Ultrasensitive Band‐Folded Transparent Resonances Enabled by a Double‐Lattice Metasurface

High‐ Q electromagnetically induced transparency (EIT)‐like resonances in all‐dielectric metasurfaces are hindered by poor field overlap with the environment due to strong optical confinement. Here, this limitation is overcome using a double‐lattice monocrystalline silicon metasurface, where Brillouin‐zone band folding enables collective coupling between a narrow electric‐quadrupole lattice resonance and a broad magnetic‐dipole mode. The resulting Fano interference yields an EIT‐like transparency window with 75% of the near‐field energy extending into the surrounding medium. Under continuous‐wave optical pumping, the resonance exhibits reversible spectral shifts, enabling low‐power all‐optical switching. In bulk refractive‐index sensing, we experimentally achieve a figure of merit (FOM) exceeding 330/RIU at 920 nm–the highest reported for dielectric metasurfaces at this wavelength. Full‐wave simulations further predict that scaling the design to the telecom O‐band increases the FOM to over 9800/RIU, substantially outperforming state‐of‐the‐art plasmonic and dielectric sensors. Our lattice‐engineering strategy simultaneously enhances the Q ‐factor, environmental sensitivity, and dynamic tunability without requiring strictly symmetry substrates. This work establishes a scalable platform for ultra‐sensitive label‐free biosensing, energy‐efficient modulation, and integrated slow‐light photonics.