The name is absent

15

metal antenna), the sampled region is defined by the size of the optical probe and of
the non-linear crystal rather than by the size of an antenna. So, instead of using a
small electro-optic crystal and a focused THz beam, one can use a large (cm²) crystal
and a large-area beam. The spatially varying polarization rotation of the probe beam
can be measured using crossed polarizers and a CCD camera [29]. Since a CCD
detector array can refresh at video rates, this technique can be used to generate
terahertz movies [4,34,35]. It does, however, require considerably more optical power
in the femtosecond probe beam. Generally, an amplified femtosecond laser system is
required. Also, one still requires a mechanical delay line to vary the delay between
the probe pulse and the THz pulse in order to obtain a measurement of the terahertz
electric field.

X.-C. Zhang and co-workers have also developed a chirped-pulse technique for
measuring an entire THz waveform along a line (ID imaging) in a single shot, entirely
dispense of the delay line [36-38]. However, this system has a limited temporal
resolution, and also requires an amplifier which increases the cost and complexity of
the laser system. The only electro-optic imaging system which does not require an
amplified laser source uses a time-of-flight camera comprising an optical demodulation
detector array [39].

In parallel with the rapid progress in pulsed time-domain techniques, tremendous
strides have also been made in technologies for continuous wave (cw) terahertz systems
since the first examples [40,41]. With some exceptions [42-44], cw THz imaging
systems typically employ an incoherent device for direct detection of the THz wave,
such as a bolometer or a Golay cell. Array detectors are also available for direct (not
heterodyne) detection, including microbolometer arrays [33,34], germanium detector
arrays [45], and pyroelectric cameras. Unlike in the case of THz-TDS, the source and

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