Ultranarrow and Wavelength-Scalable Thermal Emitters Driven by High-Order Antiferromagnetic Resonances in Dielectric Nanogratings

Quasi-dark resonances with antiferromagnetic order in silicon metasurfaces

Quasi-dark resonances exhibiting antiferromagnetic order are theoretically investigated in a near-infrared metasurface composed of square slotted rings etched in a thin silicon layer on glass substrate. Access to the quasi-dark mode is achieved by reducing the symmetry of the metasurface according to the findings of a detailed group theory analysis. A thorough finite-element study reveals the key optical properties of the antiferromagnetic order quasi-dark mode, namely resonant wavelengths, quality factors, angular dispersion, and its robustness against optical extinction losses. It is demonstrated that the thickness of the silicon metasurface can adjust the asymmetry degree of the resonant Fano lineshape without affecting substantially its quality factor. Furthermore, tuning of the resonant wavelength can be achieved without significant modification of the Fano lineshape by controlling the angle of incidence of the impinging planewave. Overall, the work presents an all-dielectric, near-infrared metasurface for the excitation of sharp resonances with antiferromagnetic order, which can find use in emerging applications based on this particular configuration of artificial optical magnetism and/or strong field confinement and light-matter interaction.

Evolution and Nonreciprocity of Loss-Induced Topological Phase Singularity Pairs

Singular behaviors are increasingly being found in structures supporting bound states in the continuum (BICs), while the nonreciprocity with spectral phase singularity has not been reported. Here, we demonstrate the origin, evolution, and application of topological phase singularity pairs (TPSPs) resulting from BICs in nonreciprocal and non-Hermitian systems. The nonreciprocity contributes to creating accidental BICs asymmetrically, each of which can split into TPSP with topological charges ±1 in reflection phases by inserting loss. The formation, annihilation, and revival processes of these TPSPs can be selectively controlled via either material or radiative loss. The criteria to predict both number and angular positions of asymmetric BICs are established. Near-complete violation of Kirchhoff's law of thermal radiation has been correspondingly manifested over a wide angular range. The unveiled physics synergizing nonreciprocity and topology will bring new opportunities in multidisciplinary areas like thermal science, magneto-optics, or topological metasurfaces.