Self-assembly of hexatic optical patterns in ferroelectric crystals: Rediscovery of the Schwarz-Hora effect

We have discovered the laser-induced “freezing” of charged defects into two-dimensional hexatic arrays in lead magnesium niobate. This situation arises when, due to simultaneous thermal focussing and photorefractive phenomena, a holographic grating is produced that allows phase-matched light scattering. The diffraction angle for the hexagonal pattern is Φ = 0.20°, which agrees reasonably well with the theoretical value of 0.45° from Firthʹs equation.