a Grating Coupling Study of Gallium-Arsenide - Aluminum Gallium-Arsenide Multiquantum Well Waveguides.

GaAs-Al(,x)Ga(,1-x)As multiquantum well waveguides (MQW) are investigated using the grating coupling technique. The effective index of the waveguide mode is used as a probe of the quantum well properties, since the mode index reflects the nature of the complex dielectric function. Photoluminescence, excitation, and absorption spectroscopy are used to optically characterize the MQW samples. All spectra confirm the presence of the two-dimensional (2D) room temperature excitons and also show that the grating fabrication process is non-destructive to the excitons and the MQW waveguide core. Techniques for the fabrication and characterization of GaAs ridge waveguides and holographic gratings are described. Attenuation measurements reveal that low-loss straight and curved dielectric waveguides can be fabricated when dry etch techniques are employed. The results of grating coupling to single mode MQW slab waveguides are presented. Large polarization dependent effects of the 2D excitons are observed in the TE/TM waveguide mode dispersion relation. By treating the MQW as a layered composite dielectric, we deduce the oscillator strengths of the heavy and light hole excitons. The background dielectric constant of the MQW, for fields polarized parallel and perpendicular to the plane of the MQW layers, is also derived. The optical properties of the MQW core are found to be anisotropic and strongly dependent on the state of polarization of the optical field. Valence band mixing effects in undoped MQW samples, and the possibility of observing such effects, are also discussed. The application of the grating coupling technique to the measurement of induced refractive index changes is considered for two specific cases. Nonlinear grating coupling is investigated using nonresonant picosecond optical excitation of the MQW. We also examine electrooptic effects in a parallel field geometry using a metal-clad ridge waveguide with an etched grating coupler. Results are discussed in light of the limitations and problems encountered in the above experiments.