To fully capitalize on the interplay between 𝜒(2) and 𝜒(3) effects, low-loss devices where both nonlinearities are strong are needed. Mengjie Yu and coworkers now show that monolithic integrated lithium niobate (LN) waveguides are ideally suited for the task. By leveraging groundbreaking recent advances in the fabrication of integrated LN components, the authors realize high-confinement waveguides whose dispersion profile is carefully engineered to optimize nonlinear interactions. Thanks to the strong nonlinearity and favorable dispersion characteristics of the fabricated waveguides, a coherent SCG spanning more than two octaves is achieved—with just 100-picojoule-level pump pulses. Moreover, as the supercontinuum spectrum overlaps with the SHG components, the authors are able to directly measure the carrier-envelope offset frequency of the pump laser, attaining a signal-to-noise ratio of 30 dB. These results beautifully illustrate the potential of the monolithic integrated LN photonic platform, and could prove particularly important for the development of chip-scale optical frequency combs.
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