Abstract
The α-phase waveguides directly produced in one fabrication step only are well known for preserving both the excellent nonlinear properties and the ferroelectric domains orientation of lithium niobate substrates. However, by using the piezoresponse force microscopy (PFM), we present a coherent study on ferroelectric dipoles switching induced by the fabrication process of α-phase waveguides on Z-cut congruent lithium niobate (CLN) substrates. The obtained results show that the proton exchange process induces a spontaneous polarization reversal and a reduction in the piezoelectric coefficient d33. The quantitative assessments of the impact of proton exchange on the piezoelectric coefficient d33 have been quantified for different fabrication parameters. By coupling systematic PFM investigation and optical characterizations of α-phase protonated regions and virgin CLN on ±Z surfaces of the samples, we find a very good agreement between index contrast (optical investigation) and d33 reduction (PFM investigations). We clearly show that the increase in the in-diffused proton concentration (increase in index contrast) in protonated zones decreases the piezoelectric coefficient d33 values. Furthermore, having a high interest in nonlinear performances of photonics devices based on PPLN substrates, we have also investigated how deep the spontaneous polarization reversal induced by proton exchange takes place inside the α-phase channel waveguides.
Highlights
Waveguides fabricated on periodically poled lithium niobate (PPLN) substrates are already one of the most widely used devices for many nonlinear optical applications based on the quasi-phase matching (QPM) process
By using the piezoresponse force microscopy (PFM), we present, in this paper, a coherent study of ferroelectric dipoles switching that occurred during the fabrication of α-phase channel waveguides on Z-cut congruent lithium niobate (CLN) substrates
By PFM investigation, we presented a coherent study of spontaneous polarization reversal induced during the fabrication of α-phase channel waveguides on
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. An efficient nonlinear process such as second harmonic generation (SHG), spontaneous parametric down conversion (SPDC) or different variants of optical frequency conversion, requires, among others, waveguide fabrication techniques that allow preserving both the nonlinear coefficient and the domains orientation of the substrate Both planar and channel optical waveguides can be fabricated by using refractive index modification techniques of the sub-surface layer of either the lithium niobate (LN) or PPLN substrates, among which the method of proton exchange is a very popular technique. Having a high interest towards nonlinear performances of photonics devices based on PPLN substrates, we have investigated how deep the spontaneous polarization reversal induced by proton exchange takes place inside the α-phase channel waveguides. Taking into account that in oxygen-octahedra ferroelectrics, such as lithium niobate, both nonlinear optical and electro-optic coefficients are directly proportional to the spontaneous polarization [5,7], our investigation acquires considerable value
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