Bound states in the continuum (BICs) are nonradiative states embedded in the spectra of the radiation continuum. The mechanism of BICs provides efficient ways to engineer the quality factor (Q factor) of resonant modes. Recent demonstrations reveal that metallic metasurfaces can support high-Q plasmonic quasi-BICs offering unprecedented opportunities to achieve giant optical field enhancement. This allows for many applications requiring strong light-matter interaction, such as laser generation, Bose-Einstein condensation, and nonlinear enhancement.
On the other hand, focusing unpolarized light using an objective with a high numerical aperture (NA) is an important way to achieve optical field enhancement in the spatial domain. Therefore, using unpolarized tightly focused light to excite high-Q plasmonic quasi-BICs would be a promising approach to further enhance the light-matter interaction.
However, the two methods of optical field enhancement, focusing the beam and generating high-Q resonance, face the problem of incompatibility. Specifically, plasmonic quasi-BICs with high Q factor tend to have strong angular dispersion and polarization dependence. The excitation condition of plasmonic quasi-BICs is limited to using polarized collimated light with good spatial coherence. If the polarization or incident direction of the excitation light deviates slightly, the Q factor and resonant amplitude of plasmonic quasi-BICs will drop rapidly, consequently optical field enhancement will be greatly weakened. Therefore, there are great challenges to exciting high-Q plasmonic quasi-BICs by using unpolarized tightly focused light.
To address these challenges, a joint research team co-led by Professor Din Ping Tsai from the City University of Hong Kong and Professor Changyuan Yu from the Hong Kong Polytechnic University utilized the physical mechanism of BICs to achieve high-Q plasmonic quasi-BIC resonances with ultraweak angular dispersion and polarization insensitivity for the first time based on a plasmonic metasurface with deep subwavelength structure. The relevant research results were published in Photonics Research, Volume 11, No. 2, 2023 (Zhuo Wang, Yao Liang, Jiaqi Qu, Mu Ku Chen, Mingjie Cui, Zhi Cheng, Jingcheng Zhang, Jin Yao, Shufan Chen, Din Ping Tsai, and Changyuan Yu. Plasmonic bound states in the continuum for unpolarized weak spatially coherent light[J]. Photonics Research, 2023, 11(2):260).
Fig. 1. Schematic of the designed plasmonic metasurface.
The designed metasurface has a three-layer structure, with a bottom mirror layer, a middle insulator layer, and a sandwich nanostructure layer (see Fig. 1). The nano block presents a sandwich structure with silver-silica-silver stacked longitudinally, which can support plasmonic quasi-BICs dominated by a magnetic dipole, and the resonant wavelength is much longer than the array period. This deep subwavelength configuration greatly reduces the angular dispersion of the resonance. In addition, the resonance formed by the nano block has strong polarization mode degeneracy, which effectively weakens the polarization dependence of plasmonic quasi-BICs. Therefore, the proposed metasurface can support high-Q near-perfect absorption resonances with ultraweak angular dispersion and polarization insensitivity. As illustrated by Fig. 2, the simulation results demonstrate that the designed metasurface can generate high-Q (≈71) resonances of near-perfect absorption (>90%) when illuminated by unpolarized focused light produced by a high-NA (NA = 0.5) objective.
Fig. 2. Schematic of the metasurface illuminated by unpolarized focused light and the average reflection spectrum.
This proposed meta-device is simple, and it can be easily fabricated using mature micro-nano processing technology, such as electron beam lithography. The research team will carry out fabrication and measurement in the future.
"Our work finds a solution for exciting high-Q plasmonic quasi-BICs using unpolarized focused light beams. It provides a new way to achieve angular dispersion regulation based on subwavelength structures," says Chair Professor Din Ping Tsai from the City University of Hong Kong, who co-led the research with Professor Changyuan Yu from the Hong Kong Polytechnic University.
This breakthrough opens new ways to generate significant optical field enhancement by using unpolarized weak spatially coherent light. It may effectively promote the development of laser generation, Bose-Einstein condensation, nonlinear enhancement, and other fields that require strong light-matter interaction.