Efficient InSb laser with resonant longitudinal optical pumping

External quantum efficiency of > 20 percent has been achieved from tunable CW InSb lasers resonantly pumped with a CO laser. Thin InSb platelets at < 10 K were pumped in a longitudinal geometry with the InSb recombination emission paralleI to the CO radiation. Single-longitudinal-mode operation with power up to 10 mW has been achieved with a 0.1-0.3-cm^-1linewidth, mechanically or thermally tunable over several cm^-1. Magnetic-field effects on the threshold and emission frequency have also been studied.