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

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Abstract
Two InGaAs/GaAs/AlAs vertical-cavity surface emitting laser (VCSEL) structures have been studied by conventional reflectance (R) and photomodulated reflectance (PR) spectroscopies at ∼300 K and ∼80 K. Growth variations across the samples (<2%) give rise to smooth changes in the cavity 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 of measurement. The R spectra show the cavity mode but at best only a weak excitonic feature. In contrast, the PR shows two prominent and distinct signals, and there is a strong enhancement (up to 40 times) at resonance. A theory has been developed for the PR modulation of the coupled cavity and exciton modes, based on energy dependent Seraphin coefficients. This was used to fit all the PR spectra simultaneously in each complete set of position dependent measurements, using seven parameters, only one of which, the cavity mode energy, varied significantly. The resulting cavity mode and excitonic energies do not clearly show an anticrossing behavior near resonance, implying 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 the characterization of VCSEL structures near their operating temperature.