Manipulations Of Electromagnetic Waves By Reconfigurable Metasurfaces

Metasurface is a new type of artificial electromagnetic material,which is formed by periodic or non-periodic arrangement of sub-wavelength elements on a two-dimensional plane.It can flexibly control the amplitude,phase,and polarization state of electromagnetic wave through equivalent electric surface impedance,equivalent magnetic surface impedance and abrupt phase.Relying on its own advantages such as low loss,low profile,easy design and fabrication,etc.,it has received widespread attention from the scientific community and the industrial community.At present,the researches on metasurface are mainly focused on single-functional or narrow-band devices.Once the metasurface structure is determined,the relevant characteristics will be fixed.However,with the continuous development of society,people's demands are becoming more and more diversified.The design of the metasurface is from simple to complex,the functionality of the metasurface is from single to compound,the metasurface is developing towards muti-functionality and reconfigurability.In this thesis,the flexible control of electromagnetic wave is realized through integrating active components into the metasurface or multi-layer devices.The specific research contents are as follows:1.A methodology of designing self-adapting RCS reduction metasurface based on deep learning is proposed.First,the metasurface reflection spectra,obtained by simulation,are used to train the network.For the pre-trained network,forward propagation can reveal the complex relationship between optical response and metasurface structure.Then,we can use the well-trained network to implement the inverse design of the customer-defined optical response.For incident waves at any frequency within the operating frequency band,it can adaptively give the required parameters to achieve RCS reduction,and theoretical calculation and numerical simulation are used to verify the results.2.Based on the varactor diode,we propose and demonstrate a metasurface that can simultaneously achieve reconfigurable beam deflection and reconfigurable near-field focus.By tuning the bias voltage of varactor diodes on each row of the metasurface,different phase gradients are introduced to deflect the reflected beam towards the desired direction,or focus the incident plane wave to designated point by shaping a spherical reflected wavefront.Then,to break through the limitation that the focus point only can be adjusted along the central axis,a metasurface-based mirror with larger electrical size is designed to make the focus point reconfigurable on the whole work plane.3.As the above-mentioned metasurface can only be manipulated for single polarization,and cannot perform different functions for different electromagnetic wave polarization,we propose a two-dimensional tunable metasurface,in which the phase can be tuned independently when x-polarization and y-polarization electromagnetic waves are irradiated.We present and demonstrate a variety of reconfigurable functions by simulation,such as polarization conversion of reflection wave,polarization-depedent beam splitting,beam deflection,RCS reduction and so on.4.A design methodology of holographic imaging based on all-optical diffractive neural network is proposed and verified.First,a single-layer holographic imaging device is designed to verify the effectiveness and accuracy of the algorithm.Second,the cooperation of two-layer holographic imaging devices is ingeniously utlized to realize reconfigurable imaging or near-field reconfigurable focusing by modulating the incident field in amplitude.