From a comparison of the thermoreflectance (first-derivative spectroscopy) and photoreflectance [electromodulation (EM)] spectra at 300 K from a GaAs/${\mathrm{Ga}}_{0.82}$${\mathrm{Al}}_{0.18}$As multiple-quantum-well structure we have obtained important information about the mechanisms of EM for uncoupled and coupled states. For the latter EM produces a third-derivative line shape due to low-field modulation of tunneling states. For the former the two spectra are almost identical, verifying the first-derivative nature of EM at 300 K related to modulation of exciton transitions.

• Received 24 November 1987

DOI:https://doi.org/10.1103/PhysRevB.37.10919