Diffused germanium dichroic mirror for optically pumped far-infrared lasers

Abstract

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.