Abstract

Due to the possibility of realizing strong exciton-photon coupling in semiconductor microcavities, one could expect that also biexcitons are strongly modified by polariton effects, leading to formation of bipolaritons. In this work, we have investigated the nonlinear optical properties of a high-quality quantum well (QW) microcavity in time-resolved pump-probe spectroscopy at T=5K. The sample consists of a 25nm GaAs single QW in the center of a wedged λ-cavity with AlAs/AlGaAs Bragg reflectors. Due to the small heavy-hole light-hole splitting there are three polariton modes visible near zero detuning, where the ratio between the heavy-hole Rabi splitting and the linewidth is as large as 19. In the experiment, the pump spectrum is shaped in order to resonantly excite only the lower polariton in the microcavity. A well resolved pump-induced optical absorption is observed in the probe reflectivity spectrum for cross-circular polarized pump and probe photons at zero relative delay, which disappears for co-circular polarization. This is shown in the figure (see arrow) together with the reflectivity spectrum without pump, as a reference. This transition is identified as a pump-induced absorption from the lower polariton (LP) to the heavy-hole biexciton (XX), which is not significantly affected by the coupling with the cavity photons. By changing the detuning between the cavity mode and the heavy-hole exciton the LP-XX transition shifts in energy, consistently with a biexciton binding energy comparable to the one in the bare QW. The dynamics of the LP-XX transition is measured by changing the delay time between pump and probe pulses. It follows the LP lifetime given by the photonic lifetime and content in the LP, as expected. As recently pointed out theoretically [1], the structure of the bound biexciton state (i.e. binding energy and radius) is not significantly affected by the coupling with the cavity photons since the exciton-photon coupling strongly modifies only a very small region of the in-plane wave vectors in the microcavity and the constituent excitons in the biexciton extend over a region of large wavevectors. The formation of a bipolariton as a bound state of two lower cavity polaritons with small in-plane wavevectors is not resolved, in spite of the high quality of the sample. This is in agreement with the calculated exponentially small bipolariton binding due to the small effective mass in the lower cavity polaritons [1].

© 2000 IEEE