High-Q polariton modes in heterostructures with traps for dipolar excitons

Polariton modes are studied in two-dimensional traps based on quantum-well heterostructures allowing the production of a Bose — Einstein condensate of indirect excitons. The characteristic equation for the modes is derived using the boundary conditions on the exciton layer located inside a resonator formed by such a trap. The spectrum and the structure of high-Q modes are found analytically and numerically. It is shown that some of these modes become unstable at the high exciton density and long polarisation relaxation time. According to the estimates, this instability can take place in experiments on the Bose condensation of dipolar excitons, thus explaining the origin of their coherent emission.