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

CuCL晶体由于双激子的双光子共振而具有很大的光学非线性，作为发展光学器件的模型材料之一而备受关注。然而，由于用于测量γ_M的普通脉冲激光具有较宽的线宽，为了估计光学器件的非线性极化率和阐明光子噪声的来源，对双激子能级宽度(γ_M)的估计是困难的。本研究的目的是：1)通过对染料放大的钛宝石环激光器的红外光进行倍频，研制线宽很窄、峰值功率适中的紫外光光源；2)用它直接测量伽马_M；3)将这种高分辨率光谱应用于固体量子物理的研究。L。产生了极窄的线宽(1mueV)和中等功率密度(100okW/cm^2)的脉冲紫外光。利用该紫外光观察到了双激子的双光子共振。在很弱的激发密度(1kW/cm^2)下，γ_M值为24微伏。在~gt；lkw/cm~2激发密度下，γ_M值有所增加。本研究是固体材料本征电子激发态的超高分辨率非线性光谱研究的第一个成功范例，今后将取得多方面的进展。

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.