May 26, 2023 
Hybrid bound states in the continuum in terahertz metasurfaces
(Nanowerk News) The quality factor (Q) is a critical parameter that characterizes the strength of lightmatter interactions. Cavities with higher quality factors have the ability to efficiently confine light and thus enhance lightmatter interactions. This feature holds significant importance in various applications such as lasers, filters, harmonic generation, and sensors.

Different schemes have been proposed to improve quality factors in microcavities, such as microdisks, Bragg reflector microcavities, and photonic crystals. Above the light cone of band structures, bound states without radiative leakage of energy are also accessible, namely bound states in the continuum (BIC).

BIC provides a generalized method to obtain ultrahigh quality factor resonances, thereby becoming a powerful mechanism to enhance lightmatter interactions that have found applications in lowthreshold lasers, multispectral sensing, and high harmonics generation.


Fig. 1 Hybrid BIC lattices. (ac) Schematic diagram of symmetryprotected BIC lattice without radiation channel (a), uniform quasiBIC lattice with radiation channel open for all the resonators by breaking symmetry (b), and hybrid quasiBIC lattice with half radiation channel open interchanging along x axis (c). (© OptoElectronic Science)

For a typical BIC, there is a quadratic quantitative relation between Q and wave vector (k), and usually a tiny disturbance in k would lead to a rapid deterioration of Q. However, defects and disorders are inevitably introduced during processing which greatly reduce the quality factor of resonances in actual samples.

The idea of merging BIC starts with modulating the exponential coefficient between Q and k (from 2 to 6), which largely alleviates the deterioration rate of Q and provides a very effective mechanism. But this approach requires precise control of the geometric dimensions of microstructures and is only applicable to band structures that simultaneously have symmetryprotected and accidental BICs, with rather harsh requirements.

Recently, Longqing Cong's group at Southern University of Science and Technology (SUSTech) proposed a more generalized approach to improve the overall quality factors and robustness of symmetryprotected BIC. Unlike the conventional approach of achieving quasiBIC by breaking symmetry of resonators uniformly in the entire lattice of metamaterials (see Fig. 1a and b), they selectively maintain the local C2 symmetry of the entire lattice so that radiative loss could be decreased and the quality factor of the array is improved (see Fig. 1c).


Fig.2 Significant Q improvement in hybrid BIC lattices and robustness against fabrication imperfections. (a) Evolution of radiative Q versus asymmetry degree for UqBIC, HtBIC, HxBIC and HqBIC lattices. The overall quality factors are improved in hybrid unit cells with a lower radiation density. (b) Influences of fabrication imperfection on quality factors in the four scenarios. (© OptoElectronic Science)

This is a generalized method that could be extended to any symmetryprotected BIC without requirements of accurate geometric design or band selectivity. According to qualitative and quantitative analysis, the hybrid BIC lattice can achieve a quality factor more than 14.6 times higher than that of the conventional lattice (Fig. 2a). By increasing the proportional coefficient between Q and k, the quality factor robustness of the hybrid BIC metasurfaces against disorders and other disturbances is improved, thereby effectively reducing the deterioration of the quality factor in actual devices. This provides a more generalized and simple approach to achieving a highquality factor (Fig.2b).

Through reciprocal space analysis of the lattice, the hybrid BIC lattice can simultaneously fold the eigenstates of the X, Y, and M points of the uniform BIC lattice to the ?? point, so that multiple Fano resonances can be observed in the farfield radiation (Fig. 3).


Fig.3 Generalized highorder hybrid BIC. (a, b) Microscopic images of HtBIC and HqBIC metasurfaces with three and one asymmetric resonators out of four in a 2??2 supercell, respectively, and the period is 2a along both x and y axes. Scale bar, 20 µm. (c) Schematic diagram of band folding from UqBIC lattice (black) to HtBIC/HqBIC (red) in the Brillouin zone. (d) Simulated transmission amplitude spectra of the HtBIC (left) and HqBIC (right) metasurfaces at asymmetry degree of 2.97%. (© OptoElectronic Science)

Multiple highquality factor Fano resonances are very important in pulse generation, sensing, communication, etc., especially for the development of sensing and nextgeneration wireless communication based on terahertz photonics. This offers novel insights into fusing metasurfaces and terahertz photonics to facilitate their development in diverse fields. This work will further enrich the physical implication of BIC and broaden the perspective of metamaterials and terahertz photonics.

The team published their finding in OptoElectronic Science ("Hybrid bound states in the continuum in terahertz metasurfaces").
