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

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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.