Advances in Terahertz Imaging
157
4 Terahertz Imaging Cameras
For a lot of scientific as well as engineering applications, one of the primary requirements is cost-effective imaging. But unfortunately for a long time, THz frequency
range lacked an inexpensive and efficient imaging device. However, recently intensive research is going on to develop THz cameras. But before these types of cameras
become universally acceptable, they have to satisfy some requirements. Firstly, the
total power consumption by a THz camera should not be high and also the size and
weight should not be large so that they may be integrated into industrial imaging
systems. Secondly, the process of fabrication of the terahertz cameras has to be in
line with modern fabrication techniques. Thirdly, while high sensitivity of the THz
cameras is still retained, techniques need to be introduced so that systems can operate at room temperatures and thus the expensive and cumbersome cryogenic cooling
systems can be avoided. Last but not the least, a THz camera has to be highly sensitive
so it is possible to record high frame rate THz videos. However, in most available
THz cameras, only intensity can be detected, whereas ideally complex electric fields
have to be detected. However, one of the major disadvantages is the scarcity of cheap,
high-power THz sources
Generally, the responsivity and the noise-equivalent power (NEP) are two essential figures of merit of any detector. Noise-equivalent power or NEP is the measure of
the minimum power that can be detected, and is defined as the input power that gives
SNR = 1, with a 1 Hz bandwidth output. A lower NEP indicates a more sensitive detector. On the other hand, responsivity is the direct detector response (either in
amperes or volts) to the incident power of the THz radiation (in watts). It is expressed
in A/W or V/W. The total incident power is to be measured to measure the responsivity. If it is assumed that all incident THz radiation is captured by the imaging
array in a THz camera, then per pixel responsivity can be calculated by dividing the
summation of the voltages of all the pixels by the total incident power. Generally, a
higher responsivity indicates a better performing detector. In recent literatures, two
main types of THz cameras are reported. They are THz FET-based cameras and THz
thermal cameras.
4.1 Terahertz FET-Based Cameras
Rectification in the field-effect transistor (FET) is a major technological trend in
terahertz cameras. Response at frequencies much higher than the cut-off frequency
of the transistor is enabled by plasma wave excitations. Theoretical demonstration
shows that for THz detection plasma waves could be used in the FET channel [74].
A schematic representation of the proposed FET for detection of THz radiation is
illustrated in Fig. 5. From the figure, it is seen that between the source and the gate
a DC voltage U 0 is applied. However, an alternating voltage U a is caused by the
incident THz radiation. A constant drain-to-source voltage is generated due to the
157
4 Terahertz Imaging Cameras
For a lot of scientific as well as engineering applications, one of the primary requirements is cost-effective imaging. But unfortunately for a long time, THz frequency
range lacked an inexpensive and efficient imaging device. However, recently intensive research is going on to develop THz cameras. But before these types of cameras
become universally acceptable, they have to satisfy some requirements. Firstly, the
total power consumption by a THz camera should not be high and also the size and
weight should not be large so that they may be integrated into industrial imaging
systems. Secondly, the process of fabrication of the terahertz cameras has to be in
line with modern fabrication techniques. Thirdly, while high sensitivity of the THz
cameras is still retained, techniques need to be introduced so that systems can operate at room temperatures and thus the expensive and cumbersome cryogenic cooling
systems can be avoided. Last but not the least, a THz camera has to be highly sensitive
so it is possible to record high frame rate THz videos. However, in most available
THz cameras, only intensity can be detected, whereas ideally complex electric fields
have to be detected. However, one of the major disadvantages is the scarcity of cheap,
high-power THz sources
Generally, the responsivity and the noise-equivalent power (NEP) are two essential figures of merit of any detector. Noise-equivalent power or NEP is the measure of
the minimum power that can be detected, and is defined as the input power that gives
SNR = 1, with a 1 Hz bandwidth output. A lower NEP indicates a more sensitive detector. On the other hand, responsivity is the direct detector response (either in
amperes or volts) to the incident power of the THz radiation (in watts). It is expressed
in A/W or V/W. The total incident power is to be measured to measure the responsivity. If it is assumed that all incident THz radiation is captured by the imaging
array in a THz camera, then per pixel responsivity can be calculated by dividing the
summation of the voltages of all the pixels by the total incident power. Generally, a
higher responsivity indicates a better performing detector. In recent literatures, two
main types of THz cameras are reported. They are THz FET-based cameras and THz
thermal cameras.
4.1 Terahertz FET-Based Cameras
Rectification in the field-effect transistor (FET) is a major technological trend in
terahertz cameras. Response at frequencies much higher than the cut-off frequency
of the transistor is enabled by plasma wave excitations. Theoretical demonstration
shows that for THz detection plasma waves could be used in the FET channel [74].
A schematic representation of the proposed FET for detection of THz radiation is
illustrated in Fig. 5. From the figure, it is seen that between the source and the gate
a DC voltage U 0 is applied. However, an alternating voltage U a is caused by the
incident THz radiation. A constant drain-to-source voltage is generated due to the