Spontaneous Polarization Reversal Induced by Proton Exchange in Z-Cut Lithium Niobate α-Phase Channel Waveguides

The alpha-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 alpha-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 d(33). The quantitative assessments of the impact of proton exchange on the piezoelectric coefficient d(33) have been quantified for different fabrication parameters. By coupling systematic PFM investigation and optical characterizations of alpha-phase protonated regions and virgin CLN on & PLUSMN;Z surfaces of the samples, we find a very good agreement between index contrast (optical investigation) and d(33) 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 d(33) 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 alpha-phase channel waveguides.

Automated summary

In this study, the ferroelectric dipoles switching induced by the fabrication process of alpha-phase waveguides on Z-cut congruent lithium niobate substrates was investigated using piezoresponse force microscopy. The results showed that the proton exchange process caused a spontaneous polarization reversal and a reduction in the piezoelectric coefficient d(33). The study also quantitatively assessed the impact of proton exchange on the piezoelectric coefficient d(33) for different fabrication parameters, showing a correlation between index contrast and d(33) reduction.