Combined excitation of running space charge and conductivity gratings in photorefractive crystals

We report the simultaneous combined excitation of the space charge and photoconductivity gratings in photorefractive Bi12SiO20 and Bi12TiO20 crystals. The experiments are carried out using the light diffraction and non-steady-state photo-emf techniques. The crystal illumination with an oscillating interference pattern excites the running photoconductivity grating which interacts with the ac component of the applied field and gives rise to the space charge wave. Being the eigenmode of the space charge dynamics the wave reveals itself as the low-frequency resonant maximum at the frequency transfer function of the detected signals. The theory of the observed effects is constructed for the conventional one-level model of semiconductor with monopolar photoconductivity. The drift mobilities of electrons are estimated for n-type photorefractive Bi12SiO20 and Bi12TiO20 crystals: mu=(1.1-1.4)x10(-2) cm(2)/V s (Bi12SiO20, lambda=532 nm, T=296-298 K), mu=1.5x10(-2) cm(2)/V s (Bi12SiO20, lambda=442 nm, T=293 K), and mu=3.1x10(-3) cm(2)/V s (Bi12TiO20, lambda=532 nm, T=293 K).