ac photo-EMF currents detected in presence of continuous movement of the interference pattern

Summary form only given. The experiments were obtained in an ac photo-EMF configuration both for GaAs and BSO samples for /spl lambda/ = 633 nm and for 532 nm respectively. In addition to conventional periodic phase modulation with a frequency /spl Omega/, the frequency offset /spl Delta//spl Omega/ = /spl Omega//sub 2/ - /spl Omega//sub 1/ was introduced in the recording beams using two similar acoustooptic cells. Experiments with non-zero frequency offset /spl Delta//spl Omega/ (both with BSO and GaAs) show that contrary to our initial expectations the photo-EMF signal U/sup /spl Omega//(/spl Omega/) proves to be equal to zero when /spl Delta//spl Omega/ /spl sime/ /spl Omega//sub f/. Some insignificant absolute growth of the signal is, however, observable at low spatial frequencies as compared with the case of zero frequency offset (/spl Delta//spl Omega/ = 0). Both of these facts correspond well to a simple theoretical analysis of the photo-EMF signal formation under external electric field and frequency offset, performed for modulation frequencies /spl Omega/ >> /spl Delta//spl Omega/; /spl tau//sub di//sup -1/.