Research on biexcitons by ultra-high resolution two-photon solid state spectroscopy

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CuCl crystal gathers much notice as one of the model materials for developing optical devices because of the large optical nonlinearity due to the two-photon resonance of biexcitons. However, the estimation of the level width of biexciton (gamma_M) which is necessary to estimate the nonlinear susceptibility and to clarify the origin of photon-noise of optical devices is difficult because of the broad line width of the ordinary pulse laser light which is used to measure gamma_M. The purpose of this research are 1) to develop a UV light source of very narrow linewidth and moderate peak power by frequency doubling of IR light from a dye amplified Ti-sapphire ring laser, 2) to measure gamma_M directly with it and 3) to apply this high resolution spectroscopy to the research on solid state quantum physics.The results are as follows,l. Pulsed UV light of very narrow line width (1mueV) and of moderate power density (1OOkW/cm^2) was generated.2. The two-photon resonance of biexcitons was observed by this UV light.3. The value of gamma_M was obtained to be 24mueV under very weak excitation density (<1kW/cm^2).4. Increase of gamma_M was found under the excitation density of >lkW/cm^2.This research is the first successful example of the ultra-high resolution nonlinear spectroscopy on intrinsic electronic excitation state in solid state material, so various progress is expected from now on.