The applications of terahertz-frequency electromagnetic
radiation are growing rapidly, driven by developments in
sources, detectors and optics suited to the THz-frequency
region. Much of the advance is in the field of THz time-domain
spectroscopy in which ultrashort pulses of near-infrared
radiation are used in a pulse-probe arrangement
to realise both the emission and detection of the THz radiation.
Methods for ultrafast THz pulse generation include
photoconductivity, transient currents or optical rectification.
While optical rectification has been known for
some time, earlier theoretical and experimental results
have been limited to simple crystallographic directions.