Probe of symmetry reduction at domain walls by nonlinear Cherenkov measurement

Abstract

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.

Linear Cherenkov radiation in ferroelectric domain walls

“…When the incident light is perpendicular to the domain wall ( o e k k > ), the LCR cannot be generated. However, due to the modulation effect of the domain wall, the phase velocity of polarization wave can be tuned from the velocity of incident light to infinity by changing the incident angle β with respect to the domain wall [18,20].…”

“…In addition, some specific properties, such as conductive and photovoltaic characteristics also have been demonstrated on domain walls [15,17]. In the previous research, the break of mirror symmetrical property has been verified in the domain walls [18]. In this paper, based on the mirror symmetry reduction of domain walls, we theoretically predict that the linear Cherenkov radiation (LCR) could be generated in ferroelectric crystal according to the coupled wave equation.…”

“…(1), the underlined zero elements are determined by the mirror inversion symmetry with respect to y-z plane. However, in Ref [18], we have demonstrated that the mirror symmetrical property in the domain walls were broken. Hence, the underlined elements are not equal to 0.…”

Optical studies of ferroelectric and ferroelastic domain walls

“…In LiNbO 3 , the second-order susceptibility tensor of domain walls has been obtained by SHG and compared with the bulk tensor of the domains. The comparison reveals that the mirror inversion symmetry is violated and the threefold rotational symmetry is retained, indicating a symmetry reduction at the domain wall [116]. In LiTaO 3 it shows Bloch-like configurations at domain walls [106].…”

Conical third-harmonic generation in a hexagonally poled LiTaO3 crystal

“…4 In 1998, Berger extended the QPM theory from one dimension to two, proposing the concept of nonlinear photonic crystals. 5 The collinear and noncollinear reciprocal vectors in two-dimensional (2D) nonlinear photonic crystals lead to numerous interesting phenomena, such as nonlinear Talbot self-imaging, 6,7 generation of orbital angular momentum, 8,9 nonlinear generation and manipulation of airy beams, 10 efficient generation of cascaded third harmonic generation (THG), 11 nonlinear Bragg diffraction, 12,13 nonlinear Cherenkov emission, [14][15][16][17][18][19] and nonlinear Raman-Nath diffraction. 20 When an optical beam propagates in a nonlinear photonic crystal, light scattered by the domain walls and defects in the crystal could be involved in the nonlinear optical processes.…”