Interaction of running gratings of the space charge and conductivity in photorefractive Bi12Si(Ti)O20 crystals

The simultaneous excitation and nonlinear interaction of the space-charge and photoconductivity gratings are studied experimentally in photorefractive Bi12SiO20 and Bi12TiO20. The measurements are performed using the diffraction technique, which implies the illumination of the crystal by an oscillating interference pattern (lambda(r)=532 nm) along with the application of dc and ac electric fields and detection of the diffracted probe beam (lambda(p)=650 nm). Such illumination excites the running photoconductivity grating, which interacts with the ac component of the applied field giving rise to the space-charge wave. Being the eigenmode of the space-charge oscillations, this wave reveals itself as a low-frequency resonant maximum at the frequency-transfer function of the detected signal. The drift mobilities of electrons are estimated using the developed technique: mu=(1.1-1.4)x10(-2) cm(2)/V s (Bi12SiO20, T=296-298 K) and mu=2.8x10(-3) cm(2)/V s (Bi12TiO20, T=293 K).