Optimization of normal-incidence GaAs-AlGaAs multiple quantum well optical modulators

This thesis concerns the experimental and theoretical optimization of electric field-induced optical modulation in GaAs-AlGaAs multiple quantum well (MQW) structures with light incident normal to the quantum well layers. The basic modulator device consists of an epitaxially-grown p-i-n diode, which contains the quantum wells in the intrinsic region and allows the absorption and the refractive index of the MQWs to be modified by the application of small reverse bias voltages. Room temperature photocurrent and transmission spectroscopy have been used to determine electric field effects on the absorption in GaAs-AlGaAs MQWs, and also to assess the possible improvements to be made in MQW modulator performance by optimum choice of well and barrier thicknesses. The devices investigated have been fabricated from material grown by both high-resolution epitaxial techniques, namely metal-organic vapour phase epitaxy (MOVPE) and molecular beam epitaxy (MBE). The details of device fabrication and measurement are discussed briefly. From the measurements of absorption changes in various MQW devices for a range of applied bias voltages, calculations of the corresponding changes in refractive index have been carried out by the Kramers-Krönig transformation. With the information on electroabsorption and electro-refraction the potential for improved modulator performance by incorporating the MQWs within a Fabry-Perot cavity structure has been assessed, where the incident light makes multiple passes through the MQW structure, resulting in destructive or constructive interference depending on the wavelength and bias voltage. The general modelling of MQW Fabry-Perot modulators has ultimately led to the experimental investigation of an asymmetric cavity device (low front and high back surface reflectivity). This type of structure operates solely in reflection, thus avoiding the need for substrate removal, and offers large, broadband reflection changes and very high on: off ratios with low operating voltages