Broadband ultraviolet-visible frequency combs from cascaded high-harmonic generation in quasi-phase-matched waveguides

High-harmonic generation (HHG) provides short-wavelength light that is useful for precision spectroscopy and probing ultrafast dynamics. We report efficient, phase-coherent harmonic up to 9th-order (333 nm) in chirped periodically poled lithium niobate waveguides driven by phase-stable $\leq$12-nJ, 100 fs pulses at 3 $\mu$m with MHz repetition rate. A mid-infrared ultraviolet-visible conversion efficiency as high 10% observed, amongst an overall 23% of the fundamental all harmonics. verify coherence frequency combs despite complex highly nonlinear process. Numerical simulations based on a single broadband envelope equation quadratic nonlinearity give estimates within approximately 1 order magnitude over wide range experimental parameters. From this comparison we identify dimensionless parameter capturing competition between three-wave mixing group-velocity walk-off harmonics governs cascaded HHG physics. These results can inform different platforms.