Small-Scale Trapped Filaments in Intense Laser Beams

Abstract

The properties of small trapped filaments in liquids are discussed in the light of new experimental and theoretical results. The duration of the filaments is shown to be about ${10}^{\ensuremath{-}10}$ sec, after which they "blow up," apparently because of the heating and subsequent expansion of the liquid. The measured diameters range from 4 \ensuremath{\mu} in C${\mathrm{S}}_{2}$ to about 12 \ensuremath{\mu} in nitrobenzene. The power found in a small filament is the order of 10 kW, agreeing with the theoretical trapping threshold for the Kerr effect, and indicating that this is the principal mechanism responsible for initiation of the filaments. For power above the trapping threshold, filaments continue to decrease in size and increase in intensity until higher-order terms in ${E}^{2}$ cause the nonlinear refractive index to saturate. Calculation of the limiting diameters, based on saturation of the Kerr effect, yields values about one order of magnitude smaller than the measured diameters and indicates that some other mechanism determines the size. Finally, the low value of the stimulated Raman gain in most of the length of filaments, which had been observed previously, is explained on the basis of frequency broadening and dispersion.