Fabrication-tolerant modal phase matching for all-optical frequency conversion in layer-poled thin film lithium niobate waveguide

Thin film lithium niobate (TFLN) combines strong second-order nonlinearity with low propagation loss, making it an ideal platform for on-chip frequency conversion and quantum light generation. Frequency conversion is usually achieved by quasi-phase matching (QPM) via electric-field poling. However, this scheme shows very high sensitivity to the dimensions of the waveguide, poling period, and duty cycle, resulting in a lack of repeatability of the phase matched wavelength and efficiency, which in turn limits the spread of TFLN frequency converters in complex circuits and hinders wafer-scale production. Here, we propose a layer-poled modal phase matching (MPM) that shows a phase matched wavelength 5–10 times less sensitive to fabrication uncertainties. By selectively poling the bottom part of the waveguide, the second harmonic is efficiently generated on a higher order waveguide mode. We validate this approach by poling foundry-fabricated TFLN waveguides as a post-process, performing a comprehensive tolerance analysis, and comparing the second harmonic generation (SHG) performance achieved with MPM to that of conventional QPM in the same waveguides. Finally, we demonstrate how our MPM can be exploited to obtain efficient intraband frequency conversion processes at telecom wavelengths by leveraging simultaneous SHG and difference frequency generation in the same waveguide. Our layer-poled MPM strategy relaxes critical fabrication constraints and paves the way for robust, reproducible TFLN frequency converters in quantum photonics, sensing, and optical communications.