Microdomain Patterns Recorded by an Electron Beam in He-Implanted Optical Waveguides on X-Cut LiNbO 3 Crystals

We present the results of studies in planar optical waveguides fabricated by He-ion implantation with the energy of 500 keV in X-cut LiNbO 3 crystals. The thickness of the formed waveguide layer confined by the depth D of the implanted layer is of about 1.06 μm. The refractive indices as well as differences in refractive indices were evaluated for wavelengths $\lambda = 445$ , 626.5, and 650 nm. Domain gratings with the period $\Lambda = 4$ μm were recorded in these samples by electron beam irradiation with acceleration voltages U in the range from 5 to 25 kV. Gratings characteristics measured by the PFM method were obtained for different domain thicknesses T d determined by U . The optimum grating regularity is achieved when the domain growth occurs beyond the He-implanted damaged barrier, i.e., at $T_{d} \le D$ , which in the given case corresponds to $U = 10$ and 15 kV. Otherwise, ( $T_{d} > D$ ), the domain evolution is affected by the structurally damaged layer and the gratings become irregular.