Supercontinuum-Assisted Self-Third-Harmonic Generation in Ta ₂ O ₅ Waveguides Revealed by Phase Matching and Spatial-Mode Analysis

Integrated nonlinear waveguides provide a compact platform for broadband spectral broadening and on-chip frequency conversion. Here, we investigate supercontinuum generation (SCG) and third-harmonic generation (THG) in Ta2O5 ridge waveguides pumped near 1550 nm, with particular emphasis on supercontinuum-assisted self-third-harmonic generation (self-THG). Conventional pump-induced THG is first established in a 1100 nm-wide waveguide through the wavelength-tripling relation and the cubic dependence of harmonic intensity on pump power. We then show that, under stronger nonlinear broadening, the visible output can no longer be attributed solely to direct THG of the original pump. By optimizing the waveguide width, a 2000 nm-wide device under horizontal polarization produces a spectrum spanning 492-2200 nm, corresponding to approximately 2.16 octaves, together with distinct visible peaks near 492, 548, and 612 nm. Numerical analysis shows that the optimized waveguide provides a more favorable dispersion condition for generating long-wavelength infrared continuum components, while phase-matching analysis identifies candidate higher-order THG channels linking these continuum components to the observed visible peaks. Far-field mode comparison further supports that the visible output is multimodal in origin. In particular, the 612 nm peak provides the clearest evidence of self-THG, because its corresponding infrared fundamental lies within the broadened supercontinuum rather than in the original pump band. These results establish a practical framework for distinguishing direct THG from self-THG in integrated nonlinear waveguides and highlight Ta2O5as a promising platform for compact visible-infrared spectral translation.