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 LiNbO₃ 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 λ = 445, 626.5, and 650 nm. Domain gratings with the period λ = 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 ≤ 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.