The stimulated Raman effect

Abstract

The present work describes a theoretical and experimental investigation of the stimulated Raman effect excited by the focused output of a slow, Q-switched ruby laser in the self-focusing liquids benzene, nitrobenzene, chlorobenzene, and carbon disulphide. Time relationships between the exciting pulse, the transmitted laser pulse, and the time resolved stimulated Raman spectra have been studied using a high speed streak camera in conjunction with fast photodiodes. The spontaneous Raman spectra of these liquids have been investigated using a photoelectric Raman spectrometer and a method is outlined for measuring the relative values of the stimulated Raman gain coefficients. It is experimentally shown that, on a nanosecond time scale, stimulated Raman lines are not generated simultaneously but in the sequence Stokes, Anti-Stokes, and second harmonic Stokes. Further, the time delay between the start of first and second harmonic Stokes is shown to be dependent on the rate of rise of the exciting pulse. It was found that during stimulated Raman generation the laser pulse transmitted through the liquid was heavily distorted and that each liquid produced its own characteristic pulse distortion. No distortion was found in the absence of stimulated Raman generation. Time correlations were found between the duration of features of the pulse distortion and the duration of first Stokes and second harmonic Stokes. These correlations show that forward stimulated Raman generation is controlled by the transmitted laser pulse not the exciting pulse. The threshold for the onset of pulse distortion is shown to be dependent on the rate of rise of the exciting pulse whilst, in all the liquids, apart from carbon disulphide, the 'cut-off' threshold is dependent on the peak power of the incident pulse. Investigation of the pulse distortion thresholds for the range of liquids produced relative values which did not agree with those predicted from either the optical Kerr coefficient or the calculated stimulated Raman gain coefficients. A theoretical model of the self-focusing of a focused beam in a medium for which both electrostriction and the optical Kerr effect are significant is presented and has been used to explain the anomalous threshold results. Within the experimental scatter of the results obtained, this model appears to explain the observed threshold effects. Since the forward stimulated Raman generation was weak in comparison to the transmitted laser pulse and followed this distorted pulse rather than the exciting pulse, it is concluded that stimulated Raman is not the dominant mechanism in the interaction. A brief review is presented of the theory and predictions of steady and non-steady state Brillouin scattering. Although the phonon lifetimes for the other liquids appear to be too short to be of significance, the results are similar in form to those of carbon disulphide. An explanation based on multiphoton absorption is suggested to explain the anomalous behaviour of these liquids. It is concluded that the observed effects in the forward stimulated Raman process depend upon the development in time of the non-linear field dependent self-focusing of the exciting beam and the strong backward scattering process.