Second-order intersubband nonlinear-optical susceptibilities of asymmetric quantum-well structures

A theory of second-order nonlinear susceptibility based on intersubband processes in asymmetric quantum-well structures is developed. Large off-resonance nonlinearity is predicted, and its origin is explained. The calculations have shown that the second-harmonic coefficient in the 10-μm range reaches 5 × 10−9 m/V and the Pockels coefficient reaches 2 × 10−8 m/V. The dependence of second-order nonlinear coefficients on band-gap offsets, effective masses, well geometries, and doping concentration is investigated. The intersubband absorption is shown to be the main factor limiting the efficiency of quantum-well-based nonlinear-optical devices. The expressions for the absorption-limited efficiency of second-order nonlinear devices are obtained. The results show that asymmetric quantum-well structures can be efficiently used in many infrared applications.