Space/Frequency/Time-Domain Digital Coding Metasurfaces And Their Applications

Metamaterials are artificial materials and are composed of an array of subwavelength meta-atoms distributed on three-dimensional space.Various intriguing functionalities can be realized by designing the structures and distribution pattern of meta-atoms,such as negative refraction,perfect imaging and invisibility cloak.The latest development of metasurface,with reduced dimensionality,exhibits exceptional abilities for controlling the flow of lights.Subwavelength-scale particles of metasurfaces can couple incident waves to free space with controllable amplitudes,phases and polarizations,such that the transmitted waves can be manipulated flexibly by designing the response and distribution of meta-particles.In 2014,Prof.Tie Jun Cui from Southeast University extended the scope of metasurfaces from material science to digital and information category,and proposed the concepts of digital coding metasurfaces and programmable metasurfaces.The emergence of coding and programmable metasurfaces has significantly simplified the analysis and design of the metasurfaces.In addition,the programmable metasurfaces can be used to dynamically manipulate the electromagnetic waves by introducing active devices to meta-atoms,such as PIN diodes and light-controlled diodes.More importantly,the digital representation scheme and the programmable functions enable the braiding of metasurface physics and information science.Recently,a number of information-based intellectual metasurfaces has been developed,enabling the flexible harvesting of electromagnetic waves with intelligent microwave camera,new architecture for wireless communications and self-adaptive radiation formations.This dissertation focuses on the analysis of multi-dimensional digital coding and programmable metasurfaces.The main contents of the thesis are summarized as follows:1)We propose the concept of frequency coding metasurfaces,and demonstrate their characteristics and design schemes.The elements of metasurfaces have different phase sensitivities over the frequency,which can be defined as frequency coding element.It is demonstrated that the frequency coding metasurfaces can be adopted to manipulate the electromagnetic radiations differently as frequency varies,enabling intriguing functions such as single beam gradually converting to double beams,four beams,and diffuse pattern.It is also demonstrated that the frequency coding metasurface can be used to realize flexible beam scanning.2)We propose the concept of space-frequency-domain gradient metasurface,which extends the concept of phase gradient metasurface.The space-frequency-domain gradient metasurfaces have uniform phase gradient in the working spectra,and the phase gradient varies as the frequency changes.The combinations of space and frequency gradients in metasurfaces open new possibilities for more powerful controls of EM waves,thereby yielding more complicated functions such as two-dimensional flexible beam scanning and continuous transformations of vortex modes.3)We propose the information theory of digital coding metasurfaces to characterize the radiation properties of coding metasurfaces from the information perspective.Based on the combination of Shannon’s information theory and generalized uncertainty principle,we show the restrictive relation between the information of digital coding pattern and the corresponding radiation patterns.The presented theory can be adopted to reveal the theoretical upper limit of orthogonal radiation states that can be realized by the programmable metasurfaces.This theory could also provide guidance for inverse design of the digital coding metasurfaces.By investigating the information of disordered-phase modulated coding metasurfaces,we find the information invariant property of their chaotic radiation patterns,which may provide theoretical guidance for computational imaging and reduction of radar cross sections.4)We adopt the concepts of group theory to analyze the space-time coding metasurfaces from the perspective of information science.Based on this theory,we reveal two ways of information transitions induced by the space-time coding metasurfaces and calculate the corresponding information transition efficiencies,respectively.Additionally,bridged with the Shannon’s information entropy theory,we further discover the upper bound of information channel capacity of the space-time coding metasurfaces,and demonstrate the connection between the channel capacity and the intensity of converted field.The proposed theory might be helpful to lay the groundwork for future researches into the regime of information-based space-time coding metasurfaces.