Fig. 1: Concept of broadband UV to blue-green generation using sum-frequency mixing.

a The infrared component of a broadband supercontinuum (SC) pulse is mixed in a \({\chi }^{(2)}\) medium with a higher frequency, stronger pump pulse. In the absence of temporal overlap, only second-harmonic generation (SHG) occurs, with the quadratic nonlinearity suppressing weaker spectral components, and leading to gaps in the generated visible spectrum. By introducing an appropriate delay, allowing the pump and SC pulses to walk through each other, sum-frequency-mixing occurs, which blue-shifts and enhances the supercontinuum components in a way that provides gap-free UV-blue generation. b Efficient conversion of 1 GHz repetition-rate, \(\approx\)100 pJ pulses is achieved by using an MgO:PPLN ridge waveguide containing a custom-designed quasi-phasematched grating. Inset: Scanning electron-beam micrograph of a representative waveguide. c The MgO:PPLN domain-width pattern follows an aperiodic design optimized to ensure gap-free upconversion. The optimized QPM grating design has entry and exit periods of \({\Lambda }_{1}\) = 6.3 µm and \({\Lambda }_{2}\) = 2.2 µm, respectively, and a nonlinearity of \(\alpha\) = 0.48, corresponding to a structure (b) with domain sizes that become progressively smaller towards the end of the waveguide. d Simulated 30-fs pump pulse and supercontinuum (red) and resulting SHG and SFM spectrum achieved for optimal pump-supercontinuum delay (\(\tau\) = 250 fs). e Simulated delay-resolved spectra (step size, \(\triangle \tau\) = 10 fs) illustrating the additional bandwidth created by the SFM process.