Abstract-Terahertz multiple modes defined by fractal symmetry in complementary meta-atoms

Zhidong Gu, Zhenyu Zhao, Hui Zhao, Wei Peng, Jianbing Zhang, Hongwei Zhao, Rajour Tanyi Ako, and Sharath Sriram

 Schematic representation of CSRR design. (a) Fractal meta-atoms of CSRR under different symmetric conditions: O-gap, U-gap, and C-gap, respectively, and fractal levels. (b) Pattern direction of fractal meta-atom, of which the z direction is the <100>-crystallographic orientation of SI-GaAs. P: lattice period, g: gap-size, r1: outer-radius, r2: inner-radius. (c) The top-view optical image of meta-atom. (d) Diagram of terahertz transmission spectroscopy

https://www.osapublishing.org/ome/abstract.cfm?uri=ome-9-10-4138

Low quality (Q) factors of the intrinsic inductive–capacitive (LC) mode as well as the parasitic dipole oscillation mode restrict high-resolution sensing using split-ring resonators (SRR). Although the trapped Fano-mode of the high-Q factor is found in asymmetric SRR, the conventional design limits the scaling down of resonators. As such, excitation and manipulation of multiple trapped modes of SRR is significant for driving innovative designs of terahertz metamaterials and metasurfaces. In this work, we present a novel approach to manipulating multiple terahertz modes by increasing the fractal levels as well as the geometric symmetry of complementary SRR. It is found that the multiple trapped modes become achievable only in the case that the gap of adjacent fractal SRR opposes each other. By increasing the fractal level, the intrinsic resonance modes change slightly, and more trapped modes appear in between the frequency range of the two major intrinsic modes. The map of surface currents and magnetic field distribution reveal that intrinsic LC resonance in the first or second level SRR dominates the intrinsic modes. By contrast, the trapped mode arises from the hybridization of high-order localized dipole oscillation as well as the multiple localized LC resonances. These findings create new design opportunities for scalable metasurfaces across the terahertz spectrum and beyond, with ability to create high-resolution sensors.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Dual terahertz slow light plateaus in bilayer asymmetric metasurfaces

Zhenyu Zhao, Zhidong Gu, Hui Zhao, Wangzhou Shi,

Fig. 1 (a) Schematic diagram of THz radiating on the bilayer asymmetric metasurface, KTHz refers to the wavevector of incident THz pulse. ETHz and HTHz refer to the electrical components and magnetic components respectively. (b) The sandwich structure of bilayer metasurface is 125 μm × 125 μm, in which L = 98 μm, D = 2.5 μm, t= 0.2 μm, w1 = 5 μm, w2 = 6 μm, a = 32 μm, g = 6 μm, respectively.

https://www.osapublishing.org/ome/abstract.cfm?uri=ome-9-4-1608

This work theoretically proposed dual terahertz (THz) slow light plateaus by tuning the destructive interference between a toroidal magnetic momentum and magnetic dipole momentum. The metasurfaces are in a sandwich structure. A metallic cut-wire is patterned on one side of polyimide thin-film, and a rectangular split-ring resonator (SRR) on the other side with asymmetric layout. By translating the SRR along the cut-wire from the top terminal to the bottom terminal of the cut-wire, dual slow light plateaus are found in the transparency window at a certain range of displacement. A maximum of 40.4 ps group delay is achieved as the displacement achieves 9 μm. The numerical mapping of electromagnetic field indicates that the electrical dipole on metallic cut-wire results in a localized toroidal magnetic momentum, while the inductive-capacitor oscillation of SRR results in a magnetic dipole momentum. These two momentums have opposite directions, which will repel each other at certain displacement, creating the transparency windows. Furthermore, an electrical coupling takes place in between the bilayer metasurface so that the slow light achieves a maximum, with the aforementioned two mechanisms working in coincidence.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Maximization of terahertz slow light by tuning the spoof localized surface plasmon induced transparency

Zhenyu Zhao, Yana Chen, Zhidong Gu, and Wangzhou Shi

https://www.osapublishing.org/ome/abstract.cfm?uri=ome-8-8-2345

This work numerically investigates a localized terahertz (THz) slow light phenomenon by tuning the spoof localized surface plasmon-induced transparency (PIT). A binary meta-molecule supports the interaction of the spoof localized surface plasmon (spoof-LSP), which is composed of a metallic arc and a textured circular cavity of periodic grooves. By tuning the central angle θ of the arc from 90 degrees to 170 degrees, a slow light plateau is found in the transparency window at certain frequency range. A maximum of 46 ps group delay is achieved at the θ of 135. The numerical mapping of the electromagnetic field indicates a new-born dipolar spoof-LSP that appears at the transparency windows on the circular cavity with opposite polarity to the spoof-LSP on the metallic arc. These two spoof-LSPs of opposite direction lead to a fake quadrupole, which will repel each other in magnetic dipole momentum. The slow light achieves maximum with the induced spoof-LSP and is the same as the origin spoof-LSP on the metallic arc in oscillation strength. This work paves a new way for the maximization of THz slow light.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement