REVIEW ARTICLE
Materials for terahertz science and
technology
Terahertz spectroscopy systems use far-infrared radiation to extract molecular spectral information in an
otherwise inaccessible portion of the electromagnetic spectrum. Materials research is an essential
component of modern terahertz systems: novel, higher-power terahertz sources rely heavily on new
materials such as quantum cascade structures. At the same time, terahertz spectroscopy and imaging
provide a powerful tool for the characterization of a broad range of materials, including semiconductors
and biomolecules.
BRADLEY FERGUSON1,2 AND
XI-CHENG ZHANG*1
Electronics
THz
Microwaves
Photonics
Visible
X-ray
γ-ray
MF, HF, VHF, UHF, SHF, EHF
1
Center for Terahertz Research,Rensselaer Polytechnic Institute,
110 8th Street Troy,New York,12180-3590,USA
2
Centre for Biomedical Engineering and Department of Electrical
and Electronic Engineering,The University of Adelaide,South
Australia 5005,Australia
*e-mail: zhangxc@rpi.edu
100
103
106
kilo
mega
109
giga
Example
Radio
industries: communications
Radar
1012
1015
tera
???
peta
1018
exa
1021
zetta
1024
Hz
yotta
Optical
Medical Astrophysics
communications imaging
Frequency (Hz)
Recent years have seen a plethora of significant advances
in materials diagnostics by terahertz systems,as higherpower sources and more sensitive detectors open up a
range of potential uses.Applications including
semiconductor and high-temperature superconductor
characterization,tomographic imaging,label-free
genetic analysis,cellular level imaging and chemical and
biological sensing have thrust terahertz research from
relative obscurity into the limelight.
The terahertz (THz) region of the electromagnetic
spectrum has proven to be one of the most elusive.
Terahertz radiation is loosely defined by the frequency
range of 0.1 to 10 THz (1012 cycles per second).
Being situated between infrared light and microwave
radiation (see Fig. 1),THz radiation is resistant to the
techniques commonly employed in these wellestablished neighbouring bands.High atmospheric
absorption constrained early interest and funding for
THz science.Historically,the major use of THz
spectroscopy has been by chemists and astronomers in
the spectral characterization of the rotational and
vibrational resonances and thermal-emission lines of
simple molecules.The past 20 years have seen a
revolution in THz systems,as advanced materials
research provided new and higher-power sources,and
the potential of THz for advanced physics research and
commercial applications was demonstrated.Terahertz
technology is an extremely attractive research field,with
interest from sectors as diverse as the semiconductor,
medical,manufacturing,space and defence industries.
Several recent major technical advances have greatly
extended the potential and profile of THz systems.
26
These advances include the development of a quantum
cascade THz laser1,the demonstration of THz detection
of single base-pair differences in femtomolar
concentrations of DNA2 and the investigation of the
evolution of multiparticle charge interactions with THz
spectroscopy3.This article provides an overview of these
and many other important recent developments.
THZ SPECTROSCOPY SYSTEMS
Terahertz spectroscopy allows a material’s far-infrared
optical properties to be determined as a function of
frequency. This information can yield insight into
material characteristics for a wide range of applications.
Many different methods exist for performing THz
spectroscopy. Fourier transform spectroscopy (FTS) is
perhaps the most common technique used for studying
molecular resonances. It has the advantage of an
extremely wide bandwidth, enabling material
characterization from THz frequencies to well into the
infrared. In FTS the sample is illuminated with a
broadband thermal source such as an arc lamp or a SiC
globar. The sample is placed in an optical interferometer
system and the path length of one of the interferometer
arms is scanned.A direct detector such as a heliumcooled bolometer is used to detect the interference
signal. The Fourier transform of the signal then yields
the power spectral density of the sample.
One disadvantage of FTS is its limited spectral
Figure 1 The electromagnetic
spectrum.The development of
efficient emitters and detectors
within each of the spectral
regimes has resulted in the birth
of numerous industries.
The search for potential
applications of THz radiation is
steadily intensifying as materials
research provides improved
sources and detectors.
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© 2002 Nature Publishing Group
REVIEW ARTICLE
Figure 2 Illustration of a THzTDS pump probe system.
The ultrafast laser beam is split
into pump and probe beams.
The pump beam is incident on
the THz emitter to generate THz
pulses, and the THz pulses are
collimated and focused on the
target using parabolic mirrors.
After transmission through the
target, the THz pulse is
collimated and re-focused on
the THz detector.The optical
probe beam is used to gate the
detector and measure the
instantaneous THz electric field.
A delay stage is used to offset
the pump and probe beams and
allow the THz temporal profile to
be iteratively sampled.
Femtosecond
pulses
Delay stage
Beam
Probe beam
Pump beam
Parabolic
THz
detector
THz
emitter
Sample
resolution. Spectral measurements having a much
higher resolution may be made using a narrowband
system with a tunable THz source or detector. In these
systems, the source or detector is tuned across the
desired bandwidth and the sample’s spectral response is
measured directly. Both FTS and narrowband
spectroscopy are also widely used in passive systems for
monitoring thermal-emission lines of molecules,
particularly in astronomy applications.
A third,more recent technique is termed THz timedomain spectroscopy (THz-TDS).THz-TDS uses short
pulses of broadband THz radiation,which are typically
generated using ultrafast laser pulses.This technique
grew from work in the 1980s at AT&T Bell Labs and the
IBM T.J.Watson Research Center4,5. Although the
spectral resolution of THz-TDS is much coarser than
narrowband techniques,and its spectral range
significantly less than that of FTS,it has a number of
advantages that have given rise to some important recent
applications.The transmitted THz electric field is
measured coherently,which provides both high
sensitivity and time-resolved phase information.It is
also amenable to implementation within an imaging
system to yield rich spectroscopic images.A THz-TDS
system is described in Fig. 2.Typical THz-TDS systems
have a frequency bandwidth between 2 and 5 THz,a
spectral resolution of 50 GHz,an acquisition time under
one minute and a dynamic range of 1 × 105 in electric
field.Signal-processing algorithms may be used to
improve the signal-to-noise ratio of the measured
signals by almost 30% (ref.6).
THZ SOURCES
The lack of a high-power,low-cost,portable roomtemperature THz source is the most significant limitation
of modern THz systems.However,there is a vast array of
potential sources each with relative advantages,and
advances in high-speed electronics,laser and materials
research continue to provide new candidates.Sources
may be broadly classified as either incoherent thermal
sources,broadband pulsed (‘T-ray’) techniques or
narrowband continuous-wave methods.
BROADBAND THZ SOURCES
Most broadband pulsed THz sources are based on the
excitation of different materials with ultrashort laser
pulses.A number of different mechanisms have been
exploited to generate THz radiation,including
photocarrier acceleration in photoconducting antennas,
second-order non-linear effects in electro-optic crystals,
plasma oscillations7 and electronic non-linear
transmission lines8.Currently,conversion efficiencies in
all of these sources are very low,and consequently,
average THz beam powers tend to be in the nano- to
microwatt range,whereas the average power of the
femtosecond optical source is in the region of 1 W.
Photoconduction and optical rectification are two
of the most common approaches for generating
broadband pulsed THz beams. The photoconductive
approach uses high-speed photoconductors as
transient current sources for radiating antennas9.
Typical photoconductors include high-resistivity
GaAs, InP and radiation-damaged silicon wafers.
Metallic electrodes are used to bias the
photoconductive gap and form an antenna.
The physical mechanism for THz beam generation
in photoconductive antennas begins with an ultrafast
laser pulse (with a photon energy larger than the
bandgap of the material hν ≥ Eg), which creates
electron–hole pairs in the photoconductor. The free
carriers then accelerate in the static bias field to form a
transient photocurrent, and the fast, time-varying
current radiates electromagnetic waves. Several
material parameters affect the intensity and the
bandwidth of the resultant THz radiation.
For efficient THz radiation, it is desirable to have rapid
photocurrent rise and decay times.Thus semiconductors
with small effective electron masses such as InAs and
InP are attractive. The maximum drift velocity is also
an important material parameter, but it is generally
limited by the intraband scattering rate or by
intervalley scattering in direct semiconductors such as
GaAs10–12. Because the radiating energy mainly comes
from stored surface energy in the form of the static bias
field, the THz radiation energy scales up with the bias
and optical fluency13. The breakdown field of the
material is another important parameter because this
determines the maximum bias that may be applied.
Photoconductive emitters are capable of relatively large
average THz powers in excess of 40 µW (ref. 14) and
bandwidths as high as 4 THz (ref. 15).
Optical rectification is an alternative mechanism for
pulsed THz generation,and is based on the inverse
process of the electro-optic effect16.Again,femtosecond
laser pulses are required,but in contrast to
photoconducting elements where the optical beam
functions as a trigger,the energy of the THz radiation in
optical rectification comes directly from the exciting
laser pulse.The conversion efficiency in optical
rectification depends primarily on the material’s nonlinear coefficient and the phase-matching conditions.
This technique was first demonstrated for
generating far-infrared radiation using LiNbO3
(ref. 17). Much research has focused on optimizing THz
generation through investigating the electro-optic
properties of different materials, including traditional
semiconductors such as GaAs and ZnTe, and organic
crystals such as the ionic salt 4-dimethylamino-Nmethylstilbazolium tosylate (DAST) among many
others18–20. Because optical rectification relies on
coupling of the incident optical power to THz
frequencies at relatively low efficiency, it usually
provides lower output powers than photoconductive
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27
© 2002 Nature Publishing Group
antennas, but it has the advantage of providing very
high bandwidths, up to 50 THz (ref. 21). Phasematched optical rectification in GaSe allows
ultrabroadband THz pulses to be generated with a
tunable centre wavelength. Tuning up to a frequency of
41 THz is accomplished by tilting the crystal about the
horizontal axis perpendicular to the pump beam to
modify the phase-matching conditions22,23.
Energy
REVIEW ARTICLE
Injection barrier
Distance
2
1
NARROWBAND THZ SOURCES
Narrowband THz sources are crucial for high-resolution
spectroscopy applications.They also have broad
potential applications in telecommunications,and are
particularly attractive for extremely high bandwidth
intersatellite links.For these reasons there has been
significant research interest in the development of
narrowband sources over the past century24.A multitude
of techniques are under development,including
upconversion of electronic radio-frequency sources,
downconversion of optical sources,lasers and backwardwave tubes.Several comprehensive reviews of this field
are available25.
The technique most used for generating low-power
(<100 µW) continuous-wave THz radiation is through
upconversion of lower-frequency microwave oscillators,
such as voltage-controlled oscillators and dielectricresonator oscillators.Upconversion is typically achieved
using a chain of planar GaAs Schottky-diode multipliers.
Using these methods,frequencies as high as 2.7 THz
have been demonstrated26.Research also continues to
increase the frequency of Gunn and IMPATT diodes to
the lower reaches of the terahertz region using alternate
semiconducting structures and improved fabrication
techniques27.Gas lasers are another common THz
source.In these sources,a carbon dioxide laser pumps a
low-pressure gas cavity,which lases at the gas molecule’s
emission-line frequencies.These sources are not
continuously tunable,and typically require large cavities
and kilowatt power supplies,however they can provide
high output powers up to 30 mW.Methanol and
hydrogen cyanide lasers are the most popular,and
they are in common use for spectroscopy and
heterodyne receivers.
Extremely high-power THz emissions have recently
been demonstrated using free-electron lasers with
energy-recovering linear accelerators28.Free-electron
a
Active region
Injector
104.9 mm
lasers use a beam of high-velocity bunches of electrons
propagating in a vacuum through a strong,spatially
varying magnetic field.The magnetic field causes the
electron bunches to oscillate and emit photons.
Mirrors are used to confine the photons to the electron
beam line,which forms the gain medium for the laser.
Such systems impose prohibitive cost and size
constraints and typically require a dedicated facility.
However,they may generate continuous or pulsed waves,
and provide an average brightness of more than six
orders of magnitude higher than typical
photoconductive antenna emitters.Free-electron lasers
have significant potential in applications where
improved signal-to-noise ratio is essential,or in the
investigation of non-linear THz spectroscopy.Bench-top
variations on the same theme,termed backward-wave
tubes or carcinotrons,are also capable of providing
milliwatt output powers at THz frequencies,and are
commercially available.
A number of optical techniques have also been
pursued for generating narrowband THz radiation.
Original efforts began in the 1970s using non-linear
photomixing of two laser sources, but struggled with
low conversion efficiencies29. In this technique, two
continuous-wave lasers with slightly differing centre
frequencies are combined in a material exhibiting a
high second-order optical non-linearity such as DAST.
The two laser frequencies mutually interfere in the
Figure 4 A broadband THz pulse
with a frequency spectrum
extending into the infrared.a,
Temporal waveform of a THz
pulse generated using optical
rectification in a 27-µm ZnTe
emitter,and measured using
freespace electro-optic sampling
in a 30-µm ZnTe sensor.b,
Frequency spectrum of the THz
field.The absorption resonance
at 5.3 THz is due to phonon
modes in the ZnTe crystals.
b
10
2
Transmission
Electro-optic signal (nA)
4
0
1
-2
0.1
-4
0
1
2
3
0
10
20
30
THz
Time (ps)
28
Figure 3 Simplified conduction
band structure of the THz
quantum cascade laser
demonstrated by Kohler et al.
(after ref.1).Electrons are
injected through the 4.3-nm
AlGaAs injection barrier into the
level 2 energy state of the active
region.The active transition from
level 2 to level 1 results in the
emission of 4.4-THz photons.
The electrons then escape to the
subsequent injector band.A total
of 104 repeats of the basic 7 well
structure was used in the laser.
Each quantum well consists of a
thin layer (10–20 nm) of GaAs
between two potential barriers of
AlGaAs (0.6–4.3 nm thick).
The layer thicknesses and the
applied electric field were
tuned to provide the required
tunnelling characteristics.
40
50
60
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© 2002 Nature Publishing Group
REVIEW ARTICLE
a
b
Refractive
index
40
c
1.5
30
mm
Figure 5 T-ray computed
tomography image of two plastic
cylinders (after ref. 57).
a,An optical image of the test
structure. b,The target was
imaged using the tomography
system, and the refractive index
of each cross-sectional slice
was reconstructed.The central
slice is shown.The greyscale
intensity indicates the refractive
index of the two different types
of plastic. c,The cross-sectional
slices are combined to form a 3D
image.A surface-rendered
image is shown.
1.4
1.3
20
1.2
10
1.1
1
0
0
10
20
mm
material, resulting in output oscillations at the sum and
difference of the laser frequencies. Such systems can be
designed such that the difference term is in the THz
range. Tunable continuous-wave THz radiation has
been demonstrated by mixing two frequency-offset
lasers in GaAs grown at low temperature30 and by
mixing two frequency modes from a single multimode
laser. Further techniques use optical parametric
generators and oscillators where a Q-switch Nd:YAG
(neodymium:yttrium-aluminium-garnet) laser pump
beam generates a second idler beam in a non-linear
crystal, and the pump and idler signal beat to emit THz
radiation31,32. Optical techniques provide broadly
tunable THz radiation and are relatively compact
owing to the availability of solid-state laser sources.
Output powers in excess of 100 mW (pulsed) have
been demonstrated33. Optical downconversion is a rich
area for materials research because molecular beam
epitaxy and other materials advances allows the
generation of engineered materials with improved
photomixing properties34.
Semiconductor lasers are a further technique with
extreme promise for narrowband THz generation.
The first such laser was demonstrated over 20 years ago
in lightly doped p-type germanium as a result of hole
population inversion induced by crossed electric and
magnetic fields35.These lasers are tunable by adjusting
the magnetic field or external stress.Terahertz lasing in
germanium has also been demonstrated by applying a
strong uniaxial stress to the crystal to induce the hole
population inversion36.Such lasers have many inherent
limitations including low efficiency,low output power
and the need for cryogenic cooling to maintain lasing
conditions.Recently,semiconductor deposition
techniques have advanced to a level where the
construction of multiple quantum-well semiconductor
structures for laser emission is feasible.Quantum
cascade lasers were first demonstrated in 1994 based on a
series of coupled quantum wells constructed using
molecular beam epitaxy37. A quantum cascade laser
consists of coupled quantum wells (nanometre-thick
layers of GaAs sandwiched between potential barriers of
AlGaAs).The quantum cascade consists of a repeating
structure in which each repeat unit is made up of an
injector and an active region.In the active region a
population inversion exists and electron transition to a
lower energy level occurs,emitting photons at a specific
wavelength.The electrons then tunnel between
30
40
quantum wells and the injector region couples them to
the higher energy level in the active region of the
subsequent repeat unit.
Quantum cascade lasers have been demonstrated
within the infrared spectrum, but until very recently
several significant problems had prevented THz
quantum cascade lasers from being realized.
The principal problems are caused by the long
wavelength of THz radiation. This results in a large
optical mode, which results in poor coupling between
the small gain medium and the optical field, and in high
optical losses owing to free electrons in the material
(these losses scale as the square of the wavelength).
Kohler et al.1 addressed these and other problems in
their recent innovative design of a THz quantum
cascade laser operating at 4.4 THz. The laser consisted
of 104 repetitions of the basic unit (shown in Fig. 3) and
a total of over 700 quantum wells. This system
demonstrated pulsed operation at a temperature of
10 K; however, optimized fabrication promises to lead
to continuous-wave operation at liquid nitrogen
temperatures (of the order of 70 K).
THZ DETECTORS
The detection of THz-frequency signals is another area
of important active research. The low output power of
THz sources coupled with the relatively high levels of
thermal background radiation in this spectral range has
necessitated highly sensitive detection methods.
For broadband detection, direct detectors based on
thermal absorption are commonly used. Most of these
require cooling to reduce thermal background. The
most common systems are helium-cooled silicon,
germanium and InSb bolometers. Pyroelectric infrared
detectors may also be used at THz frequencies.
Superconductor research has yielded extremely
sensitive bolometers based on the change of state of a
superconductor such as niobium. Interferometric
techniques may be used to extract spectral information
using direct detectors.A single photon detector for THz
photons has recently been demonstrated38.
This detector uses a single-electron transistor consisting
of a quantum dot in a high magnetic field, to offer
unparalleled sensitivity.Although detection speeds are
currently limited to 1 ms, high-speed designs are
proposed and this has the potential to revolutionize the
field of THz detection.
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© 2002 Nature Publishing Group
REVIEW ARTICLE
In applications requiring very high spectral
resolution of the sensor,heterodyne sensors are preferred.
In these systems,a local oscillator source at the THz
frequency of interest is mixed with the received signal.
The downshifted signal is then amplified and measured.
At room temperature semiconductor structures may be
used.A planar Schottky-diode mixer has been operated
successfully at 2.5 THz for sensing applications in space39.
Cryogenic cooling is used for higher sensitivity
in heterodyne superconductor detectors.Several
superconductor structures have been used for over
20 years.The most widely used is the superconductorinsulator-superconductor tunnel junction mixer40.
High-temperature superconductors such as YBCO
(yttrium–barium–copper oxide) are touted for their
potential for higher bandwidth operation.A number of
general reviews of narrowband THz receivers are
available41.Alternative narrowband detectors,such as
electronic resonant detectors,based on the fundamental
frequency of plasma waves in field-effect transistors have
been demonstrated up to 600 GHz42.
For pulsed THz detection in THz-TDS systems,
coherent detectors are required.The two most common
methods are based on photoconductive sampling and
free-space electro-optic sampling,both of which again
rely on ultrafast laser sources.Fundamentally,the
electro-optic effect is a coupling between a lowfrequency electric field (THz pulse) and a laser beam
(optical pulse) in the sensor crystal.Simple tensor
analysis indicates that using a <110>-oriented
zincblende crystal as a sensor provides the highest
sensitivity.The THz electric field modulates the
birefringence of the sensor crystal; this in turn modulates
the polarization ellipticity of the optical probe beam
passing through the crystal.The ellipticity modulation
of the optical beam can then be analysed to provide
information on both the amplitude and phase of the
applied electric field43,44.
The use of an extremely short laser pulse (<15 fs)
and a thin sensor crystal (<30 µm) allow electro-optic
detection of signals into the mid-infrared range.
Figure 4a shows a typical mid-infrared pulsed THz
waveform. The Fourier transform of the THz pulse is
shown in Fig. 4b; the highest frequency response
reaches over 30 THz. The resonant absorption at
5.3 THz is caused by the phonon modes of the ZnTe
crystals.When thin sensors are used, extremely high
detection bandwidths, in excess of 100 THz, have
been demonstrated45.
Photoconductive antennas are widely used for
pulsed THz detection.An identical structure to the
photoconductive antenna emitter may be used.
Rather than applying a bias voltage to the electrodes of
the antenna,a current amplifier and meter are used to
measure a transient current.Ultrahigh bandwidth
detection has been demonstrated using
photoconductive antenna detectors with detectable
frequencies in excess of 60 THz (ref.46).
THZ APPLICATIONS
One of the primary motivations for the development of
THz sources and spectroscopy systems is the potential to
extract material characteristics that are unavailable when
30
Figure 6 THz image of an onion
cell membrane (after ref.60).
A THz imaging system based on
optical rectification and freespace electro-optic sampling in
ZnTe crystals 30 µm thick was
used with a bandwidth up to 40
THz.This allowed a spatial
resolution of less than 50 µm to
be achieved.The cellular
structure of the tissue membrane
is clearly visible.
1.6 mm
using other frequency bands.Astronomy and space
research has been one of the strongest drivers for THz
research because of the vast amount of information
available concerning the presence of abundant
molecules such as oxygen,water and carbon monoxide
in stellar dust clouds,comets and planets47.In recent
years,THz spectroscopy systems have been applied to a
huge variety of materials both to aid the basic
understanding of the material properties,and to
demonstrate potential applications in sensing and
diagnostics.We review several of the more recently
demonstrated applications.
MATERIAL CHARACTERIZATION
One of the major applications of THz spectroscopy
systems is in material characterization,particularly of
lightweight molecules and semiconductors.Terahertz
spectroscopy has been used to determine the carrier
concentration and mobility of doped semiconductors
such as GaAs and silicon wafers48–50.The Drude model
may then be used to link the frequency-dependent
dielectric response to the material free-carrier dynamic
properties,including the plasma angular frequency and
the damping rate.An important focus is on the
measurement of the dielectric constant of thin films51.
High-temperature superconductor
characterization is another important application of
THz spectroscopy. Several superconducting thin films
have been analysed to determine material parameters
including the magnetic penetration depth and the
superconducting energy gap. THz-TDS has recently
been used to study MgB2, the material newly discovered
to be superconducting. This material exhibits an
extremely high transition temperature of 39 K and is
currently not well understood. THz-TDS was used to
determine the superconducting-gap energy threshold
of approximately 5 meV. This corresponds to only half
the value predicted by current theory and points to the
existence of complex material interactions52.
Experiments with optical-pump THz-probe
systems can reveal additional information about
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REVIEW ARTICLE
Figure 7 A biotin-avidin T-ray
biosensor (after ref.65).
a,The test slide consisting of a
biotin thin-film,half coated with
an avidin solution.b,Illustration
of the measurement procedure
where the slide is mounted on a
galvanometric shaker and
translated back and forth in the
THz beam to allow the differential
signal to be measured.c,The THz
pulse measured after
transmission through the biotinavidin sensor with (dashed line)
and without (solid line) exposure
to 0.3 ng cm–2 of avidin
molecules chemically bound to
agarose beads in solution.
a
b
THz
c
1.0
THz electric field (au)
Biotin-avidin
Pure avidin
0.5
0.0
–0.5
THz systems are ideal for imaging dry dielectric
substances including paper,plastics and ceramics.
These materials are relatively non-absorbing in this
frequency range,yet different materials may be easily
discriminated on the basis of their refractive index,
which is extracted from the THz phase information.
Many such materials are opaque at optical frequencies,
and provide very low contrast for X-rays.THz imaging
systems may therefore find important niche applications
in security screening and manufacturing quality control.
An important goal in this context is the development of
three-dimensional (3D) tomographic T-ray imaging
systems57.Figure 5 illustrates a reconstructed cross
section and 3D-rendered image of two plastic cylinders
with differing refractive index.The system is based on
the same principles as X-ray computed tomography,but
provides a wealth of information about the material’s
frequency-dependent optical properties through
broadband,phase-sensitive THz detection58.
Interest in using THz imaging to study cellular
structure is also increasing.A fundamental limitation in
this context is the resolution of current systems.
The Rayleigh criterion limits the far-field resolution of
an imaging system to the order of the wavelength
(0.3 mm at 1 THz).For this reason,researchers are
relying on imaging in the near-field to achieve improved
spatial resolution.Using near-field techniques,similar to
those used in near-field optical microscopy,resolutions
of 7 µm have been demonstrated using radiation with a
centre wavelength of 600 µm (ref.59).An alternative
method to improve the resolution is to use higherfrequency THz pulses.Figure 6 shows a THz image of a
membrane of onion cells.The resolution of
approximately 50 µm is achieved by using very
broadband THz pulses extending into the mid-infrared.
The contrast in the image is attributed primarily to
differences in the water content of the cells and the
intercellular regions60.
BIOMATERIAL THZ APPLICATIONS
–1.0
0
10
20
30
40
50
60
Delay time (ps)
materials. In these experiments, the material is excited
using an ultrafast optical pulse, and a THz pulse is
used to probe the dynamic far-infrared optical
properties of the excited material. Leitenstorfer et al.
used an optical-pump THz-probe system to identify
the time evolution of charge–charge interactions in an
electron–hole plasma excited in GaAs using ultrafast
optical pulses. This study added experimental
evidence to quantum-kinetic theoretical predictions
regarding charge build-up or dressed quasi-particles3.
THZ IMAGING AND TOMOGRAPHY
Pulsed THz-wave imaging,or ‘T-ray imaging’, was first
demonstrated by Hu and Nuss53 in 1995,and since then
has been used for imaging a wide variety of targets
including semiconductors54,cancerous tissue55 and
flames56.The attraction of THz imaging is largely due to
the availability of phase-sensitive spectroscopic images,
which holds the potential for material identification or
‘functional imaging’.
THz systems have broad applicability in a biomedical
context.Active fields of research range from cancer
detection61 to genetic analysis.Biomedical applications
of THz spectroscopy are facilitated by the fact that the
collective vibrational modes of many proteins and DNA
molecules are predicted to occur in the THz range.THz
spectroscopy has also been heralded for its potential
ability to infer information on a biomolecule’s
conformational state.The complex refractive index of
pressed pellets consisting of DNA and other
biomolecules has been determined and shows
absorption consistent with a large density of lowfrequency infrared-active modes62,63.
DNA analysers are used to identify polynucleotide
base sequences for a variety of genetic applications.
Production of gene chips is an increasingly popular
technique where unknown DNA molecules are bound
to fluorescently labelled polynucleotides with known
base sequences. Fluorescent labelling can affect
diagnostic accuracy and increases the cost and
preparation time of gene chips.As a result, several ‘labelfree’methods are being researched. THz imaging
appears to have promise in this context. THz
spectroscopy has shown the capability of differentiating
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REVIEW ARTICLE
between single- and double-stranded DNA owing to
associated changes in refractive index64. Recently the
same group has demonstrated a THz sensing system
capable of detecting DNA mutations of a single base
pair with femtomole sensitivity2.
A further biomedical application of THz systems is
the T-ray biosensor65.A simple biosensor has been
demonstrated for detecting the glycoprotein avidin after
binding with vitamin H (biotin).A film of biotin is
deposited on a solid substrate (Fig. 7a),and half of the
biosensor slide is exposed to the environment or solution
of interest.Avidin has a very strong affinity for biotin and
binds to any biotin-containing molecules.The modified
far-infrared optical properties of the bound biotin film
can then be detected using the technique of differential
THz-TDS66.The slide is mounted on a galvanometric
shaker and the THz beam is alternately focused through
the avidin and control portion of the slide (Fig. 7b).
A comparison of the signal measured using a slide
treated with 0.3 ng cm–2 avidin solution and a plain
biotin slide revealed a detectable difference (Fig. 7c).
This detectable limit is significantly enhanced by
chemically binding the avidin molecules to agarose
beads to provide increased contrast of refractive index67.
Such techniques may find broad applications in trace gas
sensing and proteomics.
OUTLOOK
THz spectroscopy systems have undergone dramatic
changes over the past decade.Improved source and
detector performance continue to expand application
areas and facilitate the transition of THz systems from
the laboratory to commercial industry.Biomedical
imaging and genetic diagnostics are two of the most
obvious potential applications of this technology,but
equally promising is the ability to investigate material
characteristics,probe distant galaxies and study
quantum interactions.THz radiation has further
potential for revolutionary new uses,such as in the
manipulation of bound atoms,which holds potential for
future quantum computers68.Several key research areas
promise significant continuing advances in THz
technology.Central among these are current efforts
towards higher-power THz sources—which will give rise
to non-linear THz spectroscopy,with the potential to
extract additional material characteristics—highersensitivity receivers and improved understanding of the
interaction between THz radiation and materials such as
quantum structures and biomaterials.
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Acknowledgements
This work was supported in part by the US Army Research Office, the National
Science Foundation, the Australian Research Council and the Cooperative Research
Centre for Sensor, Signal and Information Processing. The authors thank D.
Abbott, D. Gray, A. Menikh, S. P. Mickan and the Australian-American Fulbright
Commission.
Correspondence and requests for materials should be addressed to X.C.Z.
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