published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

This dissertation studies the physics of room-temperature optical nonlinearities in GaAs and their application to the optical logic gates. The microscopic origins of the room-temperature optical nonlinearities in GaAs are investigated experimentally and theoretically. The data of nonlinear absorption measurement are analyzed in the framework of a semiconductor plasma theory in combination with excitation-dependent line broadening. The importance of the plasma screening of the continuum-state Coulomb enhancement and band filling are emphasized for GaAs at room temperature. Optical bistability and optical logic gating are direct consequences of the nonlinear refractive index changes in etalons. The nonlinear index changes are directly measured by a new technique of observing the Fabry-Perot transmission peak shift using the self-photoluminescence as a broad-band source. The validity of a Kramers-Kronig technique under quasi-steady state conditions is crosschecked by an independent measurement of Δn under identical pumping conditions. Thermal index changes are also directly measured to establish the criteria on the temperature stability condition that is needed for reliable operation of devices based on dispersive nonlinearities. Optical logic gates based on dispersive optical nonlinearities may be the critical components of an all-optical computer in the future. Five optical logic functions are demonstrated in a nonlinear GaAs/AlGaAs MQW etalon. Specially designed dielectric mirrors are used to observe low-energy (3-pJ) operation of optical logic gates. Parallel operation using as many as eight optical logic devices is achieved with Wollaston prisms. Toward practical devices, optical logic gating using diode lasers is demonstrated in a setup much smaller than the usual argon-laser pumped dye laser setup. The cycle time of optical logic devices is limited, not by the switch-on time, but by the switch-off time which depends on the carrier relaxation rather than the switch-on time. To reduce the carrier relaxation time windowless GaAs is employed to take advantage of the faster surface recombination of carriers at the GaAs/dielectric mirror interface compared to that at the GaAs/GaAlAs interface. The speed and effectiveness of the windowless GaAs are compared with those of the proton-bombarded GaAs as optical logic gates.

Nonlinear-optical switching and logic devices based on GaAs nonlinear Fabry-Perot etalons have been investigated theoretically and experimentally. The theoretical modeling has been performed with the first realistic and easily computed theory of GaAs nonlinear optical properties near the band edge. Both steady-state and dynamic calculations have been performed for optical bistability with GaAs etalons. High-transmission operation is predicted for certain etalon detunings from the excitation wavelength. Various logic-gate functions have simulated with the model. An investigation of differential energy gain in transient, one-wavelength operation was performed. The conclusion is that useful differential gain is not achievable in transient, one-wavelength operation if the pulse width is less than about ten times the carrier lifetime in the material. Waveguide structures with single-mode transverse confinement were designed and optical bistability was predicted for long GaAs etalons similar to cleaved waveguides. GaAs nonlinear optical devices were fabricated in forms of interest for application to optical parallel processing and guided wave signal processing. The fabrication work included etalon arrays and waveguide devices fabricated by reactive ion etching. The photolithography and reactive ion etching processes used and developed are described. Preliminary work on ultra-small quantum-confinement structures is described. Optical experiments were performed on the devices fabricated. The etalon arrays demonstrated extremely fast relaxation times for GaAs etalon devices, and demonstrated the ability to control material parameters through the fabrication process, by increasing the surface recombination rate of charge carriers. Fast optical bistability at low powers was also demonstrated in the array devices. Strip-loaded waveguides with cleaved ends were operated as optical bistable devices with conclusive evidence that the mechanism was electronic in origin. Nonlinear phase shifts of greater than $2\pi$ were observed in some waveguides. Such large nonlinear phase shifts are of great interest for the development of other nonlinear-optical waveguide devices.

Gallium arsenide (GaAs) elicited a pulmonary inflammatory response in a dose dependent manner following a single exposure. A significant influx of leukocytes (polymorphonuclear cells) was observed 24 hours after intratracheal instillation of rats and hamsters. This led to an increase in the lung/body weight ratios. An increase in pulmonary DNA and total protein accompanied these observations. Histology confirmed the presence of increased numbers of pulmonary alveolar macrophages (PAM) even 1 week after exposure to GaAs. The instillation of GaAs also appeared to produce an oxidative stress in the lung only when the animals were given the 100 mg/kg dose and not the 10 mg/kg dose. Increased glutathione peroxidase and nonprotein sulfhydryls and depletion of ascorbic acid were evidence for the oxidative stress produced in the lung. These effects were dependent on the influx of phagocytic leukocytes. Analysis of the bronchoalveolar lavage fluid (BALF) also confirmed the involvement of phagocytic leukocytes in the progression of the lesions. Acid phosphatase activities increased significantly above the control levels 24 hours after exposure. The elevation of soluble protein and alkaline phosphatase indicated that the type I pneumocyte-capillary endothelial cell interface was compromised and the type II cells were damaged, respectively. The histological evaluations confirmed this phenomenon. Alveolar wall thickening was quite characteristic of the GaAs exposure. GaAs stimulated PAM to produce the active oxygen species, superoxide anion (O₂⁻) and H₂O₂, following in vitro and in vivo exposure. The dissolution of GaAs did not produce and O₂⁻ or H₂O₂ without the presence of cells. The cytotoxicity of GaAs was comparable to other compounds that elicit collagen deposition, As₂O₃ and silica. The semiconductor properties and potential dissolution products of GaAs may both contribute to its toxicity to PAM. The differences seen in the pulmonary lesions of silica (fibrosis) and GaAs (resorption of deposited collagen) treated animals may be due to the persistence of the particles. GaAs may be cleared by dissolution and silica cannot.

Nonlinear optical properties of passive and active semiconductors are investigated experimentally and theoretically. Improvement of switching cycle time in optical nonlinear etalons to 40 ps is demonstrated, and strained-layer InGaAs/GaAs quantum well material is used in an asymmetric etalon to greatly improve switching power and contrast. Coherent energy transfer (CET) induced by injection of an external light field is demonstrated in a GaAs quantum well vertical-cavity surface-emitting laser (VCSEL). The evolution of CET induced asymmetric gain with increasing injected power is investigated experimentally and theoretically, and it is found that the CET induced effective gain peak and dip are detuned proportionally with injected power as in homogeneously broadened media and in contrast to other multi-wave effects in GaAs which are detuned proportionally with the light field. Transfer of gain modification between orthogonally polarized modes of the VCSEL and cascading of gain modification within a mode is observed and investigated. The approach of a laser to an injection locked state through increased injected power is investigated experimentally and theoretically, showing new emission frequencies produced which evolve to chaos-like behavior before reaching the phase locked state. CET induced gain modification is used to demonstrate low-power high-contrast switching between polarization modes of the VCSEL with differential gain of 3,510. Switching speed and switching bistability is observed and investigated. Injection induced modification of VCSEL transverse modes is studied experimentally and theoretically. Field defects in the resulting field are observed, and their locations are dependent on the frequency of the injected field, in contrast to the temporally evolving defects normally observed. The rich behavior of nonlinear properties, especially in gain media provide interesting results and valuable applications.

No description available.

All-optical logic gates made from GaAs etalons were studied to see if they may be useful for optical computing. We have demonstrated that GaAs etalons may produce a change in output optical signal four times larger than the change in the input signal, and that the contrast of the output signal may be as high as 10 to 1. We have cascaded two GaAs etalons, i.e. the output change in the first causes the second one to switch. We have combined two signal beams and a biasing beam onto a GaAs etalon using polarized beams for a fan-in investigation, and have demonstrated that this setup may be used as an all-optical AND gate. We have also shown that GaAs etalons function well when interfaced to optical fibers by direct butt-coupling. Interconnections between all-optical gates by optical fibers, holographic optical elements, and conventional lenses are discussed.

Graduation date: 1992

Graduation date: 1992