Photorefractive fixing method for the determination of H concentration in LiNbO3

Hydrogen is an important impurity in LiNbO3. During the crystal growth it enters the lattice, mostly in the form of OH- ions [1]. In photorefractive LiNbO3 it is generally accepted that protons are the main mobile ionic entities giving rise to fixed volume holograms. The electronic space charge field which, via the electro-optic effect causes a refractive index modulation, induces proton migration above room temperature (140°C), well below the temperature at which electron thermal detrapping starts (> 180°C). The refractive index pattern arising from the proton space charge field can not be erased by illumination anymore, thus it is called a fixed pattern.