THE BULK PHOTOVOLTAIC EFFECT OF PHOTOREFRACTIVE LINBO3-FE CRYSTALS ATHIGH LIGHT INTENSITIES

Direct measurements of the bulk photovoltaic current density j(phv) in photorefractive LiNbO3:Fe,LiNbO3:Mg,Fe and LiNbO3:Zn,Fe crystals were performed at high light intensities. Illumination of the crystals wit h light pulses of a Q-switched frequency-doubled Nd:YAG laser (pulse d uration 20 ns; light wavelength 532 nm) yielded a photovoltaic current through the crystals which charged a capacitor. Afterwards an electro meter measured the voltage of the charged capacitor. From this voltage and from capacity and exposure time the current density j(phv) was de duced. For intensities above 10(7) W m(-2) a contribution to j(phv) wh ich increased quadratically with light intensity/appeared for LiNbO3:F e crystals. Go-doping with Mg and Zn lowered the quadratic component. Our results are in good qualitative and quantitative agreement with pr evious holographic measurements and support the claim that the intrins ic defect Nb-Li(4+/5+) acts as a secondary photorefractive centre in L iNbO3 at high light intensities.