Modelling of acousto-optic and electro-optic quantum-wells modulation devices.

In this thesis, the surface acoustic waves (SAWs) and quantum well (QW) interdiffusion technologies are used to develop modulation devices including electrooptic and electro-absorption QW modulators. The modulation devices are crucial components for generating optical signals in opto-electronic circuits in the use of communication systems. The propagation of SAWs and it effects on a III-V semiconductor QW are modelled to determine the change of the QW subband structure. The change of optical properties is evaluated so that the optical modulators using SAWs can be studied and optimised. For the interdiffusion technology, the constituent atom composition after interdiffusion is modelled by solving the diffusion equations. The modifications of the QW subband structure which change the QW optical properties are studied here for the use of the QW modulators and lasers. SAWs are generated by interdigital transducers deposited on the top surface of device structures. Our aims here are to optimise the change of optical properties, including the change of the QW refractive index and absorption coefficient and to simplify the structure of the transducer by increasing and reducing the SAW wavelength and power respectively. The theoretical results show that by optimising the QW structures in a stack from a single QW to a diffused QW (DFQW) and pairs of asymmetric double QW (DQW), the optical change can be enhanced 100 times as compared to conventional bulk SAW modulators. Besides, the SAW frequency and power can be reduced to ~100MHz and few mW per SAW wavelength respectively. Recently, SAWs with a frequency of ~ 100MHz have been demonstrated in single QW by others. These suggest that SAW-QW modulators can be realised in the near future. DFQWs used as both the passive cladding regions and active region of modulation devices are studied here. The results show that an electro-optical phase modulator with disorder delineated optical confinement is comparable to existing phase modulators. We also propose to use DFQWs as the active region of optical devices to produce TE and TM modes polarisation insensitive optical properties in both AlGaAs/GaAs and InGaAsP/InP QWs. At the same time, InGaAsP DFQW have been demonstrated by others as the polarisation insensitive QW amplifiers. This indicates the potential of DFQW to realise the polarisation insensitive QW optical devices