Long-living currents induced by nanosecond light pulses in LiNbO(3) crystals.

We have obtained first solid evidence of strong charge separation that is caused by relaxing localized electrons in a polar medium: Space-charge gratings induced in highly-doped LiNbO(3):Fe crystals by interfering nanosecond light pulses at 532 nm show a highly peculiar long term behavior (buildup or/and decay) in the dark. It depends strongly on the applied electric field E(0) (ranging from -140 to +640 kV/cm) and occurs on a time scale of (1 - 100) s which is much larger than the relaxation time of photo-electrons and smaller than the dark dielectric relaxation time. All peculiarities observed are fully described by a charge-transport model that incorporates the energy relaxation of electrons within a band of localized Fe(2+) states and a long-living, field-gradient-independent "polar current" directed along the polar axis.