Bose-Einstein Condensation of Microcavity Polaritons

The strong coupling of light and excitons in a two-dimensional semiconductor microcavity results in a new eigenstate of quasiparticles called polaritons. Microcavity polaritons have generated much interest due to the wealth of interesting optical phenomena recently observed in these systems such as nonlinear emission, macroscopic coherence, and bosonic stimulated scattering. The efficiency of amplification, parametric oscillation, and coherent emission of light makes it promising for applications in coherent control, microscopic optical switching, and other opto-electronic devices. Most of all, because of their light mass and bosonic character, these particles are predicted to undergo Bose-Einstein condensation (BEC) at much higher temperatures and lower densities than their atomic counterparts.Standard methods of growing semiconductor microcavities are quite inefficient in producing well-tuned samples with strong coupling of light and excitons. Wafers with continuously varying thicknesses are often produced, leaving only tiny regions with strong coupling. In our experiments, an inhomogeneous stress is applied to the microcavity in order to actively couple naturally detuned exciton and cavity modes at fixed $k_{||} = 0$, and at the same time, create an in-plane spatial trap, potentially making BEC of polaritons possible.Our recent experiments with exciton-polaritons in the stress trap have shown compelling evidence of BEC. At the bottom of the trap where the coupling is strongest, line narrowing and nonlinear increase of photoluminescence intensity are observed. Also a single, spatially narrow condensate of polariton gas is formed analogous to the case of atoms in a three-dimensional harmonic potential. Above a critical density, we observe a massive occupation of polaritons in the ground state, spontaneous build-up of linear polarization, and macroscopic coherence of the condensate all in agreement with predictions. The results are similar to what is observed in the naturally resonant unstressed case. Comparison with the stressed trap and the nonstressed case, however, revealed that stress traps play a significant contribution in forming a polariton condensate. Furthermore, the stress trap case has shown, where the unstressed case has not, two distinct thresholds, one from photon lasing and another from a BEC transition

Dissipationless Flow and Sharp Threshold of a Polariton Condensate with Long Lifetime

We report new results of Bose-Einstein condensation of polaritons in specially designed microcavities with a very high quality factor, on the order of 10^{6}, giving polariton lifetimes of the order of 100 ps. When the polaritons are created with an incoherent pump, a dissipationless, coherent flow of the polaritons occurs over hundreds of microns, which increases as density increases. At high density, this flow is suddenly stopped, and the gas becomes trapped in a local potential minimum, with strong coherence

Coupled counterrotating polariton condensates in optically defined annular potentials.

Polariton condensates are macroscopic quantum states formed by half-matter half-light quasiparticles, thus connecting the phenomena of atomic Bose-Einstein condensation, superfluidity, and photon lasing. Here we report the spontaneous formation of such condensates in programmable potential landscapes generated by two concentric circles of light. The imposed geometry supports the emergence of annular states that extend up to 100 μm, yet are fully coherent and exhibit a spatial structure that remains stable for minutes at a time. These states exhibit a petal-like intensity distribution arising due to the interaction of two superfluids counterpropagating in the circular waveguide defined by the optical potential. In stark contrast to annular modes in conventional lasing systems, the resulting standing wave patterns exhibit only minimal overlap with the pump laser itself. We theoretically describe the system using a complex Ginzburg-Landau equation, which indicates why the condensate wants to rotate. Experimentally, we demonstrate the ability to precisely control the structure of the petal condensates both by carefully modifying the excitation geometry as well as perturbing the system on ultrafast timescales to reveal unexpected superfluid dynamics.We acknowledge grants EPSRC EP/G060649/1, EU INDEX 289968, Spanish MEC (MAT2008-01555), Greek GSRT ARISTEIA programs Irakleitos II and Apollo and the Skolkovo Foundation.This version is the author accepted manuscript. The final published version can be found on the PNAS website here: http://www.pnas.org/content/early/2014/05/30/1401988111.abstrac

Research Data Supporting "A sub-femtojoule electrical spin-switch based on optically trapped polariton condensates"

The data which are presented in the plots in text format.ERC [LINASS 320503], EPSRC [EP/L027151/1], EU INDEX [289968], Spanish MEC [MAT2008-01555], Mexican CONACYT [251808], EU [FP7-REGPOT-2013-1