The invention relates to laser materials and specifically to single crystal active media for solid state lasers operating in the middle-infrared (Mid-IR) 4-5 .mu.m range of optical spectrum, wherein a cross section of stimulated emission and laser wavelength is shifted to the range of atmospheric transparency with a single crystal orthorhombic structure with space group D.sup.24.sub.2h-Fddd. The material contains Lead, Gallium, Sulfur and is doped with Dysprosium according to general formula: Pb.sub.1-3xDy.sub.2xGa.sub.2S.sub.4, where 0.0001