High-sensitivity read-write volume holographic storage in reduced KNbO3 crystals

Reduced KNbO3 is a photoconductive ferroelectric in which holograms can be recorded by the photorefractive effect. Read-write volume hologram storage and erase sensitivities ofS−1=100 J/cm2 andS−1=84 J/cm2 (S=d(Δn)/d(I0t)‖t=0) have been measured at zero applied electric field, where the charge transport is shown to be due to diffusion of photoexcited electrons. By applying an electric field along thec-axis, the migration length of the photoexcited electrons becomes comparable to the holographic grating spacing. This leads to storage sensitivities comparable to high-resolution photographic plates. Experimental data on storage and erase sensitivity as a function of the grating spacing, applied electric field, writing light intensity and temperature are reported and interpreted on the basis of the theoretical results of Young et al. and Amodei. Changes of the intensity ratio of the writing beams by self diffraction (beam coupling), reflections from surfaces and the residual dark conductivity are assumed to cause experimental results which deviate from the theoretical models. It is shown, that in reduced KNbO3 and other ferroelectric photoconductors having photocarrier transport lengths much larger than the unit cell dimension, photovoltaic currents do not contribute significantly to the build-up of space-charges leading to the photorefractive effect.