Transcript DavidCrowleyPosterx - CWRU Physics

Polymeric Thin Films for Application in THz Generation and Detection
D. Crowley, B. Kubera, J. Shan
Department of Physics, Case Western Reserve University, Cleveland, OH 44106
Introduction
The terahertz (1012 Hz) or far-infrared region of
the electromagnetic radiation corresponds to many
fundamental excitations in solids and molecules. THz
falls between the microwave and IR regions of the
electro magnetic spectrum. Potential applications of
this radiation range from imaging to spectroscopy to
security, and much more (refer to Figure 1, below).
Over the past several years, a new approach has
been developed based on the dramatic advances in
the production of ultrafast optical pulses from mode
locked lasers. In this scheme, the femtosecond laser
pulses are used to produce and detect far-IR radiation
with controlled electric-field waveforms through the
use of a nonlinear crystal.
In this study, we attempt to make polymeric thin
films for THz generation. Polymers have shown
relatively large nonlinearities and have advantageous
phase matching properties for these processes. Lack
of phonon modes, like in traditional crystals means
less of the spectrum might be cut out. They are also
easier and cheaper to produce than conventional
inorganic crystals.
Figure 1. a) image of a tooth taken with THz radiation. b) tumor located just under the
skin – detected with THz. c) traces of explosives: not only can THz tell something is
there – but can figure out exactly what materials are present due to frequency absorption
spectrum. d) 3d constructed image from THz of a turkey bone. e) can look through
common items to find dangerous objects underneath – here there is a ceramic knife and
plastic explosive. f) THz phased image of watermark in money. THz could be used for
lots of quality, security, and medical imaging tasks.
Figure 2. THz pulse (green) being traced
out by a delayed probing pulse (red). As
delay stage moves along, probing pulse,
which is much shorter than the THz
pulse, is effected differently by different
parts of the THz waveform.
Results
Figure 4. a) (above) Disperse Red 1
molecule used in my polymers. b)
(right) Picture of one of my earlier
stage THz setups.
Thin Film Preparation
Figure 3. Picture of a basic optical THz apparatus. Ti:Sapphire mode locked laser is
split into two beam lines. Most of the laser pulse is sent along the pump line to generate
the THz radiation. The THz is collected by parabolic mirrors, collimated, and then
focused back down onto the sensor element where it meets back up with the probing
pulse. THz causes a birefringence in the detector causing the probe pulse to shift in
polarization. This shift is measured by the balance detectors.
THz Apparatus
The THz experiment implements a Ti:Sapphire
mode locked, pulsed laser to create high intensities of
near infra red light that can be used to stimulate a
nonlinear reaction resulting in THz radiation. The laser
pulses will then be split by a beam sampler to send a
majority of the power towards the emitter crystal and
the rest towards the probing line. Figure 3 (above)
shows an example of a basic THz experiment.
The pump pulse will cause THz to be emitted from
a ZnTe crystal with a process known as optical
rectification, which is effectively difference frequency
generation. THz emission quickly diverges in all
directions, but it can be collected and even collimated
by properly positioning a parabolic mirror. Another
parabolic mirror is then used to focus the THz down
onto the detecting crystal. By inserting a pellicle
(effectively a large area, thin film beam splitter
transparent to THz) into the THz line it is possible to
realign the probing pulse with the THz emission so it
can be focused in the same position on the sensor
crystal. Moving the delay stage let’s us trace the THz
pulse – note Figure 2 (bottom left).
The electric field associated with the THz
radiation causes an electro optical birefringence that
alters the probing pulse’s polarization. A polarizing
beam splitter causes different polarizations to travel in
two separate paths to a balance detector which acts
like a seesaw. Changes to the probe’s polarization by
the THz cause the balance to tilt one way or another –
allowing us to see a temporal trace of the THz.
A guest-host mixtures is implemented to create
our thin films. This requires dissolving of a dye into
some host material. In our case, we used disperse red
1 (DR1) dye (see Figure 4 above), in a PMMA host
polymer. DR1 was chosen due to its relatively high
dipole moment.
First, the PMMA is dissolved in about a 10:1 ratio
of solvent: polymer. Once this is dissolved, DR1 is also
added and dissolved having about 10% of the mass of
the PMMA that is present. Once evenly dissolved, it is
time to make the films.
There are two methods that I have tried so far.
First is spin coating. Where the polymer is laid onto a
glass (or ITO) substrate and put onto a spinning
device. This whirls about at nearly 1000rpm flinging
off excess solution and creating nice, uniform films of
thickness order 1-3 microns. These would need to be
stacked or folded to achieve desired thicknesses.
Secondly, I have tried creating wells of different
depths where I just lay the solution in and evaporate
off the solvents. This has been successful in creating
fairly uniform samples from 50-80 microns. In both
cases the samples are baked at around 100 C to
expedite the evaporation process.
Poling these films is a different story. There are
two methods here that I am also trying. Each have
similarities though. First the polymer must be heated
to around its glass transition temperature (~100 C) –
this allows the molecules to move a bit more freely.
Next an electric field must be applied to line up the
molecules (~50V/um). The sample is then cooled,
ideally leaving it in a poled and well ordered state.
Better poling results in a larger nonlinear effect to be
derived from the material. The two ways I have used is
1) to sandwich the sample between ITO sheets, and
apply a voltage or 2) use a needle in what’s known as
Corona poling to deposit charge on the surface of the
film creating an electric field across it.
References:
*Sinyukov, Alexander M. and Hayden, Michael L. “Efficient electro-optic polymers for THz applications.” J. Phys. Chem. B, Vol. 108, pg 8515. 2004. *Sinyukov, Alexander M. and Hayden, Michael L. et al. “New Materials for Optical Rectification and Electro-optic Sampling of Ultra-short Pulses in the THz Regime.” J. Polymer Sci. B. Polymer Phys. Vol. 41, pg. 2492. 2003
The initial phase of my project was to build and
establish a working THz experiment utilizing
conventional sources and detectors. This turned out
being successful, and I now have a working THz set up
employing ZnTe crystals (Figure 5a – below).
The second phase is to generate a signal using a
polymer films. Several difficulties have arisen here,
preventing a signal from being detected. The THz
signal is proportional to the thickness of the emitter,
and the ZnTe crystal is 1 mm thick. Any successfully
poled samples have been three orders of magnitude
thinner. Corona poling works great for thin samples,
but when they are that thin compared to the crystal,
and the signal to noise is 100:1 there is little hope.
Even averaging over many runs to pull the signal from
the noise came up with nothing. The thicker, samples
can not be sufficiently poled using the Corona
technique, as the same amount of charge is deposited
for a much thicker sample – lowering the applied field
by a factor of 10 or more. Any attempt to pole them
between ITO has caused sparking, and ultimately
resulting in damaged films.
Conclusions
THz radiation is a new and exciting field lending
itself to many useful applications. In order to achieve
widespread use there is a need for new emitting and
detecting elements which are better and easier to
produce. One possible answer is in electrically poled
thin film polymers combined with nonlinear optical
techniques. While I have yet to acquire any useful
results personally. Others have shown that polymers
can be just as good of emitters as conventional
crystals (Figure 5b – below).
Figure 5. a) 20 scans taken from my THz set up with a trace through the averaged
points. b) *example of a comparison polymer film THz signals compared to similar
thickness ZnTe.
*Schmuttenmaer, Charles A. “Exploring Dynamics in the Far-Infrared with Terahertz Spectroscopy.” Chem. Rev. Vol. 104, pg 1759. 2004. *J. Shan, A. Nahata, and T. F. Heinz, "Terahertz time-domain spectroscopy based on nonlinear optics," J. Nonlinear Opt. Phys. Mater. Vol. 11, No. 1, pg 31. 2002.