Some investigations into stimulated electronic Raman scattering and related processes in atomic vapours

Abstract

Stimulated electronic Raman scattering, SERS, is a simple method of efficiently generating widely tunable infrared radiation. This work describes techniques by which the tuning capability of this process may be extended. To this end several schemes in Caesium and Barium vapours have been investigated. In Barium vapour two Raman transitions were studied using fixed frequency lasers. Poor performance of one of these transitions led to the application of biharmonic pumping or 'coherent Raman mixing' to enable generation of infrared light, and was the first observation of this process in an atomic vapour. This technique was subsequently used to generate infrared light from tunable dye laser light, which gave continuous tuning from 1.16µm to 1.5µm. The process was found to be limited by atomic saturation. The effect which saturation has on the generation efficiency is considered theoretically, and predicts quite accurately the observed tuning behaviour. Two methods aimed at reducing molecular absorption effects in SERS in alkali vapours were investigated. The techniques employed either laser excitation and dissociation of the molecules or a reduction in their ground state concentration by superheating of the vapour. Some degree of success was achieved in improving the available tuning from the 6s-7s transition in Caesium. However, the inclusion of the effect of collision broadening of atomic transitions into a calculation of the Raman threshold suggests that little further improvement can be obtained on this transition. A direct measurement of the Raman threshold power on the 6s-5d5/2 transition in Caesium gave excellent agreement with theoretical predictions based on collision broadening of this Raman transition. In addition, this work gave an insight into the role photoionization plays in increasing the threshold under certain circumstances. The analysis suggests that laser pulse duration and vapour column length are important in determining whether Raman threshold can be exceeded.