  Correlations of luminescence intensity have been studied under Bose-Einstein
condensation of dipolar excitons in the temperature range of 0.45-4.2 K.
Photoexcited dipolar excitons were collected in a lateral trap in GaAs/AlGaAs
Schottky-diode heterostructure with single wide (25 nm) quantum well under
applied electric bias. Two-photon correlations were measured with the use of a
classical Hanbury Brown - Twiss intensity interferometer (time resolution ~0.4
ns). Photon "bunching" has been observed near the Bose condensation threshold
of dipolar excitons determined by the appearance of a narrow luminescence line
of exciton condensate at optical pumping increase. The two-photon correlation
function shows super-poissonian distribution at time scales of system coherence
(<~1 ns). No photon bunching was observed at the excitation pumping appreciably
below the condensation threshold. At excitation pumping increasing well above
the threshold, when the narrow line of exciton condensate grows in the
luminescence spectrum, the photon bunching is decreasing and finally vanishes -
the two-photon correlator becomes poissonian reflecting the
single-quantum-state origin of excitonic Bose condensate. Under the same
conditions a first-order spatial correlator, measured by means of the
luminescence interference from spatially separated condensate parts, remains
significant. The discovered photon bunching is rather sensitive to temperature:
it drops several times with temperature increase from 0.45 K up to 4.2 K. If
assumed that the luminescence of dipolar excitons collected in the lateral trap
reflects directly coherent properties of interacting exciton gas, the observed
phenomenon of photon bunching nearby condensation threshold manifests phase
transition in interacting exciton Bose gas.
