Enhanced high-harmonic generation with spectral tunability in nonlocal metasurfaces enabled by the excitation of quasiguided modes

Regular nonlinear metasurfaces can relax the phase matching condition in conventional nonlinear optics, but unfortunately they still suffer from unsatisfactory nonlinear conversion efficiency due to the weak interaction between light and matter occurring within individual meta-atoms. In this work, we demonstrate that the interactions can be enhanced further in nonlocal metasurfaces by harnessing the quasiguided modes (QGMs). This enhancement arises mainly from an extended interaction length within the plane of nonlocal metasurfaces, together with the large local electric field enhancement provided by QGMs. Upon excitation, the incident light will couple into the QGM and propagate laterally within the metasurface plane. The distributed nonlinear signals generated at different positions of the nonlocal metasurface will be directed towards certain directions determined by the momentum matching. Thanks to the steep dispersion of QGM and the high robustness of the Q factor against wave vector, one can spectrally tune the QGM by simply changing the incident angle. Using a static nonlocal metasurface in the form of dimerized grating on a silicon slab incorporating a period-doubling perturbation, we report the experimental demonstration of third-harmonic generation enhanced by a general factor of 5 \ifmmode\times\else\texttimes\fi{} ${10}^{3}$ with the operation wavelength easily tuned over a broad bandwidth.