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 °C. Electrons are redistributed and ions drift in the electronic space-charge pattern (“thermal fixing”). At room temperature the ions are almost immobile. A density grating (space-charge field plus inverse-piezoelectric effect) and the ionic grating yield a refractive-index modulation for neutrons (“photorefractive effect”). Neutrons (wavelength 1.39 nm) are diffracted from this grating with an efficiency up to $1.2\times {10}^{-3}.$ Usually hydrogen ions form the ionic grating. The ions responsible for charge compensation in dehydrated crystals have a much smaller coherent neutron-scattering length and might be identified as lithium.