Spin dynamics of exciton polaritons in microcavities

Abstract

In this chapter we address a complex set of optical phenomena linked to the spin dynamics of exciton polaritons in semiconductor microcavities. When optically created, polaritons inherit the spin and dipole moment from the exciting light. Their state can be fully characterized by a so-called "pseudospin" accounting for both spin and dipole moment orientation. However, from the very beginning of their life in a microcavity, polaritons start changing their pseudospin state under effect of effective magnetic fields of different nature and due to scattering with acoustic phonons, defects, and other polaritons. This makes pseudospin dynamics of exciton polaritons rich and complex. It manifests itself in non-trivial changes in polarization of light emitted by the cavity versus time, pumping energy, pumping intensity and polarization. During the first years of theoretical research on exciton-polariton relaxation the polarization has been simply neglected. Later it has been understood that the energy and momentum-relaxation of exciton polaritons are spin-dependent. It is typically the case in the regime of stimulated scattering when the spin polarizations of initial and final polariton state have a huge effect on the scattering rate between these states. It appeared that critical conditions for polariton Bose-condensation are also polarization-dependent. In particular, the stimulation threshold (i.e. the pumping power needed to have a population exceeding 1 at the ground state of the lower polariton branch) has been experimentally shown to be lower under linear than under circular pumping. These experimental observations have stimulated the theoretical research toward understanding of mutually dependent polarization-and energy-relaxation mechanisms in microcavities. The authors of this chapter have been working on theoretical description of different specific effects of polariton spin-dynamics in microcavities for years. Here we attempted to put together all fragments and to formulate a general approach to the problem that would allow then to consider a variety of particular cases. We start from reminding the main spin-relaxation mechanisms known for free carriers and excitons. We then overview the most essential experimental results in this field before to present our original formalism which allowed us to interpret the key experimental findings. We are going to discuss only the strong coupling regime leaving aside all polarization effects in VCSELs.

Parametric oscillation threshold of semiconductor microcavities in the strong coupling regime

“…Combining this result with eqns. (2), (4), (12), (19) and (20), one finds the ratio between the threshold laser intensities of χ (2) and χ (3) OPOs:…”

“…For the sake of simplicity, we shall focus our attention on the case of a circularly polarized pump beam. As the circular polarization of the polariton field is preserved by the nonlinear interactions and the longitudinaltransverse splitting 19,20 is much smaller than both the linewidth γ and the nonlinear interaction energy, the circular polarization is almost completely transferred to the signal and idler beams.…”

Proposal for a Mesoscopic Optical Berry-Phase Interferometer

“…Being a mixture of quantum well (QW) excitons and cavity photons, they possess a number of peculiar properties distinguishing them from other quasi-particles in mesoscopic systems. An important property of cavity polaritons is their (pseudo)spin [11], inherited from the spins of QW exciton and cavity photon and directly connected with the polarization of emitted photons. If one is able to control the spin of cavity polaritons, one can therefore control the polarization of emitted light, which can be used in optical information transfer.…”

Generation and Dynamics of Vortex Lattices in Coherent Exciton-Polariton Fields

“…Polaritons have two possible spin projections on the structure growth axis, ¼ AE1, corresponding to the right ( þ ) and left ( À ) circular polarizations of external photons [26]. The spin-dependent GP equation is [27]:…”