  Terahertz radiation by optical rectification has been observed at room
temperature in a hydrogen-bonded organic molecular ferroelectric crystal,
2-phenyl malondialdehyde (PhMDA). The radiated electromagnetic wave consisted
of a single-cycle terahertz pulse with a temporal width of $\sim$ 0.5 ps. The
terahertz radiation amplitude divided by the sample thickness in PhMDA was
nearly equivalent to that in a typical terahertz wave emitter ZnTe. This is
attributable to a long coherence length in the range of 130 $\sim$ 800 $\mu$m
for the terahertz radiation from PhMDA. We also discussed the possibility of
PhMDA as a terahertz wave emitter in terms of the phase-matching condition.
