Extended Bound States in the Continuum with Symmetry‐Broken Terahertz Dielectric Metasurfaces

Abstract

Excitation of the non‐radiative eigenmodes in photonics and metamaterials above their light cone, that is, the bound states in the continuum (BICs), has drawn fundamental and technological interest. However, the current studies on photonic BICs are mainly restricted to the characterization of the eigenmodes with fixed point group. Here, a fourfold rotationally symmetric (C4v) array of silicon resonators in a square lattice is investigated that reveals the traditional symmetry‐protected (Γ‐state) and accidental (off‐Γ) BICs in the momentum space. Upon symmetry‐breaking, BICs at Γ‐state transform into quasi‐states with decaying quality factors that obey the inverse‐square law α−2 as a function of the structural asymmetry parameter α. Surprisingly, at larger asymmetries, multiple new states of extended BICs are observed beyond typical inverse‐square law behavior. The structural asymmetry of a resonator provides an important degree of freedom for exploring the Friedrich–Wintgen and new accidental BICs that are topologically protected by robust polarization vortices. The extended family of sharp BIC resonances in all‐dielectric silicon metasurfaces is extremely important for scalable applications in terahertz sensing, higher harmonic generation, and ultralow‐power switching.

Photonic and Plasmonic Metasensors

“…In Figure 15, sketches of several types of recently proposed Q-BIC metamaterials and resonators are demonstrated. [424][425][426][427][428][429][430][431][432][433] In the span of a few years, the sharpness of the optically driven resonant states in Q-BIC metamaterials has stimulated researchers to create ultraprecise plasmonic and photonic metasensors based on this technology. [434] In a leading study, [422] Mocella's team explored a novel sensing mechanism by structuring an all-dielectric photonic crystal metasurface (PhCM) to sustain plasmon-like surface waves across the NIR spectrum.…”

“…i) Reproduced with permission. [430] Copyright 2020, John Wiley & Sons. j) Reproduced with permission.…”

“…Copyright 2020, John Wiley & Sons. i) Reproduced with permission [430]. Copyright 2020, John Wiley & Sons.…”

High-quality-factor dual-band Fano resonances induced by dual bound states in the continuum using a planar nanohole slab

“…More specially, BICs can be perturbed via oblique incidence or symmetry-broken nanostructures, and the QBICs can be realized as the radiation channel between the eigenstates and the free space is opened [18,19]. However, most of the dielectric nanostructures used to excite QBICs with high Q-factor are complicated, such as asymmetrical nanocrosses [20], asymmetrical nanorings [21], asymmetrical nanobars [22][23][24] and asymmetrical nanorods [25][26][27][28], which are challenging in fabrication due to the requirement of inserting the deep subwavelength slits [20][21][22][23][24] or nanoholes [25][26][27][28] into the photonic structures. Other nanostructures such as the reshaped rectangular bars [29,30] have the increased sharp edges, making them more difficult to be accurately fabricated through conventional lithographic techniques, which reduces the Q-factor and the resonance lifetime of the devices due to the opening of additional leaky channels [31,32].…”

High Quality-Factor Dual-Band Fano Resonances Induced by Bound States in The Continuum Using A Planar Nanohole Slab

“…More specially, BICs can be perturbed via oblique incidence or symmetry-broken nanostructures, and the QBICs can be realized as the radiation channel between the eigenstates and the free space is opened [14]. However, most of the dielectric nanostructures used to excite BICs with high Q-factor are complicated, such as asymmetrical nanocross [15], asymmetrical nanoring [16], asymmetrical nanobars [17][18][19] and asymmetrical nanorods [20][21][22][23], which is challenging in fabrication due to the requirement of inserting the deep subwavelength slits [15][16][17][18][19] or nanoholes [20][21][22][23] into the photonic structures. Other nanostructures such as the reshaped rectangular bars [24,25] have the increased sharp edges, making them more difficult to be accurately fabricated through conventional lithographic techniques, which reduces the Q-factor and the resonance lifetime of the devices due to the opening of additional leaky channels [26,27].…”