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Ultra-fast spinor switching in polariton condensates

Alexis Askitopoulos, Hamid Ohadi, T. C. H. Liew, Z. Hatzopoulos, Pavlos G. Savvidis, Alexey Vitalievich Kavokin, Pavlos G. Lagoudakis

Published Jan 1, 2014• 1 citation
Publisher

Abstract

We demonstrate a linear to circular polarization conversion mechanism of a confined polariton condensate created by an optical potential trap. This system features a high degree of circular polarization under linear polarization pumping and is favorable for the development of spin logic operations. The physical mechanism behind this process is explained by the spin precession in the exciton reservoir that results in a condensate of a preferred spin state. Application of a non-resonant below threshold femtosecond pulse on the spinor condensate results in an ultrafast reversal of the spin state.

DOI
10.1364/cleo_si.2014.stu3o.3

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We demonstrate a linear to circular polarization conversion mechanism of a confined polariton condensate created by an optical potential trap. This system features a high degree of circular polarization under linear polarization pumping and is favorable for the development of spin logic operations. The physical mechanism behind this process is explained by the spin precession in the exciton reservoir that results in a condensate of a preferred spin state. Application of a non-resonant below threshold femtosecond pulse on the spinor condensate results in an ultrafast reversal of the spin state.
We theoretically investigate a spinor polariton condensate under nonresonant pumping, based on driven-dissipative Gross-Pitaevskii equations coupled to the rate equation of a spin-unpolarized reservoir. We find the homogeneous polariton condensate can transit from the spin-unpolarized phase, where it is linearly polarized, to the spin-polarized phase, where it is elliptically polarized, depending on the cross-spin versus same-spin interactions and the linear polarization splitting. In both phases, we study elementary excitations using Bogoliubov approach, in a regime where the decay rate of total exciton density in reservoir crosses over from the slow to the fast limit. Depending on reservoir parameters, the global-phase mode can be either diffusive or gapped. By contrast, the relative-phase mode always possesses a gapped energy, undamped in the spin-unpolarized phase but weakly damped in the spin-polarized phase. In the spin-unpolarized phase, both modes are linearly polarized despite pumping and decay. However, in the spin-polarized phase, the mode polarization can be significantly affected by the reservoir and depends strongly on the circular polarization degree of the condensate. Interestingly, we demonstrate that the ``ghost'' branch of the Bogoliubov spectrum of the relative-phase mode can be visualized in the photoluminescence emission, distinguishable from that of the global-phase mode and thus allowing for experimental observation, when the spinor polariton condensate is elliptically polarized.
We predict the spontaneous symmetry breaking in a spinor Bose-Einstein condensate of exciton-polaritons (polaritons) caused by the coupling of its spin and orbital degrees of freedom. We study a polariton condensate trapped in a ring-shaped effective potential with a broken rotational symmetry. We propose a realistic scheme of generating controllable spinor azimuthal persistent currents of polaritons in the trap under the continuous wave optical pump. We propose a new type of half-quantum circulating states in a spinor system characterized by azimuthal currents in both circular polarizations and a vortex in only one of the polarizations. The spontaneous symmetry breaking in the spinor polariton condensate that consists in the switching from co-winding to opposite-winding currents in opposite spin states is revealed. It is characterized by the change of the average orbital angular momentum of the condensate from zero to non-zero values. The radial displacement of the pump spot and the polarization of the pump act as the control parameters. The considered system exhibits a fundamental similarity to a superconducting flux qubit, which makes it highly promising for applications in quantum computing.
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The effect of a magnetic field on a spinor exciton-polariton condensate has been investigated. A quenching of a polariton Zeeman splitting and an elliptical polarization of the condensate have been observed at low magnetic fields B<2 T. The effects are attributed to a competition between the magnetic field induced circular polarization buildup and the spin-anisotropic polariton-polariton interaction which favors a linear polarization. The sign of the circular polarization of the condensate emission at B<3 T is negative, suggesting that a dynamic condensation in the excited spin state rather than the ground spin state takes place in this magnetic field range. From about 2T on, the Zeeman splitting opens and from then on the slope of the circular polarization degree changes its sign. For magnetic fields larger than the 3 T, the upper spin state occupation is energetically suppressed and circularly polarized condensation takes place in the ground state.
The intrinsic Josephson effect is observed in a two-component polariton condensate in a strained planar microcavity under a resonant pulsed excitation such that only one of the two linearly polarized eigenstates is pumped from the outside. On reaching the threshold density, the polariton-polariton interaction involves a spontaneous breaking of the symmetry between spin-up and spin-down polaritons. As a result, the condensate acquires nearly circular polarization while the pump is strong and then passes to the Josephson oscillation regime since the pump weakens.