2ND-ORDER OPTICAL-RESPONSE FROM A BOSE-EINSTEIN CONDENSATE OF EXCITONS

A theoretical investigation of the nonlinear response of the Bose-Eins tein condensate of an electron-hole plasma in a direct-gap semiconduct or quantum-well reveals the existence of a second-order response, whic h is absent in the normal state of the plasma because of inversion sym metry. We propose a two-beam optical experiment to detect the second-o rder response as a test for Bose-Einstein condensation of excitons. Th e signal is sensitive to the intrinsic pair-coherence and is dominated by a long-wavelength collective mode inside the gap. Consequently, th e signal is estimated to be much stronger than four-wave mixing, not e ven counting the extra factor of external field needed for the latter. Copyright (C) 1996 Elsevier Science Ltd