SAW-induced birefringence in InGaAs-GaAs multiple quantum well acousto-optic modulators

A theoretical study of surface acoustic wave (SAW)-induced absorption coefficient and refractive index change in InGaAs-GaAs multiple quantum well (MQW) structures is presented. We use a two-dimensional Gaussian quadrature method to solve for the exciton oscillator strength in two-dimensional momentum space. This method accounts for the asymmetric strain and electric field induced by the SAW. The optical absorption coefficient and refractive index changes near the band gap in MQWs are calculated as a function of SAW power at different well widths. We have found that SAW-induced birefringence is significantly dependent on the intrinsic strain. We computed birefringence Δn=0.06 in 0.7 % tensile strained InGaAs-GaAs MQWs for the two different polarizations (parallel and perpendicular to the SAW propagation direction) under normal incident light at a SAW power of 10 mW/λ_SAW. This is a great improvement over our earlier compressively strained (∼-1 %) InGaAs-GaAs MQW modulators (Δn=0.01). The calculations are compared with absorption coefficient measurements on compressively strained InGaAs-GaAs MQW modulators.