In developing strategies for manipulating surface electromagnetic waves, it has been recently recognized that a complete forbidden bandgap can exist in a periodic surface-wave photonic crystal, which has subsequently produced various surface-wave photonic devices. However, it is not obvious whether such a concept can be extended to a quasi-periodic surface-wave system that lacks translational symmetry. Here, we experimentally demonstrate that a surface-wave photonic quasicrystal that lacks short-range order can also exhibit a forbidden bandgap for surface electromagnetic waves. The lower cutoff of this forbidden bandgap is mainly determined by the maximum separation between the nearest neighboring pillars. Point defects within this bandgap show distinct properties compared to a periodic photonic crystal in the absence of translational symmetry. A line-defect waveguide, which is crafted out of this surface-wave photonic quasicrystal by shortening a random row of metallic rods, is also demonstrated to guide and bend surface waves around sharp corners along an irregular waveguiding path.
An asymmetric trimeric slit metasurface structure is proposed herein, which generated independently tunable double Fano resonances (DFRs), namely, Fano1 and Fano2. Rigorous simulations and experimental scanning near-field demonstrated that the non-radiative dark mode produces Fano1, while the magnetic toroidal mode generates peak Fano2. The mechanisms of the two Fano resonances infer that interference fields can be independently tuned by different geometric parameters. The separate tuning of DFRs is simultaneously generated in the asymmetric slit metasurface. These results imply the potential applications of Fano resonances, such as lasing and sensing.
Asymmetric transmission (AT) is useful for polarization manipulation. We report narrowband AT that utilizes a triple-layered symmetric trimeric metasurface with near-field coupling of the dark mode of the Fano resonance. The coupling strength of the dark mode was tuned by using a mid-layer to break the dim AT between two slit layers. The peak transmission of linearly polarized waves and percentage bandwidth reached 0.7719 and 1.26% (numerical simulations) and 0.49 and 1.9% (experiments), respectively. Coupled-mode theory and field patterns are utilized to explain the underlying physical mechanisms of the mid-layer assisted field coupling. These results are useful for Fano-resonance-based devices.
We designed multiple degenerate modes in a single-point-defect photonic crystal cavity with x-y-asymmetry to act as bright and dark modes to produce Fano resonances. Under suitable operating parameters, the power-flows in incident-direction for two degenerated modes cancel each other, resulting high Q-factors for Fano modes. Extremely-high Q-factor of Fano resonances exceeding 10 8 with transmittance larger than 0.867 is obtained based on modes with Q-factors less than 10 4 , and in further exploration, the Q-factor can reach 3 × 10 10 . This method provides a basis to design compact, high-Q Fano devices for extremely high-sensitivity biosensing, switching and lasing.
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