This Small Business Innovation Research Program will realize substantial improvements in the p-Ge laser gain medium. Far-infrared lasers based on inter-sub-band transitions of holes in p-type Ge span the broad spectral range of 1-4 THz and are the only solid state lasers in that region. They hold promise for molecular spectroscopy, chemical sensing, secure free-space local-area communications in visually obscured environments, and inter-satellite telemetry. At present, p-Ge lasers are limited by low gain, cryogenic operation, and low duty caused by overheating. Since the laser mechanism requires extremely uniform excitation current throughout the active crystal, gain in traditional melt-grown p-Ge crystals is far from optimum because of acceptor-concentration inhomogeneity. To overcome this feature, neutron transmutation doping of pure Ge crystals is proposed. The products generated by neutron irradiation decay rapidly into stable isotopes, primarily 71 Ga, which is a usual acceptor for p-Ge laser crystals. The highly homogeneous thermal neutron flux in nuclear reactors and low neutron-absorption cross section for natural Ge isotopes provide the highest possible uniformity for the resulting acceptor distribution. Small-signal gain will increase up to 10 times, allowing higher operating temperatures, reduced dimensions, and potentially continuous-wave operation.
