Cryogenic Second-Harmonic Generation in Periodically Poled Lithium Niobate Waveguides

We demonstrate second-harmonic generation in a fiber-coupled periodically poled titanium in-diffused lithium niobate waveguide over the temperatures range from 292 to 4.4 K. We observe a reproducible shift in the phase-matched pump wavelength within the telecom band, which is at odds with the phase matching arising from existing models of the refractive indices in this temperature regime. We further investigate transient discontinuities while temperature cycling, which we attribute to the build-up and discharge of pyroelectric fields. Our results show that modifications of the Sellmeier coefficients for lithium niobate are required in the cryogenic temperature regime for our waveguides, but also establish titanium in-diffused waveguides in lithium niobate as a versatile nonlinear photonic integration platform compatible with cryogenic quantum technologies.