2.2 (left) A typical time-domain terahertz waveform, measured in
ambient air. The oscillatory features which follow the initial
single-cycle transient are the result of water vapor in the beam path.
The dotted curve shows the intensity envelope, computed from the
measured electric field E(I) using a Hilbert transform, (right) The
spectral amplitude ∣ E(ω) ∣ derived from the field shown at left by
Fourier transform, on a log scale. The vertical arrows indicate the
spectral positions of tabulated water vapor absorption lines. The
inset shows the spectral phase, also derived from the time-domain
measurement. This is essentially linear, as expected for a single-cycle
pulse. The effects of the water vapor absorption lines on the phase
are measurable, but are too small to see in this display......... 9
2.3 A schematic of a typical transmission-mode raster-scan time-domain
imaging system............................... 12
3.1 The THz Fourier imaging setup. An approximately collimated beam
from the THz transmitter illuminates an object mask, placed one
focal length away from the focusing lens. The THz receiver
raster-scans and samples the Fourier transform of the object on the
focal plane................................. 23