DISPERSION OF THE NONLINEAR REFRACTIVE INDEX OF CARBON-DISULFIDE IN THE SPECTRAL RANGE OF 9-11 MICRONS.

摘要

The nonlinear refractive index (n(,2)) of room temperature liquid CS(,2) in the wavelength range of 9 to 11 micrometers is measured. A line tunable hybrid CO(,2) TEA laser and amplifier system is used for the experiments. In these measurements the well known photoacoustic method is utilized to observe the onset of whole beam self-focusing. The photoacoustic signal in a CS(,2) cell, much longer than the confocal parameter, is monitored. The departure of the acoustic signal from linear growth marks the critical power for the onset of nonlinearity. It is experimentally verified that the phenomenon is power dependent as expected from self-focusing theory. The value of n(,2) is then calculated from the theoretical model of self focusing. Measurements of the on-axis irradiance transmitted through the nonlinear material as well as the measurements of beam distortion are used to verify the validity of the photoacoustic method. In all the measurements the on-axis intensity was smaller than the calculated threshold intensity for stimulated Brillouin scattering. The back reflection was monitored to make sure that stimulated Brillouin scattering was not playing a role in the phenomenon.;In our preliminary study, the value of n(,2) in CS(,2) at 10.6 (mu)m was measured to be an order of magnitude larger than the reorientational n(,2) in the visible and near infrared. Further study of the nonlinear refraction in CS(,2) at different CO(,2) laser lines shows that n(,2) varies by a factor of four with its maximum of (11.6 (+OR-) 2) x 10('-10) at 10.24 (mu)m. Measurements of n(,2) at different temperatures and different polarizations support calculations which show electrostriction as the dominant microscopic phenomenon responsible for the nonlinearity. At the same time the polarization dependence of the nonlinearity hints to the presence of a small reorientational Kerr effect. Other experimental and theoretical evidence rules out the presence of significant thermal defocusing in the measurements. The possible origin of the observed dispersion is discussed and future experiments are suggested.