JOINT STUDY ON OPTICS OF EXCITONS IN QUANTUM CONFINED SYSTEMS

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The purpose is to investigate the properties of exciton in quantum confined systems and to develop novel nonlinear-optical phenomena related to the excitons.1. The band-offsets in a thin ZnSe/ZnS quantum well (QW) were first determined with combined use of two-photon (TPA) and one-photon absorptions. Furthermore, a prominent enhancement of the exciton binding energy (Ex) due to confinement effect is found. Then, we have found the existence of biexciton for the first time in a II-VI QW by direct TPA,together with manifestation of a new luminescence (PL) line and the PLE spectrum. As a result, the biexcitonic binding energy (Exx) for 20 * well thickness is found to be 7 times as large as that of bulk ZnSe.2. Two novel nonlinear-optical phenomena, i.e., the "forbidden" second-harmonic generation (SHG) and hyper-Raman scattering by LO phonon both in two-photon resonance with 2P exciton are found in ZnSe/ZnS QWs. The fundamental aspects are experimentally clarified, enabling us to interpret the microscopic mechanisms.3. The properties of exciton and biexciton are experimentally studied in ZnSe/ZnSxSel-x QWs (x=0.18), epitaxially-grown on a GaAs substrate with latticematching. In addition to PL and PLE,novel methods newly-developed have been employed ; those are spectroscopies of cw Brewster-angle (BR) reflection, femtosecond transient BR reflection reflecting only free-induction decay and fourwave mixing (FWM) in the BR geometry. First, one-monolayr-terraced structure is found to be formed with each excitonic spectral width dominated by homogeneous brodening. A biexciton is found to stably exist, which is evidenced by the existence of the FWM signal in the negative delay of second pulse. It is found that Exx is almost twice enhanced as compared to that of bulk ZnSe, in contrast to that of exciton.13EA05 : 4. Novel resonant SHG and HR scattering phenomena are also found in CdS and CuBr quantum dots embedded in glasses.