Abstract-Actively tunable broadband terahertz absorption using periodically square-patterned graphene

Longfang Ye, Xin Chen, Jianliang Zhuo, Feng Han,  Qing Huo Liu

http://iopscience.iop.org/article/10.7567/APEX.11.102201/pdf

We propose an actively tunable broadband perfect absorber using a square-patterned graphene–spacer–polysilicon–spacer–metal structure. The simulated results show that the absorber can reach nearly perfect broadband terahertz absorption with over 99.5% (90%) absorbance from 1.52 (1.27) to 2.23 (2.51) THz, corresponding to the normalized bandwidth of 37.5% (65.6%) under normal incidence with the graphene Fermi level of 0.7 eV. The absorption spectra show a clear independence of the polarization and the angle of incidence. By adjusting the graphene Fermi level from 0 to 0.7 eV, the peak absorbance can be continuously tuned from 15 to 100% without shifting the absorption frequency band.

Abstract-Electrically Tunable Broadband Terahertz Absorption with Hybrid-Patterned Graphene Metasurfaces

Longfang Ye,  Xin Chen, Guoxiong Cai , Jinfeng Zhu,  Na Liu, Qing Huo Liu

http://www.mdpi.com/2079-4991/8/8/562

We numerically demonstrate a broadband terahertz (THz) absorber that is based on a hybrid-patterned graphene metasurface with excellent properties of polarization insensitivity, wide-angle, and active tunability. Our design is made up of a single-layer graphene with periodically arranged hybrid square/disk/loop patterns on a multilayer structure. We find that broadband absorption with 90% terahertz absorbance and the fractional bandwidth of 84.5% from 1.38 THz to 3.4 THz can be achieved. Because of the axisymmetric configuration, the absorber demonstrates absolute polarization independence for both transverse electric (TE) and transverse magnetic (TM) polarized terahertz waves under normal incidence. We also show that a bandwidth of 60% absorbance still remains 2.7 THz, ranging from 1.3 THz to 4 THz, for a wide incident angle ranging from 0° to 60°. Finally, we find that by changing the graphene Fermi energy from 0.7 eV to 0 eV, the absorbance of the absorbers can be easily tuned from more than 90% to lower than 20%. The proposed absorber may have promising applications in terahertz sensing, detecting, imaging, and cloaking

Abstract-Omnidirectional tunable terahertz analog of electromagnetically induced transparency realized by isotropic vanadium dioxide metasurfaces

Qiongqiong Chu, Zhengyong Song, Qing Huo Liu,

http://iopscience.iop.org/article/10.7567/APEX.11.082203

We present an isotropic active analog of electromagnetically induced transparency through conductivity tuning of vanadium dioxide at terahertz frequencies. The unit cell of the designed metasurface consists of metallic split ring resonators and a metallic cross, which have identical resonance frequencies for the excitable lowest order modes but very different linewidths. By integrating vanadium dioxide into the bottom of the metasurface, an obvious tuning of the transparency window occurs under different conductivities. Calculated results show that resonant transmission frequency of the electromagnetically induced transparency remains stable with respect to the polarization and incident angle of electromagnetic waves.

Abstract-Graphene-based hybrid plasmonic waveguide for highly efficient broadband mid-infrared propagation and modulation

Longfang Ye, Kehan Sui, Yanhui Liu, Miao Zhang, and Qing Huo Liu

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-12-15935

In this paper, a graphene-based hybrid plasmonic waveguide is proposed for highly efficient broadband surface plasmon polariton (SPP) propagation and modulation at mid-infrared (mid-IR) spectrum. The hybrid plasmonic waveguide is composed of a monolayer graphene sheet in the center, a polysilicon gating layer, and two inner dielectric buffer layers and two outer parabolic-ridged silicon substrates symmetrically placed on both sides of the graphene. Owing to the unique parabolic-ridged waveguide structure, the light-graphene interaction and subwavelength SPPs confinement of the fundamental SPP mode for the hybrid waveguide can be significantly increased. Under the graphene chemical potential of 1.0 eV, the proposed waveguide can achieve outstanding SPP propagation performance with long propagation length of 12.1-16.7 μm and small normalized mode area of ~10−4 in the frequency range of 10-20 THz, exhibiting more than one order smaller in the normalized mode area while remaining the propagation length almost the same level with respect to the hybrid plasmonic waveguide without parabolic ridges. By tuning the graphene chemical potential from 0.1 to 1.0 eV, we demonstrate the waveguide has a modulation depth greater than 51% for the frequency ranging from 10 to 20 THz and reaches a maximum of nearly 100% at the frequency higher than 18 THz. Benefitting from the excellent broadband mid-IR propagation and modulation performance, the graphene-based hybrid plasmonic waveguide may open up a new way for various mid-IR waveguides, modulators, interconnects and optoelectronic devices.

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

Abstract-Isotropic wide-angle analog of electromagnetically induced transparency in a terahertz metasurface

Zhengyong Song, Qiongqiong Chu, Qing Huo Liu

https://www.sciencedirect.com/science/article/pii/S0167577X18306025

We present a terahertz classical analog of electromagnetically induced transparency by means of single-layer metamaterial consisting of two closed square ring. The narrow transparency peak is mainly caused by the excitation of antiparallel currents in different rings. Because of symmetry configuration, the designed metamaterial possesses stable polarization-independent and wide-angle transmission spectral features. Moreover, the design has a simple structure ideally suitable for the current planar micro- and nano-fabrications.

Abstract-Broadband tunable terahertz absorber based on vanadium dioxide metamaterials

Zhengyong Song, Kai Wang, Jiawen Li, and Qing Huo Liu

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-6-7148

An active absorption device is proposed based on vanadium dioxide metamaterials. By controlling the conductivity of vanadium dioxide, resonant absorbers are designed to work at wide range of terahertz frequencies. Numerical results show that a broadband terahertz absorber with nearly 100% absorptance can be achieved, and its normalized bandwidth of 90% absorptance is 60% under normal incidence for both transverse-electric and transverse-magnetic polarizations when the conductivity of vanadium dioxide is equal to 2000 𝛺−1𝑐𝑚−1$2000{\Omega }^{-1}c{m}^{-1}$. Absorptance at peak frequencies can be continuously tuned from 30% to 100% by changing the conductivity from 10 𝛺−1𝑐𝑚−1$10{\Omega }^{-1}c{m}^{-1}$ to 2000 𝛺−1𝑐𝑚−1$2000{\Omega }^{-1}c{m}^{-1}$. Absorptance spectra analysis shows a clear independence of polarization and incident angle. The presented results may have tunable spectral applications in sensor, detector, and thermophotovoltaic device working at terahertz frequency bands.

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

Abstract-Broadband absorber with periodically sinusoidally-patterned graphene layer in terahertz range

Longfang Ye, Yao Chen, Guoxiong Cai, Na Liu, Jinfeng Zhu, Zhengyong Song, and Qing Huo Liu

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-10-11223

We demonstrate that a broadband terahertz absorber with near-unity absorption can be realized using a net-shaped periodically sinusoidally-patterned graphene sheet, placed on a dielectric spacer supported on a metallic reflecting plate. Because of the gradient width modulation of the unit graphene sheet, continuous plasmon resonances can be excited, and therefore broadband terahertz absorption can be achieved. The results show that the absorber’s normalized bandwidth of 90% terahertz absorbance is over 65% under normal incidence for both TE and TM polarizations when the graphene chemical potential is set as 0.7 eV. And the broadband absorption is insensitive to the incident angles and the polarizations. The peak absorbance remains more than 70% over a wide range of the incident angles up to 60° for both polarizations. Furthermore, this absorber also has the advantage of flexible tunability via electrostatic doping of graphene sheet, which peak absorbance can be continuously tuned from 14% to 100% by controlling the chemical potential from 0 eV to 0.8 eV. The design scheme is scalable to develop various graphene-based tunable broadband absorbers at other terahertz, infrared, and visible frequencies, which may have promising applications in sensing, detecting, and optoelectronic devices.

© 2017 Optical Society of America

Abstract-Plasmonic waveguide with folded stubs for highly confined terahertz propagation and concentration

Longfang Ye, Yifan Xiao, Na Liu, Zhengyong Song, Wei Zhang, and Qing Huo Liu

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-2-898

We proposed a novel planar terahertz (THz) plasmonic waveguide with folded stub arrays to achieve excellent terahertz propagation performance with tight field confinement and compact size based on the concept of spoof surface plasmon polaritons (spoof SPPs). It is found that the waveguide propagation characteristics can be directly manipulated by increasing the length of the folded stubs without increasing its lateral dimension, which exhibits much lower asymptotic frequency of the dispersion relation and even tighter terahertz field confinement than conventional plasmonic waveguides with rectangular stub arrays. Based on this waveguiding scheme, a terahertz concentrator with gradual step-length folded stubs is proposed to achieve high terahertz field enhancement, and an enhancement factor greater than 20 is demonstrated. This work offers a new perspective on very confined terahertz propagation and concentration, which may have promising potential applications in various integrated terahertz plasmonic circuits and devices, terahertz sensing and terahertz nonlinear optics.

© 2017 Optical Society of America

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