Quantum beats between light and dark polariton states in semiconductor microcavities

Abstract

We observe experimentally pronounced beats of the intensity of photoluminescence from a bottleneck region of a microcavity in the strong coupling regime. These beats are extremely sensitive to the pumping intensity and vanish for the weak pumping. We show theoretically that the process of polariton-polariton scattering leading to the mixing between bright and dark polariton states is responsible for this effect.Exciton-polaritons in semiconductor microcavities have become an object of intensive experimental and theoretical studies in the recent decade. Cavity polaritons are unique excitations in solid state systems that conserve the coherence over tens of picoseconds despite both elastic and inelastic scattering events. An extremely low decoherence rate of the polaritons is because of the enhanced strength of light-matter coupling in microcavities that dominates over excitonic coupling with the phonon bath. Thanks to this effect many interesting phenomena could have been observed in the microcavities as e.g. stimulated scattering of exciton-polaritons [1] and possibly even their Bose-condensation [2]. Here we report an experimental evidence of a new quantum effect linked to the long-living optical coherence in microcavities. We have observed the quantum beats between optically active and forbidden polariton states coupled by a polariton-polariton scattering process.Exciton-polaritons, being composite particles, retain both the properties of excitons and photons. In particular, they have the same spin-structure as excitons. In quantum wells (QWs), the lowest energy level of a heavy hole lies typically lower than any light-hole energy level, and thus the exciton ground state is formed by an electron and a heavy-hole. The entire exciton spin thus has the following projections on the structure axis: 1 ± and 2 ± allowed for the ground state. The states with the spin projection 1 ± are optically active (bright), as they can be excited by right or left circular polarized light beam. On the other hand, the optical excitation of 2 ± states is forbidden by the selection rules. These are so-called spin-forbidden dark states. Dark and bright excitonic states are typically split by a few tens of eV µ . Mixing and coherent excitation of these states may result in oscillations of their populations, i.e. quantum beats. The mixing can be achieved by application of an in-plane magnetic field as has been experimentally demonstrated in QWs [3] and recently in microcavities [4]. Neither mixing nor beats have been observed in the absence of the field till now, to the best of our knowledge. In our microcavity system, extremely pronounced quantum beats between bright and dark states appear in the non-linear regime while no magnetic field is applied. On the other hand, in the linear regime we do not see any beats.

Time-Resolved Emission from Semiconductor Microcavities

“…[19]. Figure 3 shows the calculated time evolution of the photoluminescence in the weakand strong-pumping regimes.…”