Spin-Josephson coupling and the phase diagram of polariton condensate arrays

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• Prof. Jeremy Baumberg NanoPhotonics Centre, Cavendish Laboratory, University of Cambridge
• Monday 04 March 2019, 15:30-16:30
• Ryle Seminar Room (930) .

If you have a question about this talk, please contact Kayleigh Dunn.

Polaritons are light-matter quasiparticles which exist within wavelength-scale microcavities that couple confined photons with semiconductor excitons. Over the last decades these bosons have been found to exhibit nonequilibrium Bose-Einstein condensation, with interaction energies sufficient to give room temperature condensates. Here we will show how advances now allow arrays of interacting condensates to be explored, and show the crucial role of magnetization in some situations. We show the unusual spontaneous symmetry breaking for the magnetisation of a polariton condensate.[1,2] Unpolarised incoherent pumping generates randomly spin-up or spin-down magnetised condensates on each realisation, which remain stable for seconds, but can be rapidly switched sub-ns. By applying an electrical field perpendicular to the quantum-well plane we precisely tune the polarisation of the condensate emission. We utilise this to realise an electrical spin-switch, operating at record ultra-low switching energies of the order of attojoules and switching speeds that are only limited by the condensate dynamics (hundreds of picosecond).[3] The direction of the spin of two neighbouring condensates can also be controllably aligned (ferromagnetic) or anti-aligned (antiferromagnetic) by optically tuning their coupling strength. Building on this, we recently realized an optically controlled lattice of spin-polarized polariton condensates where we observe ferromagnetic and antiferromagnetic phases, and at the crossover between these two phases an unusual paired-spin phase.[4] We also observe new array interactions of condensates, showing how coherence evolves across the entire array. These properties, which are due to the non-equilibrium and non-linear nature of polariton condensates, give strong hopes for a whole variety of optoelectronic devices as well as outlining a completely new route towards utilising non-linear non-equilibrium coupled polariton condensates for complex simulations.

Rev. X 5, 031002 (2015), H Ohadi et al, Spontaneous spin bifurcations and ferromagnetic phase transitions in a spinor exciton-polariton condensate
116, 106403 (2016), H Ohadi et al, Tunable Magnetic Alignment between Trapped Exciton-Polariton Condensates 
Mat. 15, 1074 (2016), A. Dreismann et al, A sub-femtojoule electrical spin-switch based on optically trapped polariton condensates
119, 067401 (2017), H Ohadi et al, Spin Order and Phase Transitions in Chains of Polariton Condensates

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