Linear and nonlinear optical properties of direct gap Si-Ge superlattices

Optical properties of Si-Ge superlattices grown on virtual substrates are investigated with a view to engineering optimum characteristics. The oscillator strength and transition energy, both across the fundamental gap and between the conduction sub-bands, are examined as a function of the choice of substrate and well/barrier width. In the former case, the transition energy is shown to be tunable in the range 0.6-1.0 eV which covers the technologically important optical fibre window at 1.55 μm. Intersub-band transitions occur at wavelengths in the far infrared. A strong nonlinear optical response can also be achieved via a novel virtual excitation mechanism. Growth parameters are presented for systems in which this response is maximised while keeping the absorption to a minimum