Study Of Proton Exchange On Lithium Niobate Crystals Through Infrared Spectra

Lithium niobate, LiNbO3(LN), is widely used in electro-optical switch, optical waveguide, laser and information storage in integrated optics. It is a promising candidate material for fabricating integrated optical devices. Currently, the photonics structure based on LN has attracted more and more attention and most of photonics microstructures are formed by a complex and time-consuming lithographic and metal mask technology.However, some simple photonics structures can be made by photo-assisted technology,such as photo-control domain inversion and photoinduced silver deposition on LN. In this paper, we design a method for preparing photonics microstructure—photo-assisted proton exchange(PAPE) and the subsequent chemical etching(PACE). Proton exchange(PE) and chemical etching can be controlled selectively by light wave, achieving photonics microstructures on LN surface.First, as temperature and time are important factors influencing the proton exchange,proper temperature regulation can ensure to fabricate proton exchange waveguide with low optical loss. In this paper, the proton exchange-process was performed by varying the temperature and the exchange duration. Then the characteristic of OH- absorption bands were observed in the infrared absorption spectra and infrared reflection spectra. It can be seen that the intensity of the OH- absorption bands become higher as the exchange temperature increase. And the position of OH-peaks move to high wave number as the temperature increases. We can obtain the information of the structure and chemical bonds of a near-surface region with proton exchange-depth by analysing the IR reflection spectra of different incident angles.Second, 455 nm laser is found effective for the photo-assisted proton exchange on Y-cut and Z-cut Fe:LiNbO3. Stand-alone FT-IR microscope(Bruker LUMOS) was employed to analyze the spatial distribution of proton concentration. The analysis of the integration of the O-H vibration band, plotted as function of spatial axis(x and y-axis) in2 D and 3D show that the proton incorporation happens intensively at the laser-irradiated region while quite few protons are detected outside this region, indicating that the proton exchange process can be controlled selectively by the laser spot and that the laser irradiation has an effect on accelerating proton exchange. Moreover, the comparison resultof etch topography between +Z and-Z, indicates that the +Z surface tends to be more easily etched under irradiation than the –Z surface, which is quite different from the etching resistance demonstrated by the +Z surface in the conventional LN etching experiments.We study the photo-assisted proton exchange and the subsequent chemical etching results and analyse the trajectory model of the photo-excited electrons. Here in this case, we post explanations of the selective proton exchange and chemical etching mechanisms. Both thermal effects and photogalvanic effects of photo-excited electrons are suggested to account for this results. Under irradiation, the photo-excited electrons in Fe-doped LN transport along one fixed direction, inducing a current toward the –Z surface(i.e. PVE).This current leads to the accumulation of the electrons in the +Z surface and to the congregation of the positive charged traps in the –Z surface. The excessive negative charges in the +Z surface may attract the protons in the acid for charge compensation, resulting in the acceleration of the proton exchange and the subsequent chemical etching. Accordingly,the excessive positive charges in the –Z surface will repel the protons and induce the etching resistance of the –Z surface.