Sum and difference frequency mixing in semiconductor heterostructures

Sum-frequency mixing in a nonlinear waveguide using counter-propagating light beams was first demonstrated more than a decade ago. The subsequent development of quasi-phase matching (QPM) techniques in Al_xGa_1-xAs heterostructure waveguides has improved the conversion efficiencies such that a variety of applications based on second-harmonic (SH) generation are now feasible. Difference-frequency mixing in an Al_xGa_1-x As waveguides can also be used to generate tunable CW microwave radiation. The optimization of devices based on QPM SH generation requires the capability to spatially modulate the second-order nonlinearity χ⁽²⁾. Within the Al_xGa_1-x As material system, a number of approaches to controlling χ ⁽²⁾ have been explored. The simplest approach is to vary the concentration. Once appropriate materials have been found, the geometry of the heterostructure must be designed to optimize device operation