Abstract

Nonlinear channel waveguides can serve as building blocks for all-optical processing devices such as directional couplers, Mach-Zehnder interferometers, and ring channels. An attractive nonlinear medium for such devices is an excitonic system such as a MQW structure, since the nonlinearity is large and the operating wavelength is compatible with solid-state lasers. In contrast to electronic or bandfilling nonlinearities, the calculation of the parameters of an excitonic system is complicated by the fact that the absorption as well as the nonlinear refractive index are a function of the local intensity and the detuning frequency. We have used a theoretical model of a MQW structure which takes into account the broadening and the bleaching of the exciton resonance line. We find that the conventional figure of merit, n₂/α, is inadequate to describe the required parameters for designing a nonlinear channel waveguide. Rather, one should calculate the total phase shift $\Delta \Phi ={\displaystyle {\int }_0^{L\left(I_{\pi }\right)}k}\Delta n[(I(x)]dx$ where I_πis 1/e of the incident intensity. Taking into acccount the Urbach tail as the background absorption, one obtains realistic parameters for constructing a nonlinear channel waveguide in GaAs MQW structures. We have also calculated the nonlinear response of submicrometer spheres of GaAs or MQW structures embedded in a linear dielectric. Applications of the pure and diluted excitonic systems for nonlinear channel waveguide devices will be discussed, and their relative merits compared.

© 1985 Optical Society of America