Pinning and Depinning of the Polarization of Exciton-Polariton Condensates at Room Temperature

Abstract

A room temperature polariton condensate realized in a microcavity with embedded GaN quantum wells emits linearly polarized light at threshold with the plane of polarization pinned to one of the crystallographic axes. With increasing pumping power, a depinning of the polarization is observed resulting in a progressive decrease of the polarization degree of the emitted light. This depinning is understood in terms of polariton-polariton repulsion competing with the static disorder potential effect. The polarization behavior differs from that of conventional lasers where the polarization degree usually increases as a function of pumping power.

Incoherent excitation and switching of spin states in exciton-polariton condensates

“…3,4,7,11 . The dynamical equations for the condensate wavefunctions corresponding to the left ψ − and right ψ + hand circular polarization states are written as follows (σ = ±) 8 :…”

“…It is usually assumed that all memory of the polarization of the incoherent pump is lost 3,11 . However, as mentioned above, it was shown that the creation of a spin-polarized exciton reservoir is possible with a spin-imprinting optical pump for sufficiently short-lived excitons.…”

“…A remarkable range of spinrelated phenomena has been explored in systems with a coherent resonant optical pump, which offers the possibility to directly inject the spin state of exciton-polaritons and explore the ultrafast spin-switching dynamics [3][4][5][6] . On the contrary, the non-resonant incoherent excitation responsible for the spontaneous formation of coherent exciton-polariton condensates is only beginning to be experimentally explored in the context of spin-dependent effects.…”

“…The reason for this disparity is a long-held belief that the incoherent reservoir (uncondensed hot excitonpolaritons), inevitably created by the incoherent excitations far above the energy of the condensed excitonpolariton quasiparticles, lacks spin-selectivity, i.e., owing to the fast spin-relaxation processes within the reservoir, the pump polarization cannot affect the spin of the condensed polaritons. Thus, the theoretical modelling of an incoherently excited exciton-polariton system with spin degrees of freedom usually treats the reservoir as either fast 3 or slow 7 response scalar density state, without any spin discrimination.…”

Stimulated emission of terahertz radiation by exciton-polariton lasers

“…Stimulated scattering of exciton polaritons into the lowest energy state leads to so-called polariton lasing, recently observed in GaAs, CdTe, and GaN based microcavities. 13,14 If the scattering from the upper to the lower polariton branch was accompanied by the emission of terahertz photons, polariton lasers would emit terahertz radiation, and this emission would be stimulated by the population of the lowest energy polariton state. However, at first glance, this process is forbidden since an optical dipole operator cannot directly couple the polariton states formed by the same exciton state.…”