  We propose a way to increase the lifetime of two-dimensional direct excitons
and show the possibility to observe their macroscopically coherent state at
high temperatures. For a single GaAs quantum well embedded in photonic layered
heterostructures with subwavelength period, we predict the exciton radiative
decay to be strongly suppressed. Quantum hydrodynamic approach is used to study
the Berezinskii-Kosterlitz-Thouless crossover in a finite exciton system with
intermediate densities. Below the estimated critical temperatures, drastic
growth of the correlation length is shown to be accompanied by a manyfold
increase of the photoluminescence intensity.
