OPTICAL Stark EFFECTS AND OPTICAL NONLINEARITY AT LOW-DIMENSIONALLY CONFINED EXCITONS IN LAYERED CRYSTALS

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In this study, several mechanisms of nonlinear optical responses have been clarified especially in the low-dimensionally confined exciton systems at stacking disorders in layered crystals. A theoretical research on basic processes in a highly excited exciton system has been done. Main results obtained are as follows: 1. Transient absorption spectra at the qusi two-dimensional exciton system localized at specific stacking fault in BiI_<delta> have been investigated up to the pico second time region by means of the pump-probe method. (1) The optical Stark effect by the pump-field was clearly observed. The spectral change was analyzed on the basis of "dressed exciton model". As a result, extraordinarily large non-linear susceptibility was obtained reflecting a large electric-dipole moment in this system. (2) It turns out that even after the pump-field, really excited high-density excitons interact with each other resulting in the spectral change characteristic in the two-dimensional syste … More m. (3) From the degenerate four-wave-mixing measurements, the dephasing mechanism was made clear: under the low excitation, the intra-band LA-phonon scattering governs the dephasing process, while for the high excitation, exciton-exciton scattering becomes dominant in the process. (4) Other nonlinear optical responses: the bi-exciton state coexists with the high density exciton phase, and the occurrence of bistability with respect to the density was observed.2. Besides the above results, we observed the induced Raman lines in BiI_3, and SBI_3 crystals. Furthermore, we found that under the hydrostatic pressure, a new exciton transition appears in SBI_3, which gives a new information on quantum confined exciton states.. We studied an electron-hole system in a direct gap semiconductor which is coherently excited by a high-intensity laser. Using a mean-field approximation, we have showed that the many body effects via the screened Coulomb interactions bring about the density bistability with respect to the frequency of the laser field, in which a low density state is regarded as the exciton gas and a high density state is the BCS state of electron-hole Cooper-pairs. Less