Polariton Bose-Einstein condensate from a Bound State in the Continuum

Abstract

Exciton-polaritons are weakly interacting bosonic excitations in semiconductor heterostructures that have already shown to display condensation and superfluidity, even at room temperature. Here we show that Bose Einstein condensation of polaritons can occur in a planar waveguide by exploiting coupling to a quasi-bound state in the continuum (BIC). The combination of the ultra-long BIC lifetime and the high finesse and tight confinement of the waveguide geometry allow to achieve a record low threshold energy density. Moreover condensation is reached in a saddle point of the reciprocal space, with peculiar topological dispersion features that give rise to a collimated, spatially confined 1D emission band. The spatial confinement of the BIC state in particular is entirely due to the polariton nonlinearities and appears to be a truly original effect of the polariton BIC. Owing to the largely simplified fabrication as compared to bulky microcavity structures, these results may open a new route to energy-efficient polariton condensation at room temperature in cost-effective heterostructures, ultimately suited for the development of polaritonic integrated optical circuits. Moreover, bringing together bosonic condensation and symmetry-protected photonic eigenmodes could uncover new ways for imparting topological properties onto macroscopic quantum states.

Fabrication of Nanostructured GaAs/AlGaAs Waveguide for Low-Density Polariton Condensation from a Bound State in the Continuum

“…The exciton was introduced in the simulations with a Lorentzian resonance in the GaAs dielectric constant, with a Rabi splitting Ω = 13.9meV [8]. The bound state in the continuum appears as a saddle point of the dark state as shown in [29], which creates a direct path for polaritons to thermalize from the exciton reservoir. The fabricated gratings are 50µm wide, 300µm long with a filling factor 70% -75%.…”

“…This result, even though limited by the spectrometer resolution and inhomogeneous broadening, clearly highlights the reduction of exciton inhomogeneous broadening which ultimately corresponds to a longer exciton-polariton lifetime. A more detailed discussion about the BIC lifetime is presented in [29] where a lower limit is set by means of polariton propagation under the grating. While the effect on the excitonic component is clear, no significant shift in the polariton dispersion was observed after the aluminum oxide deposition, a sign that the lower Al 2 O 3 refractive index (n = 1.6 for thin film) compared to the photonic mode effective refractive index (n = 3.359) does not alter the designed structure behaviour.…”

“…A similar waveguide configuration with multiple QWs was used to investigate the enhancement of dipolar polariton interactions [8,[26][27][28]. More recently, the same heterostructure led to the achievement of a Bose-Einstein condensate from a BIC [29] while also proving the topological charge possessed by the condensate. In this work we show the achievement of a low-density condensate in a GaAs/AlGaAs waveguide by tuning the parameters of a low-loss shallow grating and then we study the effect of the reduction of excitonic losses on the threshold.…”

Realization of Polaritonic Topological Charge at Room Temperature Using Polariton Bound States in the Continuum from Perovskite Metasurface

“…[31][32][33][34][35][36][37][38][39][40][41] Very recently, the strong coupling regime between photonic BICs and excitonic resonances has been theoretically suggested, [42][43][44][45] with two experimental demonstrations. [46,47] The result of such a coupling is the formation of polariton-BICs (pol-BICs): hybrid excitations that are completely decoupled from the radiative continuum. This scenario is quite different from the textbook architecture in which the strong coupling regime is engineered by embedding quantum wells at the antinode positions of planar microcavities, which always exhibit substantial nonradiative losses.…”

“…As state-of-the-art, nonlinear behaviors of pol-BICs [46] and the use of pol-BICs to trigger Bose-Einstein Condensation have been recently demonstrated. [47] However, most of these demonstrations are conducted at cryogenic temperatures. Therefore, making pol-BICs with room temperature operation would be the next important step for the development of polaritonic devices based on BIC concepts.…”