Novel two-photon reasonant nonlinear-optical phenomena and their application for exploring electronic structures in solids

在凝聚态物质中发现了一些由激发态双光子共振引起的新的非线性光学现象。这些现象是：1)在通常的偶极近似下，雾几何中禁止的共振二次谐波产生；2)LO声子的共振超拉曼散射；3)布鲁斯特角组态中的四波混频现象。对它们的轮廓进行了详细的实验研究，从而阐明了它们的机理。利用这些现象作为新的光谱工具，阐明了包括双激子在内的几种量子受限半导体中的电子结构，如II-VI复合量子阱(QW)和量子点(QD)。特别是首次测量了超薄的ZnSe/ZnS量子阱中的能带偏移量和激子结合能ex，发现了由于限制而引起的Ex的显著增强。首次证明了双激子的存在，这是首次在两种材料中发现双激子的存在。结果还观察到双激子结合能的显著增强。为此，还发展了新型的光谱技术，即双光子吸收、连续布鲁斯特角反射和飞秒脉冲在布鲁斯特角几何中的瞬时(时间分辨)反射，这些反射主要反映相干激发的自由感应衰减。因此，所有这些新发展的现象都被证明为探索电子结构，特别是在量子受限系统中提供了有用的工具。目前的成果已经或将以8至10篇论文的形式发表。

A few novel nonlinear-optical phenomena due to two-photon resonance with excited state in condensed matters are found. Those phenomena are 1) resonant second-harmonic generation in the fogeometry forbidden in the usual dipole approximation, 2) resonant hyper-Raman scattering by LO phonon, and 3) four-wave mixing phenomenon in the Brewster-angle configuration. Their profiles were experimentally studied in detail, and thereby the mechanisms have been clarified. By using these phenomena as novel spectroscopic tools, the electronic structures including biexciton in several quantum-confined semiconductors such as II-VI compound quantum wells (QWs) and quantum dots (QDs) are clarified. In particular, the band-offsets and the excitonic binding energy EX in an ultra-thin ZnSe/ZnS QW have been for the first time determined ; a prominent enhancement of Ex due to confinement is found. The existence of a biexciton is evidenced for the first time in both ZnSe/ZnS and ZnS/ZnSSe QWs. As a result, striking enhancement of the biexciton binding energies are also observed. For these purposes, new-types of spectroscopic techniques, i.e., two-photon absorption, cw Brewster-angle reflection, and transient (time-resolved) reflection of a femtosecond pulse in the Brewster-angle geometry, which dominantly reflects free-induction decay of coherent excitation, have also been developed.Thus, all those phenomena newly developed have been demonstrated to provide useful tools for exploring electronic structures, particularly in quantum-confined systems. The present outcomes have been or will be published in 8 to 10 papers.