Sharp electromagnetic resonances play an essential role in physics in general and optics in particular. The last decades have witnessed the successful developments of high-quality (Q) resonances in microcavities operating below the light line, which however is fundamentally challenging to access from free space. Alternatively, metasurface-based bound states in the continuum (BICs) offer a complementary solution of creating high-Q resonances in devices operating above the light line, yet the experimentally demonstrated Q factors under normal excitations are still limited. Here, we present the realizations of quasi-BIC under normal excitation with a record Q factor up to 18 511 by engineering the symmetry properties and the number of the unit cells in all-dielectric metasurface platforms. The high-Q quasi-BICs exhibit exceptionally high conversion efficiency for the third harmonic generation and even enable the second harmonic generation in Si metasurfaces. Such ultrasharp resonances achieved in this work may immediately boost the performances of BICs in a plethora of fundamental research and device applications, e.g., cavity QED, biosensing, nanolasing, and quantum light generations.

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Localized-surface plasmon resonance is of importance in both fundamental and applied physics for the subwavelength confinement of optical field, but realization of quantum coherent processes is confronted with challenges due to strong dissipation. Here we propose to engineer the electromagnetic environment of metallic nanoparticles (MNPs) using optical microcavities. An analytical quantum model is built to describe the MNP-microcavity interaction, revealing the significantly enhanced dipolar radiation and consequentially reduced Ohmic dissipation of the plasmonic modes. As a result, when interacting with a quantum emitter, the microcavity-engineered MNP enhances the quantum yield over 40 folds and the radiative power over one order of magnitude. Moreover, the system can enter the strong coupling regime of cavity quantum electrodynamics, providing a promising platform for the study of plasmonic quantum electrodynamics, quantum information processing, precise sensing and spectroscopy.Metallic nanostructures confine light on the subwavelength scale due to the collective excitation of electrons known as localized-surface plasmon resonances (LSPRs). Ultrasmall mode volumes of the plasmonic resonances down to cubic nanometers make them a powerful platform for fundamental studies and novel applications, such as spaser [1,2], superlens [3] and quantum plasmonics [4,5]. An emerging field is to study the nanoscale light-matter interaction between plasmonic mode and few or even single quantum emitters (e.g., atoms, molecules or quantum dots, etc). For example, in the weak coupling regime LSPRs are widely used to enhance fluorescence [6][7][8] and Raman scattering [9][10][11] and to achieve unidirectional emission [12]; in the strong coupling regime the coherent hybridizations between plasmonic resonances and quantum emitters have also been investigated both theoretically [13][14][15][16][17][18] and experimentally [19][20][21][22][23][24][25].Two approaches have been taken to achieve singleemitter strong coupling -lowering the mode volumes and suppressing the dissipation. The former typically requires ultra-fine geometries with nanometer or even subnanometer precision [20,25], posing challenges on fabricating metallic structures and positioning individual quantum emitters. For the latter, the dipolar plasmonic modes dissipates through both radiation and Ohmic absorption, while the multipole modes are purely absorptive [26]. A better coherence can be achieved by reducing the excitation of multipole modes or enhancing the excitation of dipolar modes. Thus, efforts have been made to tailor the geometry of the metallic nanostructures, for example, elongating a metallic nanoparticle (MNP) to separate the dipolar and multipole resonances [27], and forming dimer or arrays to obtain stronger coupling of a certain dipolar mode [28]. An alternative approach is cancelling the coupling between the emitters and the multipole modes with emitters homogeneously distributed around the metallic structure [29]. Despite of these efforts, the ...