Multi-Functional Digital Coding Metamaterials And Metasurfaces

Full control and manipulation of electromagnetic waves for various purposes has always been the desire and affection of humankind.Unfortunately,the materials found in nature either cannot fulfill this goal or at least very difficult to do so.To fulfill this goal humankind from ancient times is trying to engineer the naturally occurring materials in various ways by using different tools available.With the development of fabrication and integration techniques now we are able to engineer,design and fabricate the naturally occurring materials,so that we can control the waves in a desired manner.In modern time the engineered materials and structures comprised of subwavelength elements used to control the waves in a desired manner are called metamaterials and metasurfaces.Metasurfaces are engineered structures which are comprised of subwavelength elements periodically distributed on a two-dimensional spatial plane.Various func-tionalities such as anomalous refraction/reflection,cloaking,perfect absorption,beam controlling,super resolution imaging,polarization conversion and many more can be realized by manipulating the electromagnetic waves through these surfaces.The building block of these engineered structures are called unit cells or more appro-priately meta-atoms which can be characterized by employing the effective medium theory,similar to the characterization of the metamaterials unit cells.If we study metasurfaces from the perspective of circuit theory then the unit cells can be re-garded as the analogue circuits as they can be characterized by effective medium parameters with continuous values.To demonstrate digital version of the above mentioned metasurfaces,Prof.Tie Jun Cui in 2014 proposed digital coding and programmable coding metasurfaces.The basic element in these types of metasurfaces is called digital coding unit cell or digital coding meta-atoms which can provide discrete phase and/or amplitude responses and can be mapped on binary numbers 0,1,00,10,and so forth.Coding metasurfaces which are combination of 0,1 elements can manipulate the EM waves through a pre-designed coding sequence.The emergence of these digital metasur-faces have eased the design procedure and offered new degrees of freedom to fully control and manipulate the EM waves.Most importantly the idea of digital coding metasurfaces have bridged the physical and digital worlds,where information sci-ence can be processed on metasurfaces.In this regard,many digital algorithms for example convolution operation can be performed on these metadevices.This dissertation is aimed to investigate coding metasurfaces with a special focus on multi-functional digital coding metasurfaces.This study includes,theoretical investigation,simulations,fabrications,experimental verification and application of digital multi-functional coding metasurfaces.Four distinct and unique unit cells are proposed,designed,explained and applied to demonstrate a series of novel meta-devices and the corresponding functionalities.This dissertation is summarized as follows:1.A dual band 2-bit coding metasurface is proposed where independent manip-ulation of reflected EM waves can be performed in the two operating bands.Full structures are designed and simulated for various functionalities,which include beam steering of spatially propagating waves to opposite half planes in reflection geometry.The other tasks demonstrated are PW-SW conver-sion in lower band and beam deflection in higher band independently.Two structures are fabricated for experimental verification where the simulated and experimental results have very good agreement.2.Multiple vortex beams carrying orbital angular momentum(OAM)with dis-tinct topological charges and deflected vortex beams are demonstrated in two different bands by taking advantage of dual band coding metasurface.More-over,negative and retro reflections are also demonstrated in one band and specular or anomalous reflection in the other band at various oblique inci-dences.Experiments are performed for steered vortex beams and negative and retro reflection after fabricating two different meta devices.In all the cases the measured results have very good agreement with simulated results.3.The unit cell of the dual band coding metasurface is further optimized and applied to generate distinct holograms in the two operating bands.Weighted Gerchberg Saxton algorithm is employed to project various phase only holo-grams in this frequency multiplexed structure.Two designs are fabricated for experimental verification where two letters or symbols completely different are projected from the same aperture.The demonstrated results are in excellent agreement with simulated results.4.A thin transmission type digital coding metasurface is proposed which can operate in dual band with the feature of switching the band of operation by tuning the incident linear polarization.Full structures with 1-bit and 2-bit resolution are designed simulated and fabricated for various functionalities including vortex beam carrying OAM with different topological charges,fo-cusing,anomalous refraction and beam shaping.For experimental verifica-tion three different structures are fabricated and tested in various anechoic chambers for the applications of beam shaping,Vortex beam generation and focusing.All the experimental results in both operating bands(Ku-band)and(Ka-band)have excellent agreements with their theoretical counterparts.5.An amplitude/phase adjustable digital coding metasurface is proposed and demonstrated to control the EM wave in transmission geometry.After suc-cessful design of unit cell with 1-bit amplitude and 2-bit phase discretization full structures are then designed by careful distribution of these meta-atoms in the 2D spatial planes.The idea is applied to modulate the transmitted power of vortex beams both in normal and deflected case.Experiments are performed by fabricating two different samples for '0' amplitude state and '1' amplitude states.All the results have very good agreement with their simulated versions.6.An Ultra thin multi-bit transmission type coding metasurface is demonstrated.The other features of this metasurface are broad band operation,angular sta-bility and high efficiency.Various full structures are simulated for the gen-eration of highly efficient and non-diffractive vortex beams,including steered and multiple vortex beams from the same aperture.Two different samples are then fabricated and experiments are performed which have excellent agree-ments with simulated results in all the cases.