Differential gain, differential index, and linewidth enhancement factor for a 4 μm superlattice laser active layer

We describe temporally and spectrally resolved measurements of the material differential gain, differential refractive index, and linewidth enhancement factor for a multilayer superlattice intended for use in midwave-infrared semiconductor lasers. We find good agreement between measured quantities and theoretical predictions based on a superlattice K⋅p formalism. The superlattice was designed for suppression of Auger recombination and intersubband absorption, and we find that the strategies employed in this process result in other characteristics that are desirable in a semiconductor laser gain medium. Specifically, for carrier densities and wavelengths appropriate to threshold in an optimized cavity configuration, this structure has a differential gain of approximately 1.5×10−15 cm2, a value comparable to that reported for near-infrared strained quantum wells. The peak gain and peak differential gain are nearly spectrally coincident, leading to a small value for the differential index. The large differential gain and small differential index result in a linewidth enhancement factor of less than one. This indicates that filamentation in high-power lasers based on this superlattice should be suppressed and that this structure is attractive for use in midwave-infrared lasers designed for spectrally pure operation.