Abstract
A theoretical and experimental investigation into the characteristics of photoelastic optical waveguides in bulk silicon and Si1-xGex/Si heterostructures is presented. This is the first experimental demonstration of this type of waveguide in these material structures. The bulk silicon structures are also the first demonstration of channel waveguides defined using only photoelastic confinement. The photoelastic constants of silicon and Si1-xGex, which give the change in refractive index with strain, are calculated from the strain-induced shifts in the energy band structure of silicon and germanium which modifies their extinction coefficient, from which the strain-induced refractive index changes are found from the Kramers-Kronig relations. A finite element model of the waveguide structures is presented which uses the calculated photoelastic constants to determine the refractive index profiles of the waveguides. Subsequently, finite difference calculations are used to calculate the optical mode profiles of the waveguides. Photoelastic waveguides are fabricated by depositing SiNy stressor films onto bulk silicon and Si1-xGex/Si heterostructures which are subsequently cleaved and polished to produce waveguide facets before narrow stressor stripes are defined from the SiNy films using photolithography and wet etching. The characteristics of the waveguides are investigated at wavelengths of 1.15mum and 1.523mum. Measurements show that there is always one guiding region outside each edge of the stressor stripe. The Si1-xGex/Si heterostructures also allow a third mode to be confined under the centre of the stressor stripe, and the relative intensity and the distance between the guided modes is controlled by the stripe width, in good accordance with the modelling results. These structures are interesting in that up to three guiding regions can be defined by the deposition of one stressor stripe on the waveguide surface, which provides a particularly simple and compact way of fabricating waveguide couplers. An interferometer is used to study the force generated by the SiNy stressor layers. It is shown that the as-deposited stressors produce low and poorly defined stresses, although significant forces of up to 2-3.106 dyn/cm are measured after rapid thermal annealing of the structures. Annealing of photoelastic waveguides in bulk silicon show a corresponding increase in photoelastic confinement which produces waveguides with excess losses of down to 4.3dB/cm. Photoelastic waveguides in Si1-xGex/Si heterostructures, due to the additional confinement from the heterojunction, are reported with zero excess losses. At 1.15mum, the band-edge absorption increases the waveguide propagation losses by up to several dB/cm, and the waveguides show multimode behaviour, making these structures unsuitable for applications at this wavelength. At 1.523mum, however, measurements show low excess propagation losses and single-mode behaviour, and they exhibit a low degree of birefringence. The simple fabrication process and compact design of these structures make them appropriate for optoelectronic intragration, and several possible applications for photoelastic waveguides in optical devices are suggested.