  With a low emittance and short-bunch electron beam at a high repetition rate
realized by a superconducting linac, stimulated excitation of an optical cavity
at the terahertz spectrum range has been shown. The electron beam passed
through small holes in the cavity mirrors without being destroyed. A sharp
resonance structure which indicated wide-band stimulated emission via coherent
diffraction radiation was observed while scanning the round-trip length of the
cavity.
