It is half a century since Franken and co-workers first reported
their landmark observations of optical second harmonic
generation (SHG), using the recently developed ruby laser . The
principle of frequency doubling became fully comprehensible with
subsequent theoretical analysis ; the rapidly ensuing, parallel
development of experiment and theory essentially marked the
birth of the diverse field we now call nonlinear optics. In SHG
the pairwise annihilation of photons from an input beam provides
for the creation of individual photons with twice the energy:
commonly, each output photon has not only double the optical
frequency but also double the photon momentum. Laserlike output
is produced when the harmonic light develops in phase with the
input, and in a real, dispersive material, one generally has to
circumvent a potential phase mismatch by the use of birefringent
or alternately layered materials. The pump laser input then
provides for the forward emission of a coherent and strongly
directed harmonic outp