Advances in Terahertz Imaging
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radiation other than heating, no health risks are known. Also, this type of radiation
has higher sensitivity to water content and dust than mm-waves, but can still pass
through different materials which are generally opaque with respect to visible light
waves. Terahertz waves can pass through dielectric materials, like fabric, leather,
wood, plastic, paper, etc. Also, metals reflect terahertz waves highly. Owing to these
facts, THz imaging is being used in biomedical [2–4] imaging, agricultural [5, 6]
applications, security and defence, technical diagnostic systems and industrial quality
assurance systems.
In recent times, Terahertz technology has been selected as a major emerging
technology out of ten emerging technologies which are expected to change the world
by MIT Technology Review. It is predicted that the terahertz technology market will
grow to $570 million in 2021 [7].
2 Terahertz Imaging Components
THz imaging systems typically require a source, different components and a detector. The source generates the THz radiation, whereas detectors detect the radiation.
Components mainly include lenses, mirrors and polarizers, which manipulate the
radiation. Terahertz mirrors are usually made of metal, and lenses are typically made
of plastics.
However, detectors are perhaps the biggest problem in THz technology. At present,
mostly photonic detectors are being used for research but again they require cooling. Among electronic solutions, Schottky diode-based heterodyne detectors have
reached 3 THz and beyond [8]. Another significant problem is to generate THz signals with useful power levels. Quantum cascade lasers can generate several mW of
power in the 5 THz range, but they require cooling to liquid nitrogen temperatures
or below. Six major types of terahertz sources are listed by Lewis [9], which can be
broadly classified as thermal [10, 11], solid-state electronic [12, 13], vacuum electronic [14–16], mechanical excitation [17, 18], lasers [19, 20] and sources pumped
by lasers [21, 22]. The choice of the terahertz source depends on the type of imaging
technique utilized. But it is to be remembered that sources generating low power
actually needs more demanding detection devices like cryogenic cooled sensors.
Now, it is seen that output power for most of the devices falls in the mid-terahertz
range. However, of late different THz wave generation techniques are already available commercially, viz. THz IMPATT diodes, backward-wave oscillators, optically
pumped terahertz lasers, quantum cascade lasers, etc.
However, a major limitation of terahertz imaging is the time taken to form a THz
image. Image formation is normally done by acquisition of data serially for each
pixel. This process is usually slow as it requires mechanical scanning of either the
object [23] or of the terahertz illuminating beam [24]. As an alternative, if an array of
parallel detectors can be used, the image acquisition speed can be enhanced. Usually,
for frequencies below 100 GHz, conventional antenna structures with integrated
electronic amplifiers at each pixel are used for array detectors [25, 26]. However,
in spite of recent progress, it is still a big challenge to fabricate such integrated
multi-pixel devices in the terahertz range.
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radiation other than heating, no health risks are known. Also, this type of radiation
has higher sensitivity to water content and dust than mm-waves, but can still pass
through different materials which are generally opaque with respect to visible light
waves. Terahertz waves can pass through dielectric materials, like fabric, leather,
wood, plastic, paper, etc. Also, metals reflect terahertz waves highly. Owing to these
facts, THz imaging is being used in biomedical [2–4] imaging, agricultural [5, 6]
applications, security and defence, technical diagnostic systems and industrial quality
assurance systems.
In recent times, Terahertz technology has been selected as a major emerging
technology out of ten emerging technologies which are expected to change the world
by MIT Technology Review. It is predicted that the terahertz technology market will
grow to $570 million in 2021 [7].
2 Terahertz Imaging Components
THz imaging systems typically require a source, different components and a detector. The source generates the THz radiation, whereas detectors detect the radiation.
Components mainly include lenses, mirrors and polarizers, which manipulate the
radiation. Terahertz mirrors are usually made of metal, and lenses are typically made
of plastics.
However, detectors are perhaps the biggest problem in THz technology. At present,
mostly photonic detectors are being used for research but again they require cooling. Among electronic solutions, Schottky diode-based heterodyne detectors have
reached 3 THz and beyond [8]. Another significant problem is to generate THz signals with useful power levels. Quantum cascade lasers can generate several mW of
power in the 5 THz range, but they require cooling to liquid nitrogen temperatures
or below. Six major types of terahertz sources are listed by Lewis [9], which can be
broadly classified as thermal [10, 11], solid-state electronic [12, 13], vacuum electronic [14–16], mechanical excitation [17, 18], lasers [19, 20] and sources pumped
by lasers [21, 22]. The choice of the terahertz source depends on the type of imaging
technique utilized. But it is to be remembered that sources generating low power
actually needs more demanding detection devices like cryogenic cooled sensors.
Now, it is seen that output power for most of the devices falls in the mid-terahertz
range. However, of late different THz wave generation techniques are already available commercially, viz. THz IMPATT diodes, backward-wave oscillators, optically
pumped terahertz lasers, quantum cascade lasers, etc.
However, a major limitation of terahertz imaging is the time taken to form a THz
image. Image formation is normally done by acquisition of data serially for each
pixel. This process is usually slow as it requires mechanical scanning of either the
object [23] or of the terahertz illuminating beam [24]. As an alternative, if an array of
parallel detectors can be used, the image acquisition speed can be enhanced. Usually,
for frequencies below 100 GHz, conventional antenna structures with integrated
electronic amplifiers at each pixel are used for array detectors [25, 26]. However,
in spite of recent progress, it is still a big challenge to fabricate such integrated
multi-pixel devices in the terahertz range.