Self-Phase Modulation and "Rocking" of Molecules in Trapped Filaments of Light with Picosecond Pulses

Abstract

The spectral properties of trapped filaments of light in many liquids have been studied in detail using, as an excitation source, a ruby laser operating in different regimes. It has been found that the regular periodic structures in the spectra, which are typical of the self-phase modulation process, can be obtained with high reproducibility under excitation with laser pulses of $\approx 5$ psec. These properties have been interpreted using the method of stationary phase, which makes it possible to derive the temporal behavior of the nonlinear refractive index $\delta n$ in the filaments in a fairly simple way. Other conclusions deduced from this method about the structure of the spectra have been confirmed by experiment. By using a functional relation between $\delta n$ and the optical field intensity $A^2$ based on a model given by Starunov the relaxation time ${\tau }_1$ of $\delta n$ and the temporal behavior of the pulse intensity have also been derived. The value of ${\tau }_1$, which is consistently of the order of a few tenths of a picosecond, gives a strong indication that a "molecular rocking" is the main mechanism for trapping with picosecond excitation. The optical pulse derived has a time width of $\approx 2.5$ psec and an asymmetrical shape. The self-consistency of the method employed has been checked by using the derived pulses $\delta n\left(t\right)\mathrm{}$ and $A\left(t\right)\mathrm{}$ to calculate, by computer, the spectral power density of the optical pulse. The spectrum so calculated fits the experimental one fairly well.