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-Demonstration of group delay above 40 ps at terahertz plasmon-induced transparency windows

Zhenyu Zhao, Hui Zhao, Rajour Tanyi Ako, Jianbing Zhang, Hongwei Zhao, and Sharath Sriram

. (a) Schematic diagram of a unit cell of proposed binary MM, in which p = 420 µm, r = 90 µm, w = 4 µm, h = 60 µm, g = 15 µm, respectively. l is the length of the metallic arc. (b) Microscopic images (VHX-500, Keyence Inc.) of unit cell of as-fabricated binary MM. (c) A schematic diagram showing terahertz transmission and group velocity delay through a periodic array of binary MM, as measured using a THz-TDS. KTHz refers to the wave vector of incident THz pulse. EX and HY refer to the electrical and magnetic component of the incident electromagnetic waves, respectively. XYZ are the coordinate axis in free space.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-19-26459

Herein, we demonstrate one of the highest terahertz group delay of 42.4 ps achieved experimentally at 0.23 THz, on a flexible planar metamaterial. The unit cell of metasurface is made up of a textured closed cavity and another experimentally concentric metallic arc. By tuning the central angle of the metallic arc, its intrinsic dipolar mode is in destructive interference with the spoof localized surface plasmon (SLSP) on textured closed cavity, which results in a plasmon-induced transparency phenomenon. The measured transmittances of as-fabricated samples using terahertz-time domain spectroscopy validate numerical results using extended coupled Lorentz oscillator model. It is found that the coupling coefficient and damping ratio of SLSP relies on the radius of the ring structure of textured closed cavity. As a consequence, the slow light maximum values become manoeuverable in strength at certain frequencies of induced transparency windows. To the best of our knowledge, our experimental result is currently the highest value demonstrated so far within metasurface at terahertz band.

© 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-Detection of iron corrosion by terahertz time-domain spectroscopy

Hui Zhao, Di-bo Wu, Hong-lei Zhan, Kun Zhao

China Univ. of Petroleum (China)

Qing Sun

National Institute of Metrology (China)

Proc. SPIE 9795, Selected Proceedings of the Photoelectronic Technology Committee Conferences held June and July 2015, 97953J (November 5, 2015); doi:10.1117/12.2214316

http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=2469464

The iron tablets, which were exposed in salt spray with different periods, were investigated in the 0.2~2.0 THz using reflection-type terahertz time-domain spectroscopy (THz-TDS) in vacuum environment at room temperature. The sample signals are attenuated in comparison to the reference signals with increasing the corrosion time. The THz spectroscopy peak EP and reflectivity (R) of samples strongly depended on corrosion time t with EP ∝ t-1 and R ∝ t-1. The THz characteristics of iron sheets in salt spray indicate that reflection THz-TDS will contribute to the development of non-destructive testing of corrosion in pipelines.
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