1. ~ 1 ~
Investigation of an effective low-cost THz TDS system
Conor Wilman
School of Physics and Astronomy, University of Manchester, Rank Prize Report, September 2016.
An experimental set-up was designed and constructed for the generation of a THz field for use in TDS. This involved
utilising optical laser diodes incident on Au antennas mounted on LT-GaAs. Additional experiments were carried out to
investigate the properties of the LT-GaAs sample and the mounted antennas.
1. INTRODUCTION
The THz frequency range has been found to be
one of great potential utility with applications in
medicine, security, industrial quality assurance, and
materials imaging and in the time domain spectroscopy
(TDS) technique.[1]
It is the latter that will be the focus of
this report.
The main obstacle to established high-quality
modern THz apparatus is their use of femtosecond lasers
for generation of the THz field. These lasers tend to be
expensive and thus act as a barrier to companies and
institutions that wish to use THz in some manner but
cannot afford the cost. A number[2][3][4]
of groups have
published papers on systems that avoid this problem by
using commodity multimode laser diodes (MMLDs) with
frequency mode spacing in the THz region to excite
carriers in photoconductive antennas (PCAs) printed
lithographically onto a semiconductor substrate, which
have been shown to work to a varying degree of success.
The method of THz generation used in this experiment
was adapted from these papers with a focus on a quality
good enough for rudimentary undergraduate research
while still being cost-effective.
2. EXPERIMENT
The main experiment was set up as according to
Figure 1.
The bow-tie antennas consist of a lithographically
defined Au 3010 µm dipole and a 10 µm gap, a design
based on previously used antennas. The antenna
substrates are low temperature (LT)-gallium arsenide
(GaAs) which has a fast carrier recombination time,
making the material suitable for continuous wave THz
operation. The antennas were mounted onto a custom
designed copper coated printed circuit board (PCB) so
that they may fit to a standard 1” lens mount, allowing for
simpler alignment of the laser and THz beams. Each
antenna was then tested with a range of bias voltages in
the light of the room and in complete darkness to check
that they operated correctly and that there was an Ohmic
relation between the voltage and current.
After set-up and alignment, the emitter antenna
was initially biased with 10 V at 92.8 kHz and a lock-in
amplifier was connected to the detector antenna to
enhance the signal-to-noise ratio. Teflon plano-convex
lenses with a focal length, f, of 100 mm were used to
collimate the THz field.
Two MMLDs were tested during the experiment.
The first operated with a central wavelength of 658 nm at
a power output of 40 mW. The second operated with a
central wavelength of 808 nm at 200 mW. Use of an
aspheric lens allowed for beam collimation in each case.
An existing research-grade THz-TDS system was
utilised to analyse the LT-GaAs sample before printing
the antennas as its specific properties were not known.
The system comprised of a Ti:Sapphire amplifier with an
output of ~700 mW at 800 nm and a laser repetition rate
of 1 kHz, which produced a THz field strength in air of
0.6 kV/cm and a bandwidth of 3 THz with a maximum at
1 THz. Scans of the sample were made at low and at
maximum power for comparison. An additional scan
without the sample was made to act as a reference. Data
was extracted using a double lock-in amplifier technique
and a custom program. The mathematics of the data
extraction is well documented elsewhere.[3][5]
3. RESULTS and DISCUSSION
MMLD
THz generation
Figure 1: Schematic of the apparatus. The solid angled lines indicate
mirrors, the dashed angled line indicates a 50:50 beam splitter, the
arrows indicate the laser beam directions, and the "THz generation"
section contains the emitter and detector and several lenses for
focussing the laser and THz beams. The dashed box is the delay stage
where the two angled mirrors represent a retroreflector on a motor
driven translation stage.
Figure 2: Top: Plot of the full carrier decay curve due to brief
illumination of the GaAs sample at low beam power.
Bottom: Fitting of the exponential curve is shown by the dashed
line, the gradient of which provides the carrier lifetime.

2. ~ 2 ~
The results of the TDS scans of the LT-GaAs
sample showed that at maximum power the carrier
lifetime, τ, was 3.75 ps while the low power scan yielded
a τ of 3.54 ps. The scan showing the decay of excited
carriers is Figure 2 and the TDS scan in the Fourier
domain at low power is shown in Figure 3. The
difference in carrier lifetimes was likely due to the
increased number of carriers produced at the higher power
beam filling the material’s defects, thus increasing τ. The
mobility, µ, at low energy was calculated to be 3700
cm2
V-1
s-1
and thus, the carrier density, 𝑁, was calculated
to be 2.50e+17 cm-3
. These values are within ranges
found in previous literature.[6]
Unfortunately, due to variations during antenna
production, the two antennas were not identical. The
resistance of one antenna was found to be 1.30GΩ
(2.65GΩ) in the light (dark) respectively. The other
antenna gave resistances of 0.23GΩ (2.52GΩ) in the light
(dark). Under laser illumination, these values became
31.4 MΩ and 39.2 MΩ for each device respectively. The
similarity of the resistances of both antennas in darkness
confirms that the resistance changes under light
conditions are due to the antennas themselves and not
material defects.
Unfortunately, even after careful realignment and
calibration of the main experimental set-up, a signal
similar to that of Figure 2 was not produced as expected.
This occurred for both laser diodes. Here, we can only
speculate as to the reasons why this might have been the
case as attempts to rectify the issue involved multiple
realignments and changing the detection system from one
lock-in to two to ensure signal-to-noise was maximised as
much as possible.
Turning to the paper by Morikawa et al[4]
, the
differences between set-ups were only in minor variables,
such as the antenna shape and emitter bias amplitude, that
should have no major impact on the ability to detect a
signal with the system detailed here.
The geometry of the 808 nm laser beam differed
from the 658 nm beam in that it had a square profile.
Collimating this beam properly using an aspheric lens was
not possible and focussing the beam onto the antennas
would not have been accurate.
Given that this technique has been shown to work
by other groups, the theoretical background of the
experiment holds up. Thus, failure is entirely contained
within the parameters and equipment used. The likely
factors attributing to this are equipment failure or a
combination of using a low bias voltage, low laser power,
poor quality antennas, and lack of hyper-hemispherical
lenses all contributed to a signal-to-noise below the
measurement level of the lock-in.
This experiment can then act as a basis for
improvement if the project were to be attempted again in
future. A high-voltage source and a laser diode with
known characteristics that can be collimated easily would
be the first points to address.
The design of the antenna itself is a major factor
contributing to the THz field strength and detector
sensitivity. M. Tani et al carried out a review of the
commonly used PCAs grown on LT-GaAs: the Hertzian
dipole; the strip line antenna; and the bow-tie antenna.
The latter was chosen in this experiment as it was shown
to be superior to the others in terms of signal amplitude
and emitted radiation amplitude at low bias voltages and
pump power.
There has been recent research into plasmonic
and interdigitated antennas, with the plasmonic antenna
potentially producing fields with power 50 times greater
than standard antennas. The techniques used to produce
these antennas, however, require electron beam
lithography, the facilities for which were not available for
use.
4. CONCLUSIONS
This experiment did not succeed in completing its
main objective of constructing a working THz TDS
system with optical laser diodes despite careful design and
testing of the emitter and detector.
A number of ideas for resolving the issues
presented in this set-up and potential improvements on it
have also been outlined.
5. REFERENCES
[1] – Tonouchi M., Nat. Photonics 1, 97-105 (2007)
[2] – Probst T., Rehn A., Koch M., Opt. Soc. Am. 23,
21972-21982 (2015)
[3] – Scheller M., Koch M., Opt. Soc. Am. 19, 5290-5296
(2011)
[4] – Morikawa O, Fujita M, Takano K, Hangyo M., J.
Appl. Phys. 110 (2011)
[5] – Parkinson P., Univ. of Oxford, Doctoral Thesis
(2008)
[6] – NSM Database, last accessed on 13.09.16,
http://www.ioffe.ru/SVA/NSM/Semicond/GaAs/index.html
Figure 3: The Fourier domain waveform obtained from the research-
grade system during a low power scan of the LT-GaAs sample. The
power of the optical pump was 5.5mW. The time domain data was
zero-padded for regions either side of the main pulse to reduce
interference.