Frequency conversion for infrared generation in monolithic semiconductor waveguides

Widely tunable mid infrared radiation achievable using quantum cascade lasers (QCLs) often requires external cavities and several QCL chips to cover a large bandwidth similar to the range reported here (∼ 1000s nm). The cost and mechanical stability of these designs leaves room for alternative more rugged approaches, which require no cavities to achieve very broad band tunability. While difference frequency generation (DFG) will unlikely match the power levels achievable from QCLs, it can provide spectral brightness and extremely wide tunablity, which can be valuable for numerous applications. Recently, we have demonstrated that dispersion engineering techniques can be used for phase matching of second order nonlinearities near the bandgap in monolithic waveguides. In this work we demonstrate an extremely simple structure to grow and fabricate, which utilizes dispersion engineering not only to achieve phase matching but also to expand the tuning range of the frequency conversion achieved in a waveguide through difference frequency generation. Frequency conversion in monolithic AlGaAs single-sided Bragg reflection waveguides using χ⁽²⁾ nonlinearities produced widely tuneable, coherent infrared radiation between 2-3 μm and 7-9 μm. The broad tunability afforded by dispersion engineering and possible current injection, waveguide width chirping and temperature tuning makes it possible to produce a single multi-layer substrate to generate mid-IR signals that span μms in wavelength.