Anti-Stokes Generation in Trapped Filaments of Light

Abstract

The angles of emission and frequency shifts of anti-Stokes radiation from small-scale trapped filaments of intense laser light in a number of liquids are examined and interpreted theoretically. For a given order of vibrational anti-Stokes radiation, a plot of angle of emission versus frequency resembles sections of nested parabolas which open towards higher frequencies, with apices a few tens of wave numbers below the unshifted anti-Stokes frequency, and very little intensity extending above this unshifted frequency. Various modulation mechanisms which smear out the Stokes and anti-Stokes frequencies are discussed, as well as the wave-vector matching conditions which appear to produce the angular distribution of radiation. Relatively simple approximations predict that the maximum extent towards lower frequencies of parabolas associated with the first anti-Stokes is one-half the vibrational frequency. The second and third anti-Stokes can extend twice and three times as far, respectively. In addition, a single set of parabolas is predicted for the first anti-Stokes radiation, but two sets for the second and third anti-Stokes. Theoretical expectations seem to agree reasonably well with observations.