Conduction band and trap limited mobilities in Bi12SiO20

The mobility of photoexcited charge carriers in photorefractive insulators can be measured with a holographic time-of-flight technique.1 By illuminating the crystal with two interfering 30 ps laser pulses at the wavelength of 532 nm, we create an instantanuous sinusoidal pattern of photoexcited charge carriers. A strong electric field E0 is applied across the crystal causing the sinusoidal pattern of charge carriers to drift with a velocity μE0, where μ is the mobility. With a proper choice of the interference fringe spacing Λ, the superposition of this drifting charge pattern on the complementary pattern of photo-ionized traps creates an observable oscillating space charge field. We probe this oscillation by diffracting a weak cw He-Ne beam from the refractive index grating that is created via the electro-optic effect. The period Pt of the observed oscillation is the time required for photoexcited charge carriers to drift over one spatial period Λ.