New Nonlinear Optical Processes in Molecules at Infrared Frequencies.

Abstract : This final report details progress in the study of optical nonlinearities in molecules at infrared wavelengths. The objectives have been to investigate the nonlinear optical properties of triply resonant and two photon or Raman resonant molecules. The techniques of third harmonic generation, multiphoton absorption, and degenerate four wave mixing have been used, with a CO2 TEA laser as the excitation source. An outstanding accomplishment has been the successful application of third harmonic generation techniques to study collisionless multiphoton excitation in sulfur hexafluoride. Depending on the excitation frequency, one or two-photon processes have been identified which deplete the population of the discrete vibrational energy levels into the quasi-continuum. Similar third harmonic generation experiments have yielded lower conversion efficiency in ammonia than in sulfur hexafluoride. Only a single rotational level in ammonia is found to be resonant, so the susceptibility is only minimally enhanced. A second area of work is the study of the two photon Raman resonant molecule CD4. Third harmonic generation results were obtained for CD4 at room and cryogenic temperatures. Problems of fundamental wavelength absorption were eliminated and the nonlinearity was enhanced at 193K.