【2020】【Paper Notes】Metasurfaces: Multifunctional and Programmable——

前言

类型
$太赫兹+超表面$
期刊
$frontiersinPhysics$
作者
$HanWeiTian,HaiYangShen,XinGeZhang,XinLi,WeiXiangJiang,TieJunCui$
时间
$2020$

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研究目的

超表面

By the pre-arranged with different electromagnetic response of artificial unit,Provide multi-function wave operation

Super surface is three-dimensional super material in the 2 d geometry evolution,Keep the advantage of the complex electromagnetic operation（These operations are difficult to use traditional process to realize）,Makes up the defect of the metamaterial（体积大、High profile、Manufacturing difficulty）

• Super surface advantages：Slim profile、Significant lightweight、易于制造、An acceptable loss、Freedom to manipulate THZ wave

With the effective medium theory,Inversion of the dielectric constant and magnetic permeability metamaterials are different,Generalized thin layer transition conditions（GSTC）And the general law of, more suitable for the analysis and design of super surface

Effective medium（近似）理论（EMA或EMT）：
In order to study the properties of the porous medium,And assume a single-phase medium,The nature and multiphase medium in macro average the same,This is referred to as the assumption of single-phase medium of multiphase medium"Effective medium".
用来描述Composite materials macroscopic propertiesThe analysis of the model or theory.The theory of the nature of the components in composite materials will be calculated by average properties of composite materials to come.Due to the different constitute the parameters of the composite material of each component and often uneven,Completely accurate calculation is almost impossible.因此,Effective medium approximation theoryComposite material as a whole,Approximate calculate its parameters and properties.目前,This theory has been able to give an acceptable approximation

Generalized thin layer transition conditions（GSTCs）：
A used in ultra-thin layer structure（Electromagnetic field boundary conditions）分析的方法.Based on the derivative of Dirac distribution form of discontinuity on either side of the ultra-thin layer structure,Using polarization said ultra-thin layer structure、The electromagnetic parameters,从而将“微观”Link structure and macroscopic polarization,Equivalent method is used to describe thin 层 的 整 体 电 磁 特 性

The generalized law of, er（光的折射定律）：
$n_{1}sin{\theta }_1=n_{2}sin{\theta }_2$

By feature configuration arrangement under the artificial and the wavelength of the surface of the super unit,Can obtain the continuous phase section、Arbitrary amplitude section、The surface impedance distribution、Free modulation incident electromagnetic wave

Now super surface in the sound field、微波、毫米波、Light field achieved many functions：高效传输、轨道角动量OAM生成、雷达散射截面RCS减小

• Multifunctional super surface：紧凑,小型化
• Coding super surface：The electromagnetic response is characterized by digital bits,Provides a numerical sequence and the direct connection between super surface
• 可编程超表面：Is an extension of the coding super surface form.By importing digital coding sequence to activate the adjustable component,Realize the function of dynamic switch

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THz

由于THzSpectrum contains most of the biological and chemical molecular characteristic frequency,因此可以用THzDetect the rich biological and material information

THz衰减小、穿透力强、高信噪比,Suitable for imaging

THzWide instantaneous bandwidth to promote the realization of high-speed communication

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Passive super surfacepassive metasurface

Passive super surface has been used in beam deflection、波束形成、Object masking、聚焦、吸收、The polarization transformation
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The beam deflection

Super surface to achieve beam deflection：Through periodic etching surface unit,External wave vector was applied to transmission wave and reflected wave,Implement polarized light beam

图左
$$n_{t}sin{\theta }_t-n_{i}sin{\theta }_i=\frac{{\lambda }_0}{2\pi }\frac{d\phi }{dx}$$

其中$n_t$Is a super surface above the medium the refractive index of,$n_i$Is below the medium the refractive index of,${\lambda }_0$Is the work wavelength,$d\phi /dx$Is given by the super surface phase gradient

图右
$$sin{\theta }_r-sin{\theta }_i=\frac{{\lambda }_0}{2\pi n_i}\frac{d\phi }{dx}$$

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Lorentz beam

In order to obtain the complex electromagnetic mode,要The phase distribution、Amplitude distribution、Polarization transformation more variable than beam deflection,Instead of monotonous gradient situation

Complex electromagnetic model of electromagnetic response,Can replace optical element（偏转器、分流器、波片）,Reduce the system complexity, maintain appropriate forming effect

Lorentz beam：
洛伦兹-The change of the gaussian beam？？？
The diffraction characteristics of,使用场景广泛,But need multiple optical element to produce

2017年,Guo using cross unit consisting of a single super surface,证明了在THzCircular polarized incident under radial polarization spectrum in lorentz beam：

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Multi-function passive super surface

Although fixed structure on the surface of the passive super,But function multiplexing can realize

Passive super surface can be divided into频率复用超表面、极化复用超表面

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极化复用（The polarization multiplexing）的原则

1. PB相位：Can be done by selecting the surface unit structure.基于PBPhase of the cell structure does not rely onMagnetic resonance,So can keep stable in the broadband phase change、振幅响应
2. 各向异性：除了PBPhase polarization selectivity of outside,Based on the anisotropy of surface structure depends on thePolarization independence,为了实现多种功能,Need a polarization has nothing to do with the response of anisotropic unit structure

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Active super surfaceactive metasurface

Adjustable function

Under the microwave frequency,PIN二极管、Varactor diodes and transistors have been widely used in,Under different bias signals with optional response,In the terahertz spectrum,Due to the small electrical size,It is difficult to obtain encapsulating semiconductor,The parasitic effect of complex is a big challenge

Can be used as a designTHzOn the surface of the material：

• Doped semiconductor
• 二氧化钒$V{O}_2$
• 液晶LC
• Transparent conductive oxideTCO
• 石墨烯
• 二维电子气2DEG
• Halide perovskite
• 量子点（The exciton bound in three directions in spaceSemiconductor nanostructures.有时被称为“人造原子”、“超晶格”、“Super atom”或“Quantum dots atom”）
• 超导体

激励手段：

• 励磁方式
• Thermal environment
• 光泵

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Dynamic capabilities multiplexing

In the terahertz and spectrum,Small size and function of immature technology lead to reuse face huge difficulties

略

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Coding super surfaceCoding metasurfaceAnd programmable super surfaceProgrammable metasurface

Coding super surface

2014In the twoMetamaterials digital representation的概念

Giovampaola和Engheta提出Digital materials：
Contains functions represent two different dielectric constant of“Metamaterials a”And by a metamaterial space given by the mixed“Metamaterials bytes”的定义

Cui提出的编码、Digital and programmable metamaterials：
The phase response digital,Rather than effective parameters.例如,对于1bit 的情况,相位响应0和πMarked as coding state0和1,对于2bit 的情况,Four coding status to00,01,10,11分别对应0,0.5π,π,1.5π

后者具体来说,

By the normal incidence of lighting$N\times M$Coding super refraction or reflection on the surface of the far field function,可以表示为
$$\begin{gathered} f(\theta ,\phi )=f_u(\theta ,\phi )\sum _{m=1}^N\sum _{n=1}^{M}exp\{-i\{\phi (m,n)+ \\ kPsin\theta [(m-0.5)cos\phi +(n-0.5)sin\phi ]\}\} \end{gathered}$$
其中$\theta$是仰角,$\phi$是方位角,$k$是波数,$f_u(\theta ,\phi )$Is a far field function per unit of super surface

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Directional said as

$$D(\theta ,\phi )=4\pi |f(\theta ,\phi ){|}^2/{\int }_0^{2\pi }{\int }_0^{\pi /2}|f(\theta ,\phi ){|}^{2}sin\theta d\theta d\phi$$

Compared with digital materials,编码概念More suitable for simplified based on the mutation of electromagnetic response surface design

In the terahertz spectrum,Encoding method has been used to implement without interference of electromagnetic operating,And simplify the super surface design

Will ring unit as1,Blank cell as a0可以构造一个1bit Coding super surface
在同一时期,This paper proposes a new coding units etching closed minkowski ring,To design based on the geometrical adjustment1位、2位和3An encoding super surface

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可编程超表面

Programmable active form on the surface of the surface is coding ultra,Need to be flexible unit to realize real-time state transition

通过输入预存储的编码序列,Specific location of the unit cell to show the localEMElectromagnetic response,And can in time to generate globalEMElectromagnetic response toWave processing

Cui Put forward the first programmable microwave super surface,And the introduction of field programmable gate array (fpga)（FPGA）,By connecting to each unit of the pins with different bias voltage.Coding sequence of multiple inputFPGA,Output voltage serial driverEMResponse to the transformation of the,To generate different scattering pattern

In the terahertz spectrum,The study of programmable super surface is less,This is due to not onlyDifficult to achieve the unit cell of tunable sex、Difficult to connect complex offset line

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结论

The paper introduced on the surface of the passive super多功能性,Through frequency reuse、The polarization multiplexing implement.
Although passive surface in regulating functions have a high degree of freedom,But fixed prototype makes the adjustable impossible
With functions of active、Multiplex active super surface,Can provide high performance,Can also have a high level of integration

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问题

Vortex beam with what produce？
OAMMacro performance for electromagnetic vortex phenomenon,Specific for the beam in the plane of the perpendicular to the direction of propagation with spiral phase characteristics.而OAMWith infinite multiple modal,Between all the modal and orthogonal to each other,For vortex wave communication and detection provides natural conditions.理论上,The vortex wave can improve communication capacity of infinite

OAMCan be generated using self-similar fractal antenna

What is the orbital angular momentumOAM？
一个光子,Can have two kinds of momentum,Linear momentum and angular momentum.And angular momentum are divided into two kinds of,The spin angular momentum and orbital angular momentumOAM.

Faraday rotation effect：In a magnetized plasma,Two different circular polarized wave phase velocity,After a distance,Composite wave polarization plane continuously into the direction of shaft rotation before

What is minkowski ring？
An antenna structure

Metamaterials have inversion of dielectric constant and magnetic permeability is what mean？
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