Stimulated Raman emission and absorption

Department of Physics, University of Toronto, Toronto, Canada.

Abstract

Since the first observations of stimulated Raman scattering in 1962, the outstanding problems have been concerned with explanations of the observed gain, line width and directional properties of the emitted radiation. Significant discrepancies between experiment and theory have been found, including for example measurements of gain as high as 100 times that expected, line widths as large as 500 cm^-1 and angles of emission 50 percent larger than predicted. Recently these difficulties have been somewhat clarified with the discovery of selffocusing of the laser beam during the generation of stimulated Raman radiation and with the identification of two independent classes of stimulated Raman radiation with different spectral and directional properties.

The present paper is concerned with measurements of (1) the frequencies and line widths of pure liquids and mixtures, and (2) the directional characteristics of anti-Stokes and Stokes emission in liquids and solids.

It is found that with л single-mode » excitation there are slight frequency shifts (~ 1 cm^-1) and small broadening (~ 5 cm^-1), and in some cases narrowing, of spectral lines with varying concentrations in mixtures. However, with « multiple-mode » excitation the measured shifts and broadening are one or two orders of magnitude larger. The direction of anti-Stokes emission in diamond has been measured and found to be in agreement with theory, just as in calcite, the only other solid whose directional dependence has been studied. Further measurements of the emission angles in benzene, toluene and nitrobenzene under various experimental conditions, however, do not agree with either the simple momentum conservation relation or with present theories which include the effects of the « active region ».