Multi-functional Digital Coding Metasurfaces And Their Smart Sensing Applications

Metamaterials are electromagnetic structures designed artificially in the subwavelength scale,which are usually in periodic or non-periodic arrangements.The widespread popularity of early metamaterial research was originated from the novel electromagnetic properties,such as zero index of refraction,negative index of refraction,negative permittivity,and negative permeability.The special electromagnetic properties that do not exist in nature have led to a variety of international researches on metamaterials,such as wave absorbers,cloaking and illusion devices,abnormal reflection and refraction,as well as other novel electromagnetic phenomena and devices.Different from the previous bulky and complex three-dimensional structures,metasurfaces,as two-dimensional versions of metamaterials,have further led the research boom with their advantages of low profile,low loss,easy processing and excellent electromagnetic control performance.To explore the deep integration of digital information and metamaterials,in 2014,Prof.Tiejun Cui and coworkers proposed the concept of coding metamaterials and programmable metamaterials,introducing digital coding into the metamaterial design,further forming a new perspective on digital design and regulation of metamaterial.On one hand,the design concept of digital coding has spawned a large number of digital coding metamaterial and metasurface designs,such as phase,amplitude,frequency,polarization,vortex wave,non-reciprocity,and other various coding forms.On the other hand,in the metamaterial platform,the theoretical research direction in combining the electromagnetic physics and digital informationization has been opened up,and the information metamaterial theories such as convolutional coding theorem,information entropy of coding metasurface,addition and multiplication theorem have been proposed.In addition to the passive coding metamaterial,field programmable metamaterials integrated with active devices largely extend the design freedom of the coding metamaterials to space-time dimensions.The programmable metamaterial such as space-time coding metasurface and self-adaptive smart metamaterial will promote the developments of information metamaterials to a higher level of intelligence and cognizability.This thesis systematically studies the digital coding and programmable metasurfaces,as well as the programmable control forms of information metasurfaces,covering the phase,amplitude,polarization,gain,and non-reciprocal properties.The thesis also proposes a smart metasurface with adaptive control capabilities,a smart sensing metasurface,and a physical artificial intelligence learning machine.The main contents and innovations of the thesis are summarized as follows.1)A method of designing a gradient index lens by optimizing feed and index distribution simultaneously is proposed.Based on this method,two lens antennas with special beams are designed,fabricated and verified,respectively realizing an ideal plane-wave beam and a fan-beam.The introduction of feed optimization provides a new degree of freedom for the design of the optical path in the lens and the amplitude and phase distribution on the aperture surface.The design based on the dielectric perforation structure also provides the lens with broadband operating characteristics.2)An information metasurface based on the joint coding of polarization and orbital angular momentum(OAM)modes is proposed.By introducing two orthogonal parameters of linear polarization and OAM mode into the design of the encoding metamaterial,the coding metasurface can improve the capacity and reliability of the information carried by the main beam.Compared with purely phase-mapped far-field information coding,the orthogonal coding method of combining polarization and OAM mode can significantly reduce the information loss in beam transmissions.This work designed and verified the independent coding using two orthogonal linear polarizations and five orthogonal OAM modes,which improved the ability of metasurface information characterization and transmission.3)This work proposes a polarization programmable information coding metasurface.Based on PIN diodes,a polarization-programmable coding element with independently control is designed to realize programmable control on the reflection amplitude of specific polarization at a specific frequency point.The x-and y-polarized components can be realized by reflecting on the overall scattering field of the metasurface.The customized control of the polarization component by arranging different coding sequences,can realize a variety of linear polarization deflection angles on the metasurface scattering beam.This work designed and verified the different deflection angles of the composite polarization of the scattered field under six coding sequences,laying a foundation for polarization information encoding and transmission.4)An information metasurface with programmable non-reciprocity is proposed.By bidirectionally integrating amplifier transistors on the transmission structure in a specific direction,a transmission metasurface with nonreciprocal characteristics is realized.A supercell composed of two opposite non-reciprocal units is designed,and the non-reciprocal programmable regulation is realized by controlling the working state of the amplifier,this work achieved four states including forward and backward non-reciprocal transmission and two-way reciprocal transmission.More importantly,based on this transmission metasurface structure,the gain control characteristics of amplifier transistors are further studied,which can represent the node weight control in neural networks.By constructing a multi-layer programmable gain-control metasurface,an intelligent physical learning machine with neural network operation and training functions is realized.5)A smart metasurface with self-adaptive function is proposed.Based on the traditional2-bit programmable metasurface,this work further ingrated sensors,microprocessors and intelligent feedback algorithms,to jointly establish a closed-loop control system that can make decisions independently.The intelligent metasurface is capable of actively sensing attitude changes and real-time regulation of its own beam to achieve beam staring,beam scanning,and multi-function beam control.The intelligent feedback algorithm can calculate the control beam angle and the required coding pattern in real time based on the sensing data.This intelligent platform is also highly extensible,and can integrate sensors such as light,sound,and heat to improve sensing dimensions and functions.6)A dual-polarization programmable metasurface with smart sensing function is proposed.Based on the dual-polarization 1-bit programmable element,the sensing structure for the intensity and polarization direction of the incident electromagnetic wave is further integrated.For electromagnetic waves of different intensities and polarizations,the smart sensing metasurface can implement customized beam functions.This work designed and verified three sets of combined applications with different functions and polarizations including dual-beams,quad-beams,and random scattering.