Symmetry‐Protected Dual Bound States in the Continuum in Metamaterials

Abstract

to access extreme confinement of photons into micro-or nanoscale region [3,10,11] above the light line within the radiation continuum. In general, EM wave is usually interpreted in terms of frequency spectrum, and it propagates as a spectral continuum above the light line in different media. Different orders of resonances may be found in the propagating continuum at specific frequencies described by i n n n 0  ω ω γ = + , where the real part indicates resonance frequencies and the imaginary part indicates the leakage rates of the n-order mode. [12] For a strongly confined mode, the leakage rate is significantly reduced, and goes to zero if photons are perfectly trapped in a so-called bound state in an ideal scenario. [13] However, such a bound state in the continuum (BIC) is not observable from the spectrum due to the nonradiative feature with vanishing spectral linewidth. As an embedded eigenvalue of the photonic system, [14] it becomes visible in the spectrum with a finite leakage rate as a quasi-BIC by introducing external perturbations. Such a BIC-inspired mechanism for light confinement allows a general strategy to access extremely high quality factor (Q, Q = ω 0 /2γ) resonance in optical cavities.Dielectric photonic crystals (PhCs) are an excellent platform to explore the BICs which largely avoid the intrinsic Ohmic loss for extremely high Q generation by exciting displacement current. [15,16] Nevertheless, the geometric size of unit cells in PhCs is usually at the scale of resonance wavelength, and resonant energy is thus loosely confined in the resonator cavity with a relatively large mode volume (V). In this condition, Purcell factor proportional to Q/V is largely reduced in the traditional PhCs despite a large Q, which hinders the efficient spontaneous emission rates in the context of cavity quantum electrodynamics. [17] An efficient approach to densely confine photons is by using metamaterials whose building blocks are in the subwavelength scale, and the resonant mode volume could be further reduced to a deep subwavelength scale by carefully designing the resonator geometry and mode properties. Here, we demonstrate dual symmetry-protected BICs in a subwavelength metamaterial that are independently induced by orthogonal polarizations of incident light. The features of the subwavelength BICs are experimentally observed in the terahertz spectrum by slightly breaking the C 2 symmetry with large Q factors. We also show the inverse square dependence of Q factor on the structural asymmetry parameter which describes a Bound state in the continuum (BIC) is a mathematical concept with an infinite radiative quality factor (Q) that exists only in an ideal infinite array of resonators. In photonics, it is essential to achieve high Q resonances for enhanced light-mater interactions that could enable low-threshold lasers, ultrasensitive sensors, and optical tweezers. Hence, it is important to explore BICs in different photonic systems including subwavelength metamaterials where symmetry-protected dual BICs exist. Th...

Terahertz sensing of 7 nm dielectric film with bound states in the continuum metasurfaces

“…The properties of the quasi-BIC Fano resonance, i.e., Q factor and amplitude of the resonance are determined by the structural asymmetry 'd' values. 35,40,44 Figure 1 Here it is noted that the red shift in quasi-BIC Fano state is very small at low analyte thickness. Such a small shift in resonance frequency could be accurately measured only by increased time scan for the terahertz pulse in the THz-TDS measurements.…”

“…In this study, we choose a typical double gap terahertz asymmetric split ring (TASR) as a building block for the metamaterial due to its design flexibility and sharp quasi-BIC Fano resonant spectral feature. 22,35,[40][41][42] A small portion of a large array of two-dimensional planar TASR metamaterial fabricated on a COC substrate is shown in Figure 1 34,35 The TASR arms act as an inductor while the electric field is confined and enhanced locally in the vicinity of the capacitive split gaps. This TASR metamaterial resides on a low refractive index COC substrate, which is a very promising dielectric material with excellent properties such as high chemical resistance, low water absorption, high flexibility, good thin film compatibility (with use of appropriate adhesion layers) and large transparency across terahertz (0.2-2.5 THz) region of the electromagnetic spectrum.…”

Spectral and temporal evidence of robust photonic bound states in the continuum on terahertz metasurfaces

“…In the absence of material losses, BICs have infinite lifetimes and zero linewidths. An approach to trap light in such remarkable electromagnetic modes is to exploit metasurfaces [2][3][4][5][6][7][8][9], i.e., subwavelength arrays (in the nondiffractive region) where only the specular reflection/transmission channels are allowed by symmetry, wherein outgoing specular channels can be suppressed by tuning the parameters of the system in various manners, leading to symmetry-protected BICs. Achieving robust BICs and quasi-BICs will be of great interest for the realization of cavities with arbitrarily high Q-factor and for applications such as lasing and sensing [10][11][12].…”

Tunable Bound States in the Continuum in All-Dielectric Terahertz Metasurfaces

“…It was revealed that dielectric metasurfaces with broken in-plane symmetry of unit cells can support high Q-factor resonance arising from the distortion of symmetry-protected BIC [25]. Such a BIC-inspired mechanism allows a general strategy to access extremely high Q resonances and giant enhancement of electromagnetic fields [26], realizing many useful functionalities including lasing and biosensing [27,28].…”