High-quality (Q-factor) metasurfaces have emerged as an exciting platform for enhancing light-matter interactions at the nanoscale and enabling a diverse range of applications, such as nanolaser, white-light LED, photoluminescence enhancement, nonlinear optics, and sensitive biochemical sensor.
A research group led by Prof. LI Guangyuan from the Shenzhen Institute of Advanced Technology (SIAT) of the Chinese Academy of Sciences has proposed a novel strategy to achieve robust ultrahigh-Q quasi-bound states in the continuum (BICs) in all-dielectric metasurfaces based on lattice hybridization.
The study was published in Advanced Optical Materials on Sept. 7.
BICs have infinite Q factors due to the vanishing leakage to the free space. Since BICs do not couple with free space light, asymmetric disturbances are usually introduced so as to transfer BICs into high-Q quasi-BICs. Conventionally, the asymmetry parameter should be less than 10% in order to obtain high Q factors. This imposes stringent requirements on the nano-fabrication processes, because multiple pattern transfers during evaporation, lithography, and etch would bring inevitable deviations on the size and shape and thus significant decrease on the measured Q factors.
In this study, the researchers proposed a new approach to tackle this challenge. They utilized the hybridization of two lattices of periodic silicon nanorods with relative displacement being the asymmetry disturbance, which is robust to pattern transfers during nanofabrication.
They further observed an interesting transition between two BICs, the electric quadrupole and the toroidal dipole BICs, which correspond to two in-plane mirror-symmetry points, even when the asymmetry parameter varies over the whole possible range.
"This significantly improves the minimum achievable Q factor of the quasi-BIC, suggesting robustness to the asymmetry parameter," said Dr. LI.
Thanks to the robustness to both design and nanofabrication, they managed to demonstrate robust high-Q quasi-BICs with the minimum Q factor of 1,317 in theory and 1,024 in experiment, and the maximum Q factor of 41,30 in experiment.
