Probe of symmetry reduction at domain walls by nonlinear Cherenkov measurement.

We report on the investigation of symmetrical properties of lithium niobate (LiNbO₃) domain walls utilizing the nonlinear Cherenkov radiation. Compared with LiNbO₃ bulk crystals, new nonzero elements of the χ⁽²⁾ tensor at domain walls are found by the Cherenkov second harmonic generation (CSHG) and Cherenkov sum frequency generation (CSFG) measurement. Experimentally, we demonstrate the symmetry reduction of domain walls, where the mirror inversion symmetry of LiNbO₃ lattice is broken while the threefold rotational symmetry still remains.