Abstract

It was recently suggested that under suitable conditions, the propagation of femtosecond optical pulses in electro-optic materials should be accompanied by the radiation of an extremely fast electromagnetic transient^1,2. This phenomenon, which arises from the inverse electro-optic effect³, produces a Cerenkov cone of pulsed radiation having a duration of approximately one cycle and a frequency in the THz range. Although closely resembling the classical Cerenkov radiation from relativistic charged particles in dielectric media, the physical basis for this effect has some unique distinguishing features. Most important, it is a nonlinear optical effect, arising from a second order nonlinear polarization, and consequently requires a lack of inversion symmetry in the host material. Also, the charge state of the effective source is neutral, being a dipole moment, rather than a point charge. The velocity of the source, however, does move at a velocity which exceeds the radiation velocity due to the additional contribution to the low frequency dielectric permittivity from the infrared lattice vibrations. This is expected to produce a characteristic cone of radiation in the form of a shock wave. Unlike classical Cerenkov radiation, however, the radiation source is spatially extended, being proportional to the intensity envelope of the optical pulse. Consequently the details of the radiation field are expected to depend sensitively on both the duration and beam waist of the optical pulse. A discussion of these effects, based on an analysis of this phenomenon, is given in references 1 and 2.

© 1984 Optical Society of America