Laser and photoluminescence spectra of InGaAsGaAs strained-layer superlattices

Photoluminescence (PL) and laser emission from optically excited In x Ga 1−x As–GaAs strained-layer superlattices (SLS) grown by molecular-beam epitaxy have been examined. In low-intensity PL, a single, narrow (≈5 nm FWHM) emission peak was observed, corresponding to the n = 1 electron to heavy-hole transition. This feature, as well as a series of higher energy transitions, was also observed in absorption and photoreflectance spectra. High-intensity PL, using a pulsed nitrogen laser for excitation, yielded an additional peak, several millielectronvolts lower in energy than the dominant low-power PL feature. Optical-gain measurements were performed using the variable stripe-length method. The stimulated emission emerging from the single cleaved end was shifted to the long-wavelength side of the low-power PL peak, and in the case of the narrower well samples, the emission consisted of two peaks separated by about 8 meV. Lasing at these gain positions was observed when ≈400 μm long chips were prepared and their lengths were pumped with a stripe of nitrogen laser light. The observed gain spectrum has been attributed to a combination of a free-carrier-induced bandgap shift and self-absorption by unpumped material deeper within the SLS waveguide.