Nonlinear optical phenomena in coherent phase of 2D exciton system

We show that spatial many-particle coherence of condensed 2D exciton system, which is represented by anomalous averages 〈âk1†⋯âkN†〉, can be transferred to exciton luminescence via the processes of coherent (simultaneous) recombination of many correlated excitons with production of the same number of correlated photons (âk† is exciton creation operator). In high-quality samples 2D momentum conservation dictates the requirement ∑iki=0 for 2D exciton momenta. Due to momentum conservation, the photons coherently produced in such a process possess zero sum 2D momentum as well. Analyzing such a many-photon spatial coherence of the luminescence we predict new nonlinear optical effects in a coherent phase of 2D exciton system. The effects are the above-mentioned processes stimulated by external laser beams. They effectively look as if the stimulated exciton system irradiates a unidirectional beam with recoil momentum. ‘Recoil’ beams constitute a photon ‘fan’ in case of single stimulating laser beam and a photon ‘lattice’ in case of two stimulating laser beams. Quantitative estimations for exciton system in GaAs/AlGaAs double quantum wells point to possible experimental observability of the effects.