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REVIEW
Copyright © 2007 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoelectronics and Optoelectronics
Vol. 2, 1–19, 2007
Organic Broadband Terahertz Sources and Sensors
Xuemei Zheng
∗
, Colin V. McLaughlin, P. Cunningham, and L. Michael Hayden
Department of Physics, University of Maryland, Baltimore County, MD 21250, USA
We review recent research using organic materials for generation and detection of broadband tera-
hertz radiation (0.3 THz−30 THz). The main focus is on amorphous electrooptic (EO) polymers, with
semiconducting polymers, molecular salt EO crystals, and molecular solutions briefly discussed.
The advantages of amorphous EO polymers over other materials for broadband THz generation
(via optical rectification) and detection (via EO sampling) include a lack of phonon absorption (good
transparency) in the THz regime, high EO coefficient and good phase-matching properties, and, of
course, easy fabrication (low cost). Our ∼12-THz, spectral gap-free THz system based on a poly-
mer emitter-sensor pair is an excellent demonstration of the advantages using of EO polymers. We
also present a model that can predict the performance of a polymer-based THz system. Both the
dielectric properties of an EO polymer and laser pulse related parameters are included in the model,
making the simulations close to real conditions. From our modeling work, the roles the dielectric
properties play in the THz generation and detection are clearly seen, providing us with a good guide
to select and design suitable EO polymers in the future.
Keywords: Electrooptic Polymer, Nonlinear Optics, Terahertz, Far Infrared, Spectroscopy.
CONTENTS
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2. Generation, Detection, and Application of
Broadband THz Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Basic Broadband THz System . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Broadband THz Generation . . . . . . . . . . . . . . . . . . . . . . . . 3
2.3. Broadband THz Detection . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. THz Time-Domain Spectroscopy . . . . . . . . . . . . . . . . . . . . 6
3. Organic Materials for THz Sources and Detectors . . . . . . . . . . . 6
3.1. Conjugated Semiconducting Polymers . . . . . . . . . . . . . . . . 6
3.2. Organic EO Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Organic EO Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Polar Molecules in Solutions . . . . . . . . . . . . . . . . . . . . . . . 9
4. Experimental Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Efficient THz Emitter Based on EO Polymer . . . . . . . . . . . 9
4.2. LAPC Emitter-Sensor Pairs Operated at ∼800 nm . . . . . . 9
4.3. DAPC Emitter and Multi-Layer LAPC
Sensor Operated at ∼1300 nm . . . . . . . . . . . . . . . . . . . . . . 10
4.4. DAST Emitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.5. Dielectric Property Characterization of
EO Polymers Using THz-TDS . . . . . . . . . . . . . . . . . . . . . . 14
5. Modeling a Polymer Emitter-Sensor Pair . . . . . . . . . . . . . . . . . . 15
6. Conclusions and Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . 17
References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1. INTRODUCTION
Terahertz (THz) radiation, with a loose definition between
0.3 THz and 30 THz (1 THz = 10
12
Hz), bridges micro-
wave and infrared (IR) radiation. A variety of excitations,
∗
Author to whom correspondence should be addressed.
such as rotational and vibrational states in molecular sys-
tems, lattice resonance in dielectric crystalline materials,
and confinement states in artificially fabricated nano-struc-
tures, occur in this spectral regime, suggesting the versatile
application of the THz radiation in chemical and biological
detection, medical imaging, and spectroscopy. However,
the lack of compact, bright THz sources and sensitive THz
detectors slows progress in this field. Even after intensive
study for nearly two decades, THz science and technology
is still in its infancy.
While tunable continuous-wave THz radiation, asso-
ciated with photomixing or quantum cascade lasers, is
useful for spectroscopy with very high frequency resolu-
tion, single-frequency imaging and remote sensing, pulsed
(equivalently, broadband) THz radiation associated with
the employment of ultrashort lasers is the optimal choice
when an overall snapshot of the spectral characteristics
of a sample in the THz regime is important.
1
For pulsed
THz systems, a wide bandwidth with a smooth frequency
response using low power laser sources would be quite
valuable in many scientific and technological arenas.
Currently, optoelectronic and all-optical techniques are
commonly employed for generation and detection of
pulsed THz radiation. The optoelectronic technique relies
on the use of photoconductive dipole antennas (PDA)
fabricated as micro-striplines or coplanar transmission
lines on photoconductive inorganic substrates.
2 3
These
PDAs have excellent sensitivity and a smooth frequency
response but a narrow useable bandwidth. The all-optical
J. Nanoelectron. Optoelectron. 2007, Vol. 2, No. 1 1555-130X/2007/2/001/019 doi:10.1166/jno.2007.005 1
REVIEW
Organic Broadband Terahertz Sources and Sensors Zheng et al.
technique uses optical rectification (OR)
4
in electro-optic
(EO) materials to generate the THz radiation and uses EO
sampling
5 6
to detect the THz radiation. This method has
good sensitivity and a large bandwidth, but the conven-
tional systems consisting of crystalline EO materials do
not have a smooth frequency response across that band-
width due in part to phonon absorption associated with the
crystalline nature of the emitters and detectors.
Xuemei Zheng is currently a postdoctoral research associate in the Physics Department
at University of Maryland, Baltimore County. She received her Ph.D., MA, and BE from
University of Rochester, the City College of New York, and Tianjin University (China),
respectively. Her research interests involve terahertz optoelectronics, nonlinear optics in
organic materials, and studies of ultrafast dynamics of photo-induced carriers in condensed
matters.
Colin V. Mclaughlin was born in Portland, Maine, on July 30, 1979. He received his BA,
majoring in physics, from Drew University in 2003. He received his MS from University
of Maryland, Baltimore Co. in 2005. His research interests include EO polymer devices for
THz applications.
P. Cunningham was born in Baltimore, MD April 29, 1982. He received his BS in applied
physics from Towson University in 2004 and his MS from University of Maryland, Balti-
more Co. in 2006. His research interests include optical-pump THz-probe studies of photo-
conductive materials and devices for solar cell and photodetector applications.
L. Michael Hayden is a Professor of Physics and the Chairman of the Department of
Physics at the University of Maryland, Baltimore County. He has a BS from the U. S. Naval
Academy and a Ph.D. from the University of California, Davis. He has had careers in the
US Navy, the private sector, and academia. His current research interests involve ultra-fast
optical studies of organic and polymeric materials, with applications to terahertz science.
He is a private pilot with single and multi-engine ratings.
Recently, some attention has been paid to the use of
organic materials for the THz generation and detection.
For both the optoelectronic and all-optical techniques,
organic materials have shown great potential and broad-
ened the material possibility. Organic EO materials have
made significant contribution to the recent development of
all-optical THz systems. For example, organic crystalline
DAST (4-N ,N -dimethylamino-4
-N
-methyl stilbazolium
2
J. Nanoelectron. Optoelectron. 2, 1–19, 2007
REVIEW
Zheng et al. Organic Broadband Terahertz Sources and Sensors
tosylate)
7–9
and 2-(-methylbeayl-amino)-5-nitropyridine
(MBANP).
10
have been shown to exhibit much higher
EO coefficients than their inorganic counterparts and
efficiently generate and detect the THz radiation. More
encouragingly, amorphous EO polymers exhibit not only
high EO coefficients but also the absence of phonon
absorption (in the THz regime). A THz system based on a
polymer emitter-sensor pair has produced spectral gap-free
bandwidth up to ∼12 THz.
11
The most exciting thing about
EO polymers is the tunability of the properties through
composite constituent modification and film processing.
This should allow these materials to establish excellent
sensitivities, extremely wide bandwidths and flat frequency
responses in the mid- and far-IR THz regimes. With slower
progress, organic PDAs based on PPV
12
and pentacene
13
have also been successfully made for the THz emission.
Compared with a large amount of work done with inor-
ganic crystalline materials, the use of organic materials in
the THz generation and detection is still under-explored.
With the great potential that the organic materials have
shown, there is a need, at this point, to review THz sources
and sensors involved with organic materials. In order to
make this review more readable to researchers who are not
quite familiar with but want to get involved in the THz
science and technology, we will give an introduction in
Section 2 to the common techniques used for THz gen-
eration and detection. We mainly focus on the all-optical
technique involved with EO materials, taking our expe-
rience into consideration. Principles of THz time-domain
spectroscopy (THz-TDS), one of the most important appli-
cations of THz radiation, are also presented in this section.
Section 3 goes over organic materials for THz sources and
sensors. Our focus is on amorphous EO polymers, but we
also briefly discuss organic semiconductors and organic
EO crystals and liquids as a complete review of this field.
In Section 4, we present the experimental results obtained
from our THz systems based on EO polymers operated
at both 800-nm and ∼1300 nm wavelength. Advantages
of using EO polymers as THz emitters and sensors are
clearly shown in this section. Experimental results using
DAST as the THz emitter are also given in this section.
Our modeling work on a polymer emitter-sensor pair is
presented in Section 5. In Section 6 we conclude and point
out challenges and future work.
2. GENERATION, DETECTION, AND
APPLICATION OF BROADBAND THz
RADIATION
2.1. Basic Broadband THz System
Broadband THz generation is intimately tied to femtosec-
ond laser sources. In fact, THz technology boomed shortly
after solid-state femtosecond lasers became widely avail-
able a little more than two decades ago that could be oper-
ated by a relative novice. Yet, compared to other spectral
ranges of electromagnetic radiation, it is still quite a recent
technology and not fully established.
A schematic of the optical arrangement of a THz system
is shown in Figure 1. An output laser beam from a fem-
tosecond laser is split into two beams, with most power
going to the pump beam to drive a THz emitter and very
little power going to the probe beam that interacts with the
THz wave under investigation in a THz detector. By vary-
ing the optical delay line in one arm, the probe pulse (with
duration much shorter than the THz pulse) sees different
parts of the THz waveform. With the delay line position
and data acquisition controlled by a computer, we can map
out the electric field (instead of power) of the THz wave.
This gated sampling technique allows for jitter-free phase
coherent detection, leading to a high signal-to-noise ratio
(SNR) and dynamic range.
2.2. Broadband THz Generation
In general, broadband THz radiation can be generated
either in an optoeletronic manner involving photogener-
ated transient currents in photoconductive antennas.
14
or
in an optical manner involving optical rectification in EO
materials.
15
The two techniques have always been under
parallel developments and boasted of different advantages.
The technique of using photoconductive antennas to
generate electromagnetic radiation can be traced back to as
early as the middle of the 1970’s when Auston generated
picosecond microwave pulses on a transmission line by
exciting the photoconductor gap bridging the electrodes of
the transmission line with picosecond pulses
2
(see Fig. 2).
The mechanism is simple: the optical pulse with the pho-
ton energy higher than the bandgap of the photoconductor
excites electrons from the valance band to the conduction
band; because of the electric field provided by the bias
across the electrodes, the injection of these photocarriers
closes the switch with the current through the switch ris-
ing rapidly (determined by the laser pulse duration) and
decaying with a time constant determined by the carrier
lifetime of the photoconductor; according to Maxwell’s
equations, Et ∝ J t/t, so the transient photocurrent
Jt radiates into the free space.
16
The big step from microwave radiation to THz radiation
was made possible by the availability of single-picosecond
Fig. 1. General optical arrangement of a THz system. Four 90
off-
axis parabolic mirrors are used to collect, collimate and focus the THz
radiation. This arrangement is suitable for spectroscopy study, for which
a sample under study can be placed at the THz focal point.
J. Nanoelectron. Optoelectron. 2, 1–19, 2007 3
REVIEW
Organic Broadband Terahertz Sources and Sensors Zheng et al.
Fig. 2. A photoconductive switch integrated in a microstrip transmis-
sion line. When a laser pulse with photon energy higher than the photo-
conductor’s bandgap energy illuminates the gap of the transmission line
biased by an external field, photogenerated carriers close the switch and
the transient current consequently radiates into free space.
and subpicoscecond photoconductors (LT-GaAs, ion-
implanted GaAs, etc.), micro-lithography (allowing fabri-
cation of smaller radiating devices and consequently
higher frequency electromagnetic radiation), and ultrashort
lasers providing shorter and shorter pulses (down to
∼12 fs, commercially available). The achievable band-
width from most PDA-based THz systems is usually a
few THz and is often attributed to the limit of the car-
rier lifetime of the photoconductor involved. In a few
cases, however, ultrabroad bandwidths (>10 THz) have
been reported. Kono et al.
17
reported a THz detection up
to 20 THz with a low-temperature-grown GaAs (LT-GaAs)
PDA gated with 15 fs light pulse. It was quite a surprising
result as the LT-GaAs they used exhibited a relatively long
carrier lifetime of ∼1.4 ps. According to the authors, the
fast response of the PDA was explained by the fast rise
in the photocurrent upon excitation by the ultrashort laser
pulse,
17
and the physical origin of the fast photocurrent
within ∼100 fs might be explained by the ballistic trans-
port of the photoexcited electrons in the biased electric
field.
18
In this picture, the PDA works as an integration
detector, so the photocurrent from the antenna should be
proportional to the time integration of the incident THz
radiation. With a post-measurement analysis where both
the number of photocarriers (a function of time) and the
integration mode of the PDA detector were taken into con-
sideration, the same authors obtained even broader detec-
tion bandwidth, up to ∼40 THz.
19
On the other hand,
ultrabroadband THz generation from LT-GaAs PDAs was
demonstrated by Shen et al.
20
Using a backward collec-
tion scheme to minimize the THz absorption by the LT-
GaAs substrate, THz radiation with frequency components
over 30 THz was achieved, and the transverse optical (TO)
phonon absorption band of GaAs was clearly identified.
The ultrabroad bandwidth might be due to the specific
scheme where the pump beam was illuminated on the edge
of one of the PDA electrodes.
21–23
For the edge illumina-
tion scheme, the transient current in the PDA results from
the dielectric relaxation of the space-charge field such that
its dynamics is not determined by the carrier lifetime.
24
So far, the broadest bandwidth from a PDA emitter-sensor
pair is ∼15 THz,
25
although a very distinguished spectral
gap related to the TO phonon band of the substrate was
exhibited. In addition to the operation complexity, another
disadvantage of this technique is the necessity of the com-
plex and expensive lithography facility.
Compared with the technique involved with PDAs, the
advantage of using nonlinear optical rectification to gener-
ate THz radiation is a possible broader bandwidth, as well
as the availability of a variety of EO materials. Pioneering
work done by Shen et al.
4
demonstrated the possibility of
using picosecond laser pulses in EO materials to gener-
ate far infrared radiation via optical rectification. Auston
et al. extended this technique by using shorter laser pulses
and observed a generated electromagnetic wave in the THz
regime.
26
Since then, many researchers have followed and
further developed this technique by exploiting numerous
materials and geometries.
7 15 27–29
Optical rectification can be understood as mixing of two
different frequency components in the frequency spectrum
of an incident ultrashort optical pulse in an EO medium.
The difference frequency mixing results in a nonlinear
polarization and consequently a radiation at the beat fre-
quency. The bandwidth of the radiation in OR is limited
by the bandwidth of the optical pulse, as well as the rele-
vant properties of the nonlinear medium. Mathematically,
the difference frequency mixing process via optical recti-
fication is describe as follows:
d
2
dz
2
+
2
c
2
E
THz
z =
4
c
2
2
P
NL
z (1)
where c is the speed of light, is the THz frequency,
is the dielectric constant of the nonlinear medium in
the THz region [ = n
2
THz
, if there does not exist
THz absorption in the NLO medium],
E
THz
z is the
propagating THz field generated in the nonlinear medium,
and P
NL
z is the nonlinear polarization propagating
along the z-axis expressed by (assuming the optical wave-
length is far away from the material’s electronic resonance
region):
P
NL
z =
eff
−
Ez · Ez − d
=
eff
· I z (2)
where is the optical frequency. It should be noted that
Iz is the autocorrelation of the optical electric field,
or, actually, the Fourier transform of the intensity profile
of the optical pulse—Izt. Dispersion and absorption of
the nonlinear medium in both the THz and optical regime
make analytically solving Eq. (1) very difficult, if not
impossible. However, analytical solutions based on certain
assumptions simplifying the problem can be obtained.
30
It is found, from these solutions, that the phase-mismatch
(the difference between the optical group index n
g
= n
opt
−
opt
dn
opt
opt
, where n
opt
and
opt
are the optical index and opti-
cal wavelength, respectively, and the THz index n
THz
in
the material) limits the amplitude and bandwidth of the
4
J. Nanoelectron. Optoelectron. 2, 1–19, 2007
