Abstract-Metamaterial absorber with independently tunable amplitude and frequency in the terahertz regime

Xin Huang, Fan Yang, Bing Gao, Qi Yang, Jiamin Wu, and Wei He

 Independently tunable metamaterials absorber for amplitude and frequency with graphene and STO layer. (a) Top view of the unit cell. (b) Schematic representation and geometrical characters of the unit cell. (c) Schematic representation of the proposed absorber, which consist of a graphene sheet, two dielectric layers and one metallic background layer, with the terahertz wave along the z-axis. The geometrical parameters are given.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-18-25902

A tunable metamaterial absorber is proposed in the terahertz regime. The amplitude and center frequency of the absorber can be tuned independently. Owing to the effective combination of graphene and strontium titanate (STO) in one metamaterial structure, the tunable properties of the amplitude and center frequency are implemented. The amplitude can be tuned by adjusting the chemical potential of graphene sheet, and center frequency can get a shift through temperature changes in the STO material. In a full-wave numerical simulation, the amplitude of the absorber can be tuned from approximately 100% to 35% with a fixed center frequency when chemical potential varies from 0.7 eV to 0.0 eV. The center frequency of the absorber can shift from 0.43 THz to 0.3 THz when temperature changes from 400 K to 200 K. The complex surface impedance of the graphene and permittivity of STO material in this research range are thoroughly examined, and the independently tunable mechanism of the absorber is explored by elucidating the electric field distribution. The influence of the oblique incidence of electromagnetic wave to the absorber is studied. The absorber can be scalable to the infrared and visible frequencies and demonstrates promising application on tunable sensors, filters, and photovoltaic devices.

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

Abstract-Polarization-independent and angle-insensitive broadband absorber with a target-patterned graphene layer in the terahertz regime

Xin Huang, Wei He, Fan Yang, Jia Ran, Bing Gao, and Wei-Li Zhang

Fig. 1 Proposed broadband metamaterial absorber with a target-patterned graphene layer. (a) Schematic of the proposed broadband tunable metamaterial absorber. (b) Top view of the unit cell, (c) Perspective view of the unit cell. The geometry parameters of the proposed structure are set as (unit:𝜇𝑚$\mu m$): 𝑝=75,ℎ=27,𝑅=23,𝑅11=24,𝑅12=35,𝑔=1,𝑛=0.2$p=75,h=27,R=23,R_{11}=24,R_{12}=35,g=1,n=0.2$.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-20-25558

We propose a broadband tunable metamaterial absorber with near-unity absorption in the terahertz regime based on a target-patterned graphene sheet. Due to gradient diameter modulation of the graphene sheet and circular symmetry of the unit cell, broadband and polarization-independent properties are achieved in the absorber. A full-wave numerical simulation is performed, and the results show that the absorber’s bandwidth of 90% terahertz absorption reaches 1.57 THz with a central frequency of 1.83 THz under normal incidence. At oblique incidence, the broadband absorption of the absorber remains more than 75% over a wide incidence angles up to 60°$°$for the transverse electric (TE) mode and 75°$°$for the transverse magnetic (TM) mode. Furthermore, tunable property is implemented and the peak absorption of the absorber can be tuned from 19% to near 100% by changing the Fermi energy of the graphene sheet from 0 to 0.9 eV via electrostatic doping. The absorber is scalable to the infrared and visible frequencies, which could be used as tunable sensors, filters and photovoltaic devices.

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

Abstract-Unifying ultrafast demagnetization and intrinsic Gilbert damping in Co/Ni bilayers with electronic relaxation near the Fermi surface

Wei Zhang, Wei He, Xiang-Qun Zhang, Zhao-Hua Cheng, Jiao Teng, and Manfred Fähnle

https://journals.aps.org/prb/accepted/c2075Y25Ia810e5b542200726ae86a14d2dd92127

The ability to controllably manipulate the laser-induced ultrafast magnetic dynamics is a prerequisite for future high speed spintronic devices. The optimization of devices requires the controllability of the ultrafast demagnetization time, \begin{figure}[htbp] } \label{fig1} \end{figure} , and intrinsic Gilbert damping, \begin{figure}[htbp] } \label{fig2} \end{figure} . In previous attempts to establish the relationship between \tauM and \alphaintr , the rare-earth doping of a permalloy film with two different demagnetization mechanism is not a suitable candidate. Here, we choose Co/Ni bilayers to investigate the relations between \begin{figure}[htbp] } \label{fig3} \end{figure} and \begin{figure}[htbp] } \label{fig4} \end{figure} by means of time-resolved magneto-optical Kerr effect (TRMOKE) via adjusting the thickness of the Ni layers, and obtain an approximately proportional relation between these two parameters. The remarkable agreement between TRMOKE experiment and the prediction of breathing Fermi-surface model confirms that a large Elliott-Yafet spin-mixing parameter b2 is relevant to the strong spin-orbital coupling at the Co/Ni interface. More importantly, a proportional relation between \tauM and \alpha \mbox{intr} in such metallic films or heterostructures with electronic relaxation near Fermi surface suggests the local spin-flip scattering domains the mechanism of ultrafast demagnetization, otherwise the spin-current mechanism domains. It is an effective method to distinguish the dominant contributions to ultrafast magnetic quenching in metallic heterostructures by investigating both the ultrafast demagnetization time and Gilbert damping simultaneously. Our work can open a novel avenue to manipulate the magnitude and efficiency of Terahertz emission in metallic heterostructures such as the perpendicular magnetic anisotropic Ta/Pt/Co/Ni/Pt/Ta multilayers, and then it has an immediate implication of the design of high frequency spintronic devices.https://journals.aps.org/prb/accepted/c2075Y25Ia810e5b542200726ae86a14d2dd92127