The name is absent

16

detector are independent devices, rather than two components of a coupled system.
In some cases, the terahertz source is tunable, in which case it is possible to combine
spectroscopic measurements with imaging just as in the time-domain systems.

One important advantage of imaging with a single-frequency source is the ability
to select the source wavelength to optimize the imaging capability. This is relevant,
for example, in the case of imaging at a stand-off distance. Stand-off imaging (i.e. at
greater than a few metres distance) is challenging because of the presence of atmo-
spheric water vapour, which significantly attenuates the terahertz beam. However, at
certain frequencies within the terahertz range, the atmospheric attenuation is mini-
mized because there are no nearby strong water vapour absorption lines. The ability
to tune the terahertz source to a water ’window’ is a considerable advantage. For
example, for a 25m stand-off, the power throughput from transmitter to receiver in-
creases by more than a factor of 10 if the atmospheric loss decreases by only 0.5 dB
m^-1. For comparison, variations of several dB m~¹ can be achieved by frequency
tuning within narrowwindows in the 4-5 THz range. Real-time imaging at a 25m
stand-off has been demonstrated using a terahertz quantum cascade laser (QCL) and
a room-temperature microbolometer array [33,46]. In this demonstration, the QCL
was held at 30 K, within the range of a thermomechanical cooler that does not re-
quire liquid cryogens, and produced ~17mW of power in pulsed operation. A few
per cent of the emitted power reached the imaging array, SufRcient for imaging with
a 1 s integration time. Future THz imaging systems with cw sources mainly rely on
the development of QCL at room temperature [47], and detector arrays with higher
sensitivity in the THz range.

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