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

TL;DR

This paper demonstrates Bose-Einstein condensation of polaritons within a bound state in the continuum (BIC), leveraging its topological protection and ultra-long lifetime to achieve low-threshold condensation in integrated photonic devices.

Contribution

It is the first to show polariton Bose-Einstein condensation in a BIC, exploiting its non-radiative nature and topological properties for efficient quantum state formation.

Findings

Polariton BEC occurs in a BIC due to its non-radiative nature.

Condensation threshold is extremely low, at a saddle point in reciprocal space.

Topological properties of BICs can be imparted to macroscopic quantum states.

Abstract

Optical bound states in the continuum (BIC) are peculiar topological states that, when realized in a planar photonic crystal lattice, are symmetry-protected from radiating in the far field despite lying within the light cone, i.e., in the energy-momentum dispersion region for which radiation can propagate out of the lattice plane. These BICs possess an invariant topological charge given by the winding number of the polarization vectors, similarly to vortices in quantum fluids, such as superfluid helium and atomic Bose-Einstein condensates. In spite of several reports of optical BICs in patterned dielectric slabs with evidence of lasing, their potential as topologically protected states with theoretically infinite lifetime has not been fully exploited, yet. Here we show Bose-Einstein condensation of polaritons, hybrid light-matter excitations, occuring in a BIC thanks to its peculiar non-radiative nature. The combination of the ultra-long BIC lifetime and the tight confinement of the waveguide geometry allow to achieve an extremely low threshold density for condensation, which is not reached in the dispersion minimum but at a saddle point in reciprocal space. By bridging bosonic condensation and symmetry-protected radiation eigenmodes, we unveil new ways of imparting topological properties onto macroscopic quantum states with unexplored dispersion features. Such an observation may open a route towards energy-efficient polariton condensation in cost-effective integrated devices, ultimately suited for the development of hybrid light-matter optical circuits