Study of Self-Focusing and Self-Phase-Modulation in the Picosecond-Time Regime

The propagation properties of a single picosecond pulse produced by a mode-locked Nd:glass laser are studied in several liquids by observing the optically induced birefringence and the changes in spectral content of the self-focused light. The spatial properties of the focal volume are studied using multiple second-harmonic (SH) probing beams which measure the induced birefringence from a single infrared pulse sequentially in many parts of the cell. These data allow a direct determination of the velocity of propagation of the high-intensity region. The formation and disappearance of high-intensity regions is also observed, as well as propagation lengths greater than 4 cm on a single pulse for the case of C${\mathrm{S}}_2$. Using temporally compressed SH pulses, we find that the induced birefringence is due primarily to the orientational Kerr effect in materials such as nitrobenzene and toluene. Results in C${\mathrm{S}}_2$ are also consistent with this mechanism and indicate that the duration of the infrared pulse in the focal volume is less than the duration of the incident pulse. The propagation properties of filaments which pass through a 1-mm glass slide are investigated, and the properties of beams which are focused within the cell by an external lens are studied. The occasional appearance of multiple filaments on some laser shots at different temporal positions in the pulse indicates the possibility of using this technique for beam wave-front diagnostics. The experimental spectra of the self-focused light were studied using beams of picosecond pulses in a number of materials, and they are all similar, showing a symmetric broadening about the laser frequency. The spectra are modulated with an interference structure which is characteristic of a phase-modulated pulse. The effective duration of the modulated pulse is consistent with the duration of the pulse in the high-intensity region as determined from the propagation data. Most of the results of the propagation and frequency-broadening experiments can be explained with a transient theory of self-focusing which results from numerical integration of the self-focusing equations with a relaxing nonlinear index. The theory, however, predicts an asymmetric frequency broadening, and therefore additional mechanisms are discussed which may account for the large amount of upshifted light which was observed.