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Exciton-Polariton Optoelectronic and Quantum Employment in Semiconductors

Final Report Summary - EPOQUES (Exciton-Polariton Optoelectronic and Quantum Employment in Semiconductors)

The EPOQUES project aimed to design, theoretically study and predict the potential of future practical devices based on excitons in quantum wells and exciton-polaritons in semiconductor microcavities. There was a strong focus on ingredients for optical circuits, where effects on ballistic spin transport were considered in detail, as well as the development of proposals for holographic arrays and the study of processes for the generation of terahertz radiation. In the theoretical modelling of such systems, new theoretical tools were also developed including a stochastic treatment of energy relaxation caused by polariton-phonon scattering and the solution of the cubic-quintic Gross-Pitaevskii equation. The results from each part of the project are detailed below.

Holographic Arrays

An original proposal for spatial light modulation with complete and independent control of the amplitude and phase (across the full 2pi range) of light was developed, suitable for holographic devices (see Fig. 1). The proposal was based on the electrical control of exciton resonances in quantum and was shown theoretically to have extremely promising characteristics. The switching time was extremely fast (being limited by external electronics), high efficiency is predicted making use of the high reflectivity of Bragg mirrors, and only small electric fields are required for operation. 100% intensity contrast is achieved and the design is based on a single compact structure with no moving parts, allowing for micron sized pixel sizes. Finally, the design was shown to be compatible with materials capable of room temperature operation.

Ref: “Holographic arrays based on semiconductor microstructures”, T C H Liew, Phys. Rev. B, 86, 235314 (2012).

Spin Structure and Spin Dynamics of indirect exciton gases

The four-component spin structure of indirect exciton gases was fully investigated, including the effects of Rashba and Dresselhaus spin-orbit coupling, applied magnetic field and sample anisotropy. Both the spatial spin texture of the ground state of indirect excitons and the patterns of spin currents (see Fig. 2) forming in non-equilibrium excitation schemes were studied. Calculations on the magnetic field dependence were compared with experimental work and shown to be in good agreement. The understanding of the behaviour of indirect exciton spin currents is essential for the development of spintronic devices based on these systems, which may benefit from long lifetimes, strong electric field sensitivity and a rich spin four-component spin structure.

Refs: “Spin-orbit coupling and the topology of gases of spin-degenerate cold excitons in photoexcited GaAs-AlGaAs quantum wells”, M Matuszewski, T C H Liew, Y G Rubo, & A V Kavokin, Phys. Rev. B, 86, 115321 (2012).

“Spin Currents in a Coherent Exciton Gas”, A A High, A T Hammack, J R Leonard, Sen Yang, L V Butov, T Ostatnicky, M Vladimirova, A V Kavokin, T C H Liew, K L Campman, & A C Gossard, Phys. Rev. Lett., 110, 246403 (2013).

“Ballistic spin transport in exciton gases”, A V Kavokin, M Vladimirova, B Jouault, T C H Liew, J R Leonard, & L V Butov, Phys. Rev. B, 88, 195309 (2013)

Amplification of Polariton Wavepackets

An essential ingredient of functional logical circuits is a mechanism of amplification to compensate any losses in the system. In collaboration with experimental groups an amplification mechanism of propagating polariton condensates was studied in detail, where a non-resonant laser excitation compensates the finite polariton lifetime.

Ref: “Propagation and Amplification Dynamics of 1D Polariton Condensates, E Wertz, A Amo, D D Solnyshkov, L Ferrier, T C H Liew, D Sanvitto, P Senellart, I Sagnes, A Lemaitre, A V Kavokin, G Malpuech, & J Bloch”, Phys. Rev. Lett., 109, 216404 (2012).

Symmetry Breaking in Polariton Lasers

The spontaneous polarisation of polariton lasers was analysed with a theoretical model based on the stochastic Fokker-Planck approach and compared to experimental results. While the polarisation has a stochastic element, it can be strongly influenced by spin-anisotropic interactions that can cause precession of the polarisation. This work revealed a previously unknown link between the polarisation of the exciting non-resonant laser and the polariton laser signal, namely a significant amount of circular polarisation can be imprinted on the polariton laser signal. The control of the polarisation is expected to allow for hybrid electro-optical devices based on polariton lasers where electrical injection still allows a well-defined coherent spin population.

Ref: “Spontaneous Symmetry Breaking in a Polariton and Photon Laser”, H Ohadi, E Kammann, T C H Liew, K G Lagoudakis, A V Kavokin, & P G. Lagoudakis, Phys. Rev. Lett., 109, 016404 (2012).

Stochastic Treatment of Energy Relaxation

A stochastic method for the treatment of energy relaxation was developed, which is generally applicable to the description of a coherent field (such as a Bose-Einstein condensate) interacting with an incoherent bath of other particles. While being useful in the modelling of a range of systems, this theory is particularly useful for the description of exciton-polariton based devices where one typically has a coherent field that undergoes a series of both spontaneous and stimulated relaxation steps via phonon interactions.

Ref: “Stochastic Gross-Pitaevskii equations for excitonic systems”, I G Savenko, T C H Liew, & I A Shelykh, Phys. Rev. Lett., 110, 127402 (2013).

Dissociation Dynamics of Vortices

The dynamics of half-quantum polarised vortices was studied with a stochastic model and compared to experimental results. Here, a spatially non-uniform polarisation splitting caused by strain was shown to cause the dissociation of full vortices into half-vortices. The work extends the understanding of the polarisation sensitivity of the system, which is an important step toward the control of polarised topological excitations. The use of topologically projected states to carry information is expected to allow for highly robust devices requiring minimal error correction.

Ref: “Dissociation Dynamics of Singly Charged Vortices Into Half Quantum Vortex Pairs”, F Manni, K G Lagoudakis, T C H Liew, R Andre, V Savona, & B Deveaud, Nature Communications, 3, 1309 (2012).

Cubic-Quintic Gross-Pitaevskii equation

Higher order interactions were studied theoretically in the cubic-quintic Gross-Pitaevskii equation. Such interactions are expected to lead to highly multistable systems that can be used for a denser coding of information using a larger alphabet.

Ref: “One-dimensional cubic-quintic Gross-Pitaevskii equation for Bose-Einstein condensates in a trap potential”, C Trallero-Giner, R Cipolatti, & T C H Liew, Eur. Phys. J. D, 67,143 (2013).

Multiphoton Polaritonics

The two-photon excitation of 2p dark excitons was shown to allow for the emission of terahertz frequency radiation caused by relaxation to 1s exciton states. Radiation at the terahertz frequency has several important applications in medical imaging, security screening and short-range communications. In addition, the presence of bistability in the system was shown to allow for ultrafast THz switches and memories.

Refs: “Nonlinear effects in multi-photon polaritonics” A A Pervishko, T C H Liew, V M Kovalev, I G Savenko, & I A Shelykh, Opt. Express, 21, 15183 (2013).

“Bistability in bosonic terahertz lasers” A A Pervishko, T C H Liew, A V Kavokin, & I A Shelykh, J. Phys. Condens Matter., 26, 085303 (2014).