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published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor 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.

Using laser pulses of duration comparable to semiconductor relaxation and dephasing times, the coherent phenomena common to two-level systems have been demonstrated in II-VI and III-V semiconductors. These light-matter interactions are characterized by electrons and holes in their initially excited states and/or a macroscopic polarization that persists after the excitation. A nonthermal carrier distribution was observed in bulk CdSe, demonstrating the extremely fast energy relaxation possible when carrier-LO-phonon scattering is included. Quantum-confined CdSe microcrystallites were then employed to limit the available decay avenues. The one and two-pair transitions were characterized, and using spectral hole-burning, energy relaxation was shown to be substantially slower than in bulk. Dipole dephasing remained very rapid. Persistent macroscopic polarization was demonstrated in semiconductors through pump and probe experiments conducted on time scales faster than the inverse linewidth of the exciton. In the first set of experiments, it was found that the exciton optical Stark shift deviates from that predicted by a steady-state theory. The shift is smaller than expected and lasts longer than the pump pulse. Both effects are explained by free evolution of the exciton's polarization. Transient exciton bleaching is observed and shown to be a result of adiabatic following. In a second study of coherent polarization, spectral oscillations at negative delay times were studied. Oscillations were measured in a wide variety of semiconductors, temporally preceding several different pump-probe phenomena. By comparison with a semiclassical theory, it was shown that spectral oscillations in the spectral region of the exciton are evidence of persistent exciton polarization. Oscillations in the band are due to four wave mixing of the pump and probe beams.

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.

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Ha Kwong-leung = 不同的生長處理對II-VI/III-V外延層的表面、結構和光學特性的影響 / 夏廣良. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2001. / Includes bibliographical references (leaves 76-79). / Text in English; abstracts in English and Chinese. / Ha Kwong-leung = Bu tong de sheng chang chu li dui II-VI/III-V wai yan ceng de biao mian, jie gou he guang xue te xing de ying xiang / Xia Guangliang. / Acknowledgments --- p.i / Abstract --- p.ii / Chapter Chapter 1 --- Introduction / Chapter 1.1 --- Brief introduction --- p.1 / Chapter 1.2 --- Background of the epitaxial growth --- p.4 / Chapter 1.3 --- Our work --- p.6 / Chapter Chapter 2 --- Experimental procedures / Chapter 2.1 --- Substrate preparation --- p.7 / Chapter 2.2 --- Metal-organic sources --- p.9 / Chapter 2.3 --- Growth conditions --- p.10 / Chapter Chapter 3 --- Characterization methods / Chapter 3.1 --- Surface morphology --- p.13 / Chapter 3.2 --- X-ray diffraction --- p.14 / Chapter 3.3 --- Optical properties / Chapter 3.3.1 --- Photoluminescence --- p.17 / Chapter 3.3.2 --- General properties of PL spectrum of ZnSe/GaAs --- p.18 / Chapter 3.4 --- Other techniques --- p.21 / Chapter Chapter 4 --- Influence on the properties of epilayers by the thickness of the low-temperature buffer layer --- p.22 / Chapter 4.1 --- Effects on the surface morphology --- p.23 / Chapter 4.2 --- Effects on the structural properties --- p.31 / Chapter 4.3 --- Effects on the optical properties --- p.35 / Chapter Chapter 5 --- Influence of thickness of epilayers on their properties --- p.39 / Chapter 5.1 --- Effects on the surface morphology --- p.40 / Chapter 5.2 --- Effects on the structural properties --- p.45 / Chapter 5.3 --- Effects on the optical properties --- p.50 / Chapter Chapter 6 --- Influence on the epilayers by passivating with different MO sources and different durations of interruption --- p.53 / Part I: Passivation by different MO sources / Chapter 6.1 --- Effects on the surface morphology --- p.55 / Chapter 6.2 --- Effects on the structural properties --- p.58 / Chapter 6.3 --- Effects on the optical properties --- p.61 / Part II: Different durations of interruption / Chapter 6.4 --- Effects on the surface morphology --- p.63 / Chapter 6.5 --- Effects on the structural properties --- p.66 / Chapter 6.6 --- Effects on the optical properties --- p.70 / Summary --- p.73 / Conclusions --- p.74 / References --- p.76

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published_or_final_version / abstract / toc / Physics / Doctoral / Doctor of Philosophy

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