Date of Award
Doctor of Philosophy in Electrical Engineering
We studied electroabsorption in GaAs/ AlGaAs Multiple Quantum Well (MQW) structures. The waveguide used for this study was designed as a phase modulator with an active layer containing six 75 Å GaAs quantum wells and 80 Å Al.35Ga.55As barriers.
Electroabsorption in MQW structures is dependent on, among other things, the light polarization, the orientation of the crystal optical axes, and the direction of the applied electric field.
In the first study we measured the spectral dependence of the quadratic electro-optic coefficient for TM polarized light (s33 near the absorption band edge. The light hole exciton resonance occurs at 820 nm for the particular device we used. The spectral range studied was from 840 nm to 880 nm. In this region, the quadratic coefficient demonstrated large values (1.2 x 10-8 cm2 /kV2 at 840 nm) near the band edge. This is about two orders of magnitude larger than that observed in bulk material. It also falls rapidly as the wavelength increases further away from the band edge.
A knowledge of this material parameter permits the calculation of the change in refractive index with an applied field, which is very useful in phase modulator design. This along with the change in the absorption coefficient as a function of the applied field (also measured) determines the operational characteristics of the phase modulator at a given wavelength.
In the second study, we investigated the effect of the crystal optical axes orientation on electroabsorption in MQW's. We reported, for the first time, a difference in the change in electroabsorption by almost a factor of 2 between [-1,1,0] and [1,1,0] oriented structures. This makes the former orientation better suited for intensity modulator/switching devices as one gets larger change in the absorption coefficient with lesser applied voltage.
We attributed that orientation dependence in the absorption coefficient between the two orientations to the anisotropy of the valence subband structure brought about by band mixing due to the quantum wells periodic potential.
In the third study, we devised a novel technique to extract the spectral dependence of the absorption coefficient in these structures from measurements of the transmitted light intensity as a function of the applied voltage at a single fixed wavelength.
Results are in very good agreement with the measurement of the spectral dependence of the absorption coefficient by varying the wavelength. This technique is a rather simple one and does not require a coherent tunable light source which is very expensive and not readily available in every lab.
Abdelkader, Hatem, "Electroabsorbtion in Multiple Quantum Well Structures" (1992). Open Access Dissertations. Paper 387.