Microdomain engineering in waveguide and layered structures based on ferroelectrics for applications in photonic elements (brief review)

Studies of ferroelectric nano- and microdomain structures formed in LiNbO$_3$-based optical waveguides are reviewed. Nanodomain structures of a given configuration have been written in an LNOI (LiNbO$_3$-on-insulator) waveguide sandwich with the polar ($Z$) orientation by the field of an atomic force microscope tip and their properties have been studied. The static conductivity $\sigma_{\text{DW}}$ of domain walls has been detected and estimated by an original method based on the characteristics of the atomic force microscopy writing of domains. The estimated value $\sigma_{\text{DW}}\approx8\times10^{-4}$ ($\Omega$ cm)$^{-1}$ is at least $12$ orders of magnitude higher than the bulk conductivity of LiNbO$_3$. Microdomain gratings with given periods have been written in He:LiNbO$_3$ and Ti:LiNbO$_3$ planar optical waveguides formed on nonpolar ($X$ and $Y$) surfaces of a crystal by the electron beam method. Studies of the nonlinear optical conversion of radiation in written structures have demonstrated that the optimal characteristics of the waveguide conversion to the second harmonic are reached when the depth of written domains $T_{\mathrm{d}}$ corresponds to the thickness of the waveguide layer. The depth $T_{\mathrm{d}}$ is specified by the accelerating voltage $U$ of a scanning electron microscope.