  Tailoring optical nonlinear effects (e.g. harmonic generation, sum-frequency
mixing, etc.) in the recently emerging all-dielectric platform is important for
both the fundamental science and industrial development of high-efficiency,
ultrafast, and miniaturized photonic devices. In this work, we propose a novel
paradigm for dynamically switchable high-harmonic generation in Silicon
nanodimer metasurfaces by exploiting polarization-controlled dual bound states
in the continuum (BIC). Owing to the high-quality factor of BIC resonances,
efficient harmonic signals including the third-harmonic generation and
fifth-harmonic generation from a direct process as well as a cascaded process
by degenerate four-wave mixing are obtained. Moreover, the BICs and their
resonantly enhanced harmonics can be switched on or off with high selectivity
respect to the fundamental pump polarization. Compared with previous reports,
our work provide a simple but effective tuning strategy by fully exploring the
structural symmetry and polarization degree of freedom rather than resorting to
additional external stimuli, which would have great advantages in smart
designing tunable and switchable nonlinear light source for chip-scale
applications.
