Study On Waveguide Property Of Ion-implanted LiNbO₃ Crystal Using Ellipsometry

Lithium niobate is a good nonlinear optical material with excellent optical, piezoelectric,electro-optic elastic properties.Waveguide structure based on Lithium niobate is widely used in the fields of telecommunication,optoelectronics,optical information procession and optical computing.As an effective method to fabricate waveguides,ion implantation has called wide attention.It can change the refractive index of the material while not change the optoelectronic properties of the material. The method can be carried out under low temperature and can accurately control the implantation dose and depth.Up to now,many waveguide structures have been formed by ion implantation in various materials such as optical crystals,glass, semiconductors and polymers.Research on ion implantation began from 70's 20^th century.First,light ions such as He,H are used to be implanted into Lithium niobate crystal with implantation dose of 10¹⁶ ions/cm² to get multi-mode waveguides.At the end of the ion track,a low refractive index optical barrier is built up because of the lattice disorder produced by the nuclear damage process of the ion implantation.The region between this optical barrier and the surface is therefore surrounded by regions of low index and is able to act as a waveguide.Later heavy ions such as O,Ni,C,Si with low dose are implanted into Lithium niobate to get waveguide structure.Ellipsometry is a good method to get material surface properties.Optical properties and depth of film can be got by analyzing the change of polarization of the light reflected by the sample.It is used in the fields of physics,chemistry,biology and optical,electronics due to its particular advantages.This paper introduces the basic principle of reflecting ellipsometry and get the parameters of several models.Lithium niobate implanted by B ions with energy of 2.5MeV and dose of 2×10¹³ ions / cm² and Lithium niobate implanted by Ni ions with energy of 3.0MeV and dose of 10¹⁴ ions/cm² and Lithium niobate crystal are studied in this paper. Single-crystal property and transmission rate are measured.The experiment shows that each sample has a optical axis vertical to its surface.Good property of single crystal is reserved after implanted.Damage from the ion implantation is tiny.The ellipsometry parameters of the ion-implanted sample are measured. Divided-layer-uniform anisotropic models are analyzed and the relationship between refractive index and depth of layers are got.Srim software is be used to simulate the process of ion implantation.A damage curve is got.The range of B ion implantation into the crystal is around the depth of 2.0μm with tiny damage and ion replacement of 23%;while the range of Ni ion implantation into the crystal is around the depth of 1.0μm with large damage and ion replacement of 55%.By establishing models for the two samples ellipsometry formulation is got.With the help of MATLAB,the refractive index dispersion formulation is got at the range of 0.3-0.8μm.Both the ordinary and extraordinary dispersion curve of ion-implanted samples have similar track to the crystal with no ion implanted,n_o and n_e of damaged layer are obviously decreased and both are lower than that of region of waveguide.So light can be refined in the region of waveguide.