Interband photorefractive effects: Theory and experiments in KNbO3.

Abstract

Near-ultraviolet light induces band-to-band electron transitions and is used to record interband photorefractive gratings in undoped ${\mathrm{KNbO}}_{3}$. The high absorption constant in the range from 500 to 6000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ dictates the use of thin crystals or of the crystal volume near the surface. The probing of the grating is usually performed in the visible. We measured a fast grating response of 10 \ensuremath{\mu}s at 1 W/${\mathrm{cm}}^{2}$ for refractive index changes of 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$. The hologram diffraction efficiency reaches \ensuremath{\eta}=1.4% in a longitudinal recording geometry with a grating approximately 200 \ensuremath{\mu}m thick. In a transverse geometry we measured \ensuremath{\eta}=60% in 7.5-mm-long crystals. The photorefractive sensitivity of ${\mathrm{KNbO}}_{3}$ is similar to that of multiple-quantum-well structures while the resolution is better due to smaller carrier mobilities and because of shorter optical wavelengths used. Interband photorefractive gratings are highly robust with respect to intensive illumination in the visible. A theoretical model that describes interband electron transitions predicts the important role of free carriers in both bands. It explains the transition from a linear to a sublinear dependence of photoconductivity on light intensity which is observed already at intensities of a few mW/${\mathrm{cm}}^{2}$. The predictions are consistent with dynamic and steady-state holographic investigations performed under variation of grating spacing, applied electric field, ultraviolet and visible light intensities, and grating readout depth.