In recent years, chalcopyrites have distinguished themselves as the nonlinear optical materials of choice for mid- to far-infrared (ir) laser applications. In particular, AgGaSe2, ZnGeP2, and CdGeAs2 have demonstrated the highest conversion efficiencies and output powers in the wavelength range beyond 4 μm. The superior performance of these crystals arises from their high nonlinear optical coefficients (39 pm/V, 75 pm/V, and 236 pm/V, respectively), their relatively large birefringence (sufficient for phase matching), and advances in crystal growth and processing that have improved transparency and eliminated cracking.
The two most direct approaches to generating laser output in the mid-ir (in particular the 3–5-μm atmospheric transmission window) are (1) shifting the output of a solid-state laser to longer wavelengths via optical parametric oscillation (OPO), or (2) doubling the frequency of a CO2 laser (9–11 μm) via second-harmonic generation (SHG). The OPO approach offers the advantage of tunability combined with potentially more compact and efficient solid-state lasers, whereas the SHG approach benefits from the greater maturity of high-power CO2 laser technology. A third process, difference-frequency generation (DFG), is also a 3-wave interaction similar to OPO that can be used to mix two photons (from two lasers or from an OPO) to produce longer wavelength photons (corresponding to the small difference frequency) over a large spectral range. The optimum chalcopyrite crystal (AgGaSe2, ZnGeP2, or CdGeAs2) for a given approach depends on a complex combination of material parameters described in the sections that follow.