Photomodulated reflectance study of InxGa1-xAs/GaAs/AlAs microcavity vertical-cavity surface emitting laser structures in the weak-coupling regime: The cavity/ground-state-exciton resonance

Two InGaAs/GaAs/AlAs vertical-cavity surface emitting laser (VCSEL) structu res have been studied by conventional reflectance (R) and photomodulated re flectance (PR) spectroscopies at similar to 300 K and similar to 80 K. Grow th variations across the samples (<2%) give rise to smooth changes in the c avity mode energy so that it can be tuned through the position of resonance with the quantum well (QW) ground-state exciton, by varying the position o f measurement. The R spectra show the cavity mode but at best only a weak e xcitonic feature. In contrast, the PR shows two prominent and distinct sign als, and there is a strong enhancement (up, to 40 times) at resonance. A th eory has been developed for the PR modulation of the coupled cavity and exc iton modes, based on energy dependent Seraphin coefficients. This was used to fit all the PR spectra simultaneously in each complete set of position d ependent measurements, using seven parameters, only one of which, the cavit y mode energy, varied significantly. The resulting cavity made and excitoni c energies do not clearly slow an anticrossing behavior near resonance, imp lying only a weak exciton-cavity coupling. The ability of PR to detect, in a nondestructive manner, both the cavity and exciton modes, and the extent to which they are in resonance, suggests it could be extremely useful in th e characterization of VCSEL structures near their operating temperature. [S 0163-1829(99)01904-9].