength, let’s say at 646 nanometers,
out to around 700 nanometers, or back into the visible around 560 nanometers. So what
you can do, is you can analyze the kinetics at a particular wavelength. Conversely, if
you want to look at a particular delay, which we can do by moving the cursor along this
axis, you can see that the spectrum in the lower left hand corner will change as a function
of delay. You can see that the spectrum at early-time delays looks like this, but as
we move further in terms of delay, you can see the spectrum evolve over time… all the
way to the end of the scan. So again, there's a lot of information encapsulated in this
two-dimensional array but you can effectively get kinetics at a particular wavelength, or
you can get spectrum at a particular delay time. In this two-dimensional array here you
have a color map, and this color map corresponds to the amplitude or the signal strength that
you have here. In this case, basically going from yellow to violet gives you negative signals.
So you can see an example of this right here, this is a negative signal. A negative signal
corresponds to where the sample actually absorbs in its ground state, so what you’re doing
is depleting or bleaching that ground state. This can be contrasted to what’s shown in
the positive sense when you go from yellow up into red which are positive signals. And
that’s shown over here in the visible portion of the spectrum. This would be indicative
of an absorptive process as opposed to a bleaching process, so this is effectively excited state
absorption. You can see we have a broad enough probe, that we have the capability of looking
in regions where we have excited state absorption as well as ground state bleaching here.
Typically, the way that this data is actually presented in a research journal or a publication,
is one of two ways: either you can look at spectra at various time delays which means
in one particular graph you'll have a multiplicity of spectra and they will be labeled with the
different time delays that they were taken at. Conversely, you could also have a series
of kinetics, meaning amplitude vs. time for a bunch of different probe wavelengths. So
in this way it allows you to separate the spectral characteristics from the temporal
characteristics.
This transient spectrometer is pretty versatile. We've been able to measure things as varied
as electronic processes in organics, charge carrier dynamics in organic photovoltaics,
carrier dynamics in semiconductors. We've been able to measure thermal dynamics in metal
dielectric stacks. And in addition to absorption spectrometry, you can also do transient reflectivity,
and you can do transient changes in polarization as well. So again, this system is very versatile
in terms of looking at different optical phenomenon as well as different types of materials.