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A new type of dichroic reflector has been developed for the far‐infrared region of the spectrum, based on n‐type germanium with a diffused conduction layer. A 111‐axis single‐crystal plate of 40‐Ω cm optical‐grade germanium was implanted with 5×1014 As+ ions/cm2 by ion bombardment at 200 keV; the donor impurity ions were subsequently diffused into the substrate at a temperature of 850 °C (held for 20 h). The measured power‐reflection spectrum displayed a shallow minimum at ∼70 cm-1, rising steeply below 50 cm-1 to reach 80% reflectivity at 10 cm-1. Computer modeling of the infrared reflectance of a thermally diffused conducting layer in n‐type germanium—with the assumption of Conwell–Weisskopf mobility variation for the conduction electrons—gave an excellent fit to the observed reflection spectrum for an integrated column density of 1014 cm-2 and 10‐μm half width, the latter being in good agreement with the value √Dt=11 μm calculated for thermal diffusion under the conditions indicated.