Pushing Optical Virus Detection to a Single Particle through a High- Quasi-bound State in the Continuum in an All-dielectric Metasurface.

Bound states in the continuum (BICs) have emerged as a powerful platform for boosting light-matter interactions because they provide an alternative way of realizing optical resonances with ultrahigh quality factors, accompanied by extreme field confinement. In this work, we realized an optical biosensor by harnessing a quasi-BIC (qBIC) supported by an all-dielectric metasurface with broken symmetry, whose unit cell is composed of a silicon cuboid with two asymmetric air holes. Thanks to the excellent field confinement within the air gap of a metasurface enabled by such a high- qBIC, the figure of merit (FOM) of the biosensor is up to 2136.35 RIU. Futhermore, we demonstrated that such a high- metasurface can push the detection limit to a few virus particles. Our results may find exciting applications in extreme biochemical sensing like COVID-19 with ultralow concentrations.