Center-of-mass quantized exciton polariton states in bulk-GaAs microcavities

We present a detailed low-temperature luminescence and reflectivity study of polariton modes in bulk GaAs microcavities. "Bulk" microcavities are structures in which the cavity spacer is, at the same time, the optically active material. High-order center-of-mass quantized exciton states and their anticrossing with the cavity optical mode are clearly resolved in the luminescence spectra. This result indicates that the cavity polariton involves the collective oscillation of all quantized exciton states with the same in-plane wave vector. Spectra of full and half (without top Bragg reflector) cavities are compared and used to test a linear dispersion model that takes into account the polaritonic spatial dispersion in the GaAs layer. We show, in addition, that the lower cavity mode displays a minimum linewidth around zero detuning similar to that observed in quantum-well-embedded cavities. We argue that in bulk microcavities the quantized exciton states can be viewed as an inhomogeneous distribution of levels similar to vertical disorder in quantum-well-based structures.