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published_or_final_version / Physics / Master / Master of Philosophy

In this thesis, the carrier localization effect, the quantum confinement Stark effect (QCSE) and nonlinear optical properties of the as-grown InGaN/GaN multiple-quantum-wells (MQWs) structure and nanopillars with diameters of 100 nm and 160 nm and height of 700 nm have been investigated with linear and nonlinear photoluminescence (PL) techniques, In order to investigate the carrier localization effect and QCSE, “S-shaped” temperature dependent PL peak positions of the samples are quantitatively simulated and analyzed with the localized-state ensemble luminescence model. It is found that after nanotexturing both the carrier localization effect and QCSE become weakened. Moreover, the smaller the pillars the weaker the two effects will be. In addition, the nanotexturing introduced the new radiative recombination pathways of carriers are confirmed on the sidewalls of the nanopillars with cathodoluminescence (CL) spectrum and panchromatic CL image. Two-photon absorption (TPA) induced PL spectra of the three samples are measured to investigate the nonlinear optical properties. A peculiar excitation-power dependence, say I~P1.53, of the PL intensity is revealed. It was proposed that a mixed excitation mechanism, namely two-step successive one-photon absorption occurring in the InGaN well layers and one-step two-photon absorption mainly taking place in the GaN barrier and buffer layers, to interpret the observed phenomenon. Besides, the steady-state energetic distribution of carriers excited via this mixed excitation mechanism is very different from that of carriers via one-step one-photon excitation. In contrast with the case of one-photon PL in the samples, the influence of carrier localization effect becomes weaker in the TPA PL of the two nanopillars. / published_or_final_version / Physics / Master / Master of Philosophy

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Two-photon optical bistability in commercial thin-film interference filters with ZnS and ZnSe spacers is observed with switching times ≳ 200 μs. The accompanying drifting of the bistability loop in time and laser-induced "damage" indicate a thermal mechanism. The problem of water vapor absorption in such filters must be addressed before the inherent potential and advantages of these devices in two-dimensional image processing applications can be realized. Trans- verse effects in GaAs superlattice etalons are observed which cannot be explained on the basis of a plane-wave analysis. The optical nonlinearity in GaAs and diffraction combine to produce drastic effects on the measured beam profiles and bistable loops, due to an intensity-dependent virtual focus. Lastly, the first observation of optical bistability due to a biexcitonic nonlinearity in CuCl etalons is reported with detector-limited switching times of 600 ps. In addition, lasing action along the pump axis in a cavity defined by the end mirrors is observed in a very thin (2.0 μm) CuCl etalon. The lasing transition is unique in that it involves the creation of a virtual excitation of biexcitons which decay to the longitudinal exciton state.

Nearly all the deficiencies of conventional vacuum evaporated coatings trace to a single physical property of condensed films: low packing density. One way to increase packing density is to bombard the growing film with ions during deposition, called ion-assisted deposition (IAD). The beginning chapters of this dissertation analyze IAD as a perturbation of the conventional vacuum evaporation process. The experimental chapters begin with an examination of the effect on moisture penetration behavior of oxygen-ion bombarding completed optical filters. Moisture adsorption and desorption is retarded after bombardment in filters composed of titania and silica, but not in those of zirconia and silica. Bombardment evidently induces a crystalline-to-amorphous transition in titania, causing the surface to swell and occluding the pores. The transition in zirconia is the reverse, and no impediment to moisture appears. Argon-ion-assisted magnesium fluoride (MgF₂) can show ultraviolet (UV) absorption. The primary mechanism is probably the formation of F-centers (single fluorine-ion vacancies), although an unsaturated oxygen bond may also be responsible. Absorption can be removed by baking and often by irradiation with UV. After baking, fluorine is lost and replaced by oxygen. Absorption-free MgF₂ films can be deposited by minimizing the substrate temperature and bombardment flux. Ion-assisted films contain up to 2% argon and up to 170 parts-per-million of tungsten from the ion gun filaments. They show a slightly higher refractive index, are much less porous, and are much more resistant to damage by abrasion and exposure to fluorine gas. Ion-assisted aluminum oxide (alumina, Al₂O₃) films show a small increase in UV absorption after argon-ion bombardment; however, a mixture of argon and oxygen ions avoids the problem. Excess oxygen is often incorporated into alumina films, and depresses both the mass density and the refractive index. IAD increases refractive index and decreases porosity. Ion-assisted alumina films are somewhat more stable in humid environments. Ion-assisted deposition has been shown by this study to cause substantial improvements in many of the physical and some of the optical and chemical properties of evaporated magnesium fluoride and aluminum oxide films.

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