We propose to use dispersion engineering of interwell excitons in coupled quantum wells with external electric and magnetic fields in order to generate coherent phonons and to detect exciton condensate. A parallel magnetic field moves the dispersion minimum of interwell excitons away from the radiative zone and thus reduces their recombination rate. Normal electric field moves an interwell excitons dispersion minimum on the energy scale. These two fields effect can be used to tune the resonance condition of the interwell excitons recombination process via an in-well excitons level, which results in acoustic phonon emission. We show, that one can change recombination rate as well as intensity and angular distribution of the interwell excitons photoluminescence in the wide range by controlling the external fields. Based on this principle we propose and theoretically evaluate a procedure to detect the condensate of interwell excitons, as well as a scheme to obtain a coherent and monochromatic phonon beam (saser). The statistics of the phonon emission from the condensate of interwell excitons is studied. Numerical estimate for GaAs/AlGaAs coupled quantum wells is provided.
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