Experimental investigation of the refractive indices of thin-film lithium niobate modified by the proton exchange method

Abstract

We have studied the refractive indices of thin-film lithium niobate (TFLN) modified by proton exchange. In order to investigate the kinetics of the proton exchange process, the process time was varied. The main experimental research methods used were prism coupling and reactive ion etching. We propose a method for determining the change in the refractive index of the HxLi1-xNbO3 epitaxial layer formed on the TFLN surface after proton exchange. The method is based on separating the contribution to the average refractive index of HxLi1-xNbO3 and of the initial TFLN. The basic assumption was that the refractive index after proton exchange varies stepwise. The thickness-averaged extraordinary and ordinary refractive indices were obtained by means of prism coupling. To distinguish the layers, the proton concentration profile was determined using depth profiling of the reactive ion etching rate. The shape of the concentration profiles differed from the profiles taken for bulk lithium niobate. From the concentration profiles, the depth of proton exchange and the diffusion coefficient were obtained. The diffusion coefficient was 0.031 mkm2/hour, which is less than the values typical for bulk lithium niobate. The paper shows that the average refractive indices vary linearly with the time of proton exchange. At the same time, the refractive indices of the epitaxial layer remain constant. The changes in the extraordinary and ordinary refractive indices due to proton exchange were 0.8 and –0.5 respectively, which corresponds to typical values for the β-phases of bulk lithium niobate.