Spectral line splitting due to exciton-photon interaction in GaAs/AlAs multiple quantum wells

We studied the reflection and photoluminescence spectra of GaAs/AlAs multiple quantum well (MQW) structures (30 periods) in the temperature range (5-140 K). Although the MQW period is ∼λ/6n [where λ is the wavelength of the (e1:hh1)1S exciton spectral range and n is the average refractive index of all the layers], these structures exhibit a line splitting that is due to the exciton interaction with resonant photons. The splitting is larger than the exciton linewidth and it is found to be enhanced particularly in samples that had their substrate removed and thus there is a strong internal reflection at the two end surfaces. The reflection spectra are analyzed by a model based on transfer matrices that describe light propagation through the MQW and expressing the energy-dependent refractive index in the exciton spectral range by a Lorentzian oscillator response function. The large values of the exciton-photon interaction strength (obtained by the model fitting) indicate that a coherent interaction between excitons in different wells is established by the resonant interaction with the photons.