Optical control of resonances in temporally symmetry-broken metasurfaces.

Tunability in active metasurfaces has mainly relied on shifting the resonance wavelength or increasing material losses to spectrally detune or quench resonant modes, respectively. However, both methods face fundamental limitations, such as a limited Q factor and near-field enhancement control and the inability to achieve resonance on-off switching by completely coupling and decoupling the mode from the far field. Here we demonstrate temporal symmetry breaking in metasurfaces through ultrafast optical pumping, providing an experimental realization of radiative-loss-driven resonance tuning, allowing resonance creation, annihilation, broadening and sharpening. To enable this temporal control, we introduce restored symmetry-protected bound states in the continuum. Even though their unit cells are geometrically asymmetric, coupling to the radiation continuum remains fully suppressed, which, in this work, is achieved by two equally strong antisymmetric dipoles. By using selective Mie-resonant pumping in parts of these unit cells, we can modify their dipole balance to create or annihilate resonances as well as tune the linewidth, amplitude and near-field enhancement, leading to potential applications in optical and quantum communications, time crystals and photonic circuits.