Neutron diffraction from thermally fixed gratings in photorefractive lithium niobate crystals

Iron-doped lithium niobate crystals are illuminated with a sinusoidal light pattern (period length 374 nm) at a temperature of about 180 degrees C. El ectrons are redistributed and ions drift in the electronic space-charge pat tern ("thermal fixing"). At room temperature the ions are almost immobile. A density grating (space-charge field plus inverse-piezoelectric effect) an d the ionic grating yield a refractive-index modulation for neutrons ("phot orefractive effect"). Neutrons (wavelength 1.39 nm) are diffracted from thi s grating with an efficiency up to 1.2 x 10(-3). Usually hydrogen ions form the ionic grating. The ions responsible for charge compensation in dehydra ted crystals have a much smaller coherent neutron-scattering length and mig ht be identified as lithium. [S0163-1829(99)50738-8].