Transition metal-doped zinc chalcogenides: Spectroscopy and laser demonstration of a new class of gain media

The absorption and emission properties of transition metal (TM)-doped zinc chalcogenides have been investigated to understand their potential application as room-temperature, midinfrared tunable laser media. Crystals of ZnS, ZnSe, and ZnTe, individually doped with Cr2+, Co2+, Ni2+, or Fe,2+ have been evaluated. The absorption and emission properties are presented and discussed in terms of the energy levels from which they arise. The absorption spectra of the crystals studied exhibit strong bands between 1.4 and 2.0 μm which overlap with the output of strained-layer InGaAs diodes. The room-temperature emission spectra reveal wide-band emissions from 2-3 μm for Cr and from 2.8-4.0 μm for Co. Cr luminesces strongly at room temperature; Co exhibits significant losses from nonradiative decay at temperatures above 200 K, and Ni and Fe only luminesce at low temperatures. Cr2+ is estimated to have the highest quantum yield at room temperature among the media investigated with values of ∼75-100%. Laser demonstrations of Cr and Cr have been performed in a laser-pumped laser cavity with a Co pump laser. The output of both lasers were determined to peak at wavelengths near 2.35 μm, and both lasers demonstrated a maximum slope efficiency of approximately 20%. Based on these initial results, the Cr2+ ion is predicted to be a highly favorable laser ion for the mid-IR when doped into the zinc chalogenides; Co2+ may also serve usefully, but laser demonstrations yet remain to be performed.