Cr/sup 2+/-doped zinc chalcogenides as efficient, widely tunable mid-infrared lasers

Transition-metal-doped zinc chalcogenide crystals have recently been investigated as potential mid-infrared lasers. Tetrahedrally coordinated Cr/sup 2+/ ions are especially attractive as lasants on account of high luminescence quantum yields for emission in the 2000-3000-nm range. Radiative lifetimes and emission cross sections of the upper /sup 5/E state are respectively /spl sim/10 /spl mu/s and /spl sim/10/sup -18/ cm/sup 2/. The associated absorption band peaked at /spl sim/1800 mm enables laser-diode pumping of the Cr/sup 2+/ systems. Laser demonstrations with ZnS:Cr and ZnSe:Cr (using a MgF/sub 2/:Co/sup 2+/ laser pump source) gave slope efficiencies up to 30%. Excited-state-absorption losses appear small, and passive losses dominate at present. Tuning experiments with a diffraction grating produce a tuning range covering at least 2150-2800 nm. Laser crystals can be produced by Bridgman growth, seeded physical vapor transport, or diffusion doping. Zinc chalcogenide thermomechanical properties of interest for medium-to-high-power operation compare favorably with those of other host materials, except for the larger refractive-index derivative dn/dT.