Evidence of nonlinear diffusion in KTP waveguides

TL;DR

This study investigates the fabrication of rubidium-exchanged waveguides in KTP, revealing nonlinear diffusion behavior influenced by stress, which challenges existing theoretical models and informs high-efficiency quantum device engineering.

Contribution

The paper provides the first detailed analysis of the nonlinear diffusion process in KTP waveguides, highlighting the impact of fabrication parameters on waveguide depth and stress effects.

Findings

Narrower waveguides are deeper than expected.

Waveguide depth variation increases with temperature and time.

Stress influences the diffusion process in KTP waveguides.

Abstract

Integrated $\chi^{(2)}$ devices are a widespread tool for the generation and manipulation of light fields, since they exhibit high efficiency, small footprint and the ability to interface them with fibre networks. Surprisingly many things are not fully understood until now, in particular the fabrication of structures in potassium titanyl phosphate (KTP). A thorough understanding of the fabrication process and analysis of spatial properties is crucial for the realization and the engineering of high efficiency devices for quantum applications. In this paper we present our studies on rubidium-exchanged waveguides fabricated in KTP. Employing energy dispersive X-ray spectroscopy (EDX), we analysed a set of waveguides fabricated with different production parameters in terms of time and temperature. We find that the waveguide depth is dependent on their widths by reconstructing the waveguide depth profiles. Narrower waveguides are deeper, contrary to the theoretical model usually employed. Moreover, we found that the variation of the penetration depth with the waveguide width is stronger at higher temperatures and times. We attribute this behaviour to stress-induced variation in the diffusion process.