A protective layer laser hardens an optical detector. The material for therotective layer is Hg.sub.1-Y Cd.sub.Y Te, where Y is selected so that the band gap of the protective layer is higher than the expected energy level for photons impinging on the protective layer. Photons with energy levels lower than the band gap are transmitted by the protective layer while photons exceeding the band gap energy level are absorbed or reflected by the protective layer. A semiconductor junction can be formed on the opposite side of the substrate from a Hg.sub.X Cd.sub.X Te layer with a band gap lower than the expected energy level, so that photons transmitted through the substrate are absorbed in the Hg.sub.X Cd.sub.X Te layer and, therefore, detected at the junction. At sufficiently high intensities where detector damage could result, the protective layer switches so that the incident photons are either absorbed or reflected, thus protecting the detector from damage.