Hybrid bound states in a continuum in terahertz metasurfaces

(Nanowerk News) The quality factor (Q) is an important parameter characterizing the strength of light-matter interaction. Cavities with a higher quality factor have the ability to confine light efficiently and thus enhance light-matter interactions. This feature is very important in applications such as lasers, filters, harmonic generators and sensors.

Different schemes have been proposed to improve the quality factor in microcavities, such as microdisks, Bragg reflector microcavities and photonic crystals. On top of the band structure of the light cone, a bound state without energy radiation leakage is also accessible, that is, a bound state in the continuum (BIC).

BIC provides a general method for obtaining very high quality factor resonances, thus being a powerful mechanism for enhancing light-matter interaction that has found applications in low-threshold lasers, multi-spectral sensing, and high-harmonic generation. Fig. 1 Hybrid BIC Grid. (ac) Schematic diagram of a symmetry-protected BIC lattice without radiation channels (a), a uniform quasi-BIC lattice with open radiation channels for all resonators with symmetry breaking (b), and a quasi-hybrid BIC lattice with half open radiation channels interchanged along x-axis (c). (© Opto-Electronic Science)

For a typical BIC, there is a quadratic quantitative relationship between Q and the wave vector (k), and usually a small perturbation of k will lead to a rapid decrease in Q. However, defects and interference inevitably occur during processing which greatly reduces the quality factor of resonance in the actual sample.

The idea of ​​combining BIC begins by modulating the exponential coefficient between Q and k (from -2 to -6), which largely reduces the rate of decline in Q and provides a very effective mechanism. But this approach requires precise control of the geometric dimensions of the microstructure and is only applicable to band structures which simultaneously have symmetry-protected and coincidence-protected BICs, with rather harsh requirements.

Recently, the Longqing Cong group at Southern University of Science and Technology (SUSTech) proposed a more general approach to improve the overall quality factor and robustness of symmetry-protected BICs. In contrast to the conventional approach of achieving quasi-BIC by breaking the resonator symmetry uniformly across the entire metamaterial lattice (see Fig. 1a and b), they selectively maintain the local C2 symmetry of the entire lattice so that radiation loss can be reduced and the array quality factor is increased (see Fig. 1c). Fig.2 Significant increase in Q in hybrid BIC gratings and resistance to fabrication imperfections. (a) Evolution of the radiative Q versus the degree of asymmetry for the U-qBIC, Ht-BIC, Hx-BIC and Hq-BIC lattices. The overall quality factor is improved in a hybrid unit cell with a lower radiation density. (b) Effect of fabrication imperfections on quality factor in four scenarios. (© Opto-Electronic Science)

This is a general method that can be extended to symmetry protected BICs without requirements for accurate geometric design or band selectivity. According to the qualitative and quantitative analysis, the hybrid BIC grating can achieve a quality factor more than 14.6 times higher than that of the conventional grating (Fig. 2a). By increasing the proportional coefficient between Q and k, the resistance of the quality factor of hybrid BIC metasurfaces to interference and other disturbances is improved, thereby effectively reducing the deterioration of the quality factor in the actual device. This provides a more general and simple approach to achieving high-quality factors (Fig. 2b).

Through lattice mutual space analysis, the hybrid BIC lattice can simultaneously fold the eigenstates of points X, Y, and M of the uniform BIC lattice to the Γ point, so that multiple Fano resonances can be observed in the far-field radiation (Fig. 3). Fig.3 Common high-order hybrid BICs. (a,b) Microscopic images of the Ht-BIC and Hq-BIC metasurfaces with three and one asymmetric resonator of four in a 2 × 2 supercell, respectively, and their period is 2a along the x and y axes. Scale bar, 20 µm. (c) Schematic diagram of the bandfold from the U-qBIC lattice (black) to the Ht-BIC/Hq-BIC (red) in the Brillouin zone. (d) Simulation of the transmission amplitude spectra of the Ht-BIC (left) and Hq-BIC (right) metasurfaces at the 2.97% asymmetry level. (© Opto-Electronic Science)

Multiple high-quality Fano resonance factors are very important in pulse generation, sensing, communication, etc., especially for the development of next-generation wireless sensing and communication based on terahertz photonics. This offers new insights into the incorporation of metasurfaces and terahertz photonics to facilitate their development in various fields. This work will further enrich the physical implications of BIC and broaden terahertz metamaterial and photonic perspectives.

The team published their findings at Opto-Electronic Science (“Hybrid bound states in a continuum in terahertz metasurfaces”).