Advances in Terahertz Imaging
159
4.2 Terahertz Thermal Cameras
Terahertz thermal cameras detect the generated heat due to incident THz radiation.
Generally, three types of THz radiation thermal detectors are available: (1) pyroelectric sensors; (2) Golay cells; and (3) bolometers. The detectors are generally used
with a modulated source for reducing the ambient noise. However, at the same time,
the possible overall maximum frame rate at room temperature is also reduced. On
the contrary, the spectral range of operation of thermal detectors is broad, and this is
an advantage.
Pyroelectric crystals are those crystals for which electrical polarizability changes
when heated. The changes can be detected by pyroelectric sensors. Actually, a temporary voltage is generated when the heat is transferred to the crystal by an absorbing
layer. Some common examples of pyroelectric crystals are lithium tantalite (LiTaO 3 ),
barium titanate (BaTiO 3 ), triglycine sulphate (TGS) and deuterated triglycine sulphate (DTGS). Among these, the highest sensitivities at THz frequencies are exhibited by TGS and DTGS [78], for which at a modulation frequency of ~10 Hz, typical
NEP and responsivity are 1nW/
√
Hz and ~1 kV/W, respectively. Fortunately pyroelectric detectors are available as array cameras commercially [79]. The pyroelectric
detector sensitivity can be improved by increasing absorption of THz radiation in
the coating layer of the crystal and by reducing the thickness of the crystal. For commercial pyroelectric cameras, minimum detectable power is typically in the range
between 50 and 100 nW [30].
The Golay cell is used mainly for infrared spectroscopy. It is a type of opto-acoustic
detector. A gas-filled enclosure with a material for absorbing infrared radiation and a
flexible diaphragm or membrane is integral part of Golay cell. The generated heat is
transferred to the expanding gas cell in Golay cells. Then, optical method of moving
mirrors is used to measure the increased pressure [80]. However, as Golay cells
depend on the movement of the membrane mechanically, they are generally slow
detectors. As the operating principle of Golay cells are quite simple, they are used
widely for measuring power of THz radiation. But to integrate Golay cells into dense
arrays of detectors is difficult, and this makes them suitable in single-pixel detection
scheme. Typical Golay cells NEP range between 0.1 and 10 nW/
√
Hz [30].
Thermal bolometers are used as individual pixels in another type of THz cameras.
Bolometer imaging arrays have generated much interest in real-time terahertz imaging as they are highly sensitive and their fabrication technology is also established. In
this technique, bolometer pixel measures the changes in resistance which is dependent on temperature [30]. The primary constituents of a bolometer as described by
Richards [81] have been shown in Fig. 6a.
The THz beam radiation power P THz falls on an absorber having heat capacity
C A . This is again connected to a heat sink at temperature T s via a thermal link
of conductance G L . The temperature of the absorber T A rises at a rate dT A /dt =
P THz /C A to the limiting value T A = T s +
P THz
G L
, with the thermal time constant
τ = C A /G L . Figure 6b shows that on turning off the illumination, the temperature
159
4.2 Terahertz Thermal Cameras
Terahertz thermal cameras detect the generated heat due to incident THz radiation.
Generally, three types of THz radiation thermal detectors are available: (1) pyroelectric sensors; (2) Golay cells; and (3) bolometers. The detectors are generally used
with a modulated source for reducing the ambient noise. However, at the same time,
the possible overall maximum frame rate at room temperature is also reduced. On
the contrary, the spectral range of operation of thermal detectors is broad, and this is
an advantage.
Pyroelectric crystals are those crystals for which electrical polarizability changes
when heated. The changes can be detected by pyroelectric sensors. Actually, a temporary voltage is generated when the heat is transferred to the crystal by an absorbing
layer. Some common examples of pyroelectric crystals are lithium tantalite (LiTaO 3 ),
barium titanate (BaTiO 3 ), triglycine sulphate (TGS) and deuterated triglycine sulphate (DTGS). Among these, the highest sensitivities at THz frequencies are exhibited by TGS and DTGS [78], for which at a modulation frequency of ~10 Hz, typical
NEP and responsivity are 1nW/
√
Hz and ~1 kV/W, respectively. Fortunately pyroelectric detectors are available as array cameras commercially [79]. The pyroelectric
detector sensitivity can be improved by increasing absorption of THz radiation in
the coating layer of the crystal and by reducing the thickness of the crystal. For commercial pyroelectric cameras, minimum detectable power is typically in the range
between 50 and 100 nW [30].
The Golay cell is used mainly for infrared spectroscopy. It is a type of opto-acoustic
detector. A gas-filled enclosure with a material for absorbing infrared radiation and a
flexible diaphragm or membrane is integral part of Golay cell. The generated heat is
transferred to the expanding gas cell in Golay cells. Then, optical method of moving
mirrors is used to measure the increased pressure [80]. However, as Golay cells
depend on the movement of the membrane mechanically, they are generally slow
detectors. As the operating principle of Golay cells are quite simple, they are used
widely for measuring power of THz radiation. But to integrate Golay cells into dense
arrays of detectors is difficult, and this makes them suitable in single-pixel detection
scheme. Typical Golay cells NEP range between 0.1 and 10 nW/
√
Hz [30].
Thermal bolometers are used as individual pixels in another type of THz cameras.
Bolometer imaging arrays have generated much interest in real-time terahertz imaging as they are highly sensitive and their fabrication technology is also established. In
this technique, bolometer pixel measures the changes in resistance which is dependent on temperature [30]. The primary constituents of a bolometer as described by
Richards [81] have been shown in Fig. 6a.
The THz beam radiation power P THz falls on an absorber having heat capacity
C A . This is again connected to a heat sink at temperature T s via a thermal link
of conductance G L . The temperature of the absorber T A rises at a rate dT A /dt =
P THz /C A to the limiting value T A = T s +
P THz
G L
, with the thermal time constant
τ = C A /G L . Figure 6b shows that on turning off the illumination, the temperature