Probe Of Symmetry Reduction At Domain Walls By Nonlinear Cherenkov Radiation

The current thesis describes for the first time in second-order susceptibility tensor of domain walls in periodically poled lithium niobate by nonlinear Cherenkov measurement.Our results further indicate that the specific symmetry properties of lithium niobate?LiNbO₃?domain walls compared with bulk medium.Domain walls,the boundaries between positive and negative domains in ferroelectrics,reveal specific properties such as conductivity,mobility and symmetry different from the bulk medium and have a promising application in nanoscale functional devices.We report on the investigation of symmetry properties of LiNbO₃ domain walls utilizing the nonlinear Cherenkov radiation.Compared with LiNbO₃ bulk crystal,new nonzero elements of the second-order susceptibility tensor at domain walls are found by the Cherenkov second harmonic generation and Cherenkov sum frequency generation measurement.Experimentally,we demonstrate the symmetry reduction of domain walls,where the mirror symmetry of LiNbO₃ lattice is broken while the threefold rotational symmetry remains.Furthermore,linear Cherenkov radiation has also been observed in LiNbO₃ domain walls,which satisfies the Cherenkov condition when the incidence is an e-polarized light and the outgoing beam is an o-polarized light employing birefringence of LiNbO₃.Similar to nonlinear Cherenkov process,the linear polarization wave confined by domain walls also carry the information of domain walls and the linear susceptibility is the key to analyze the symmetry reduction of domain walls.