Development of Chiral Negative Refractive Index Me

DevelopmentofChiralNegativeRefractiveIndexMetamaterialsfortheTerahertzFrequencyRegime3052IEEETRANSACTIONSONANTENNASANDPROPAGATION,VOL.55,NO.11,NOVEMBER2021DevelopmentofChiralNegativeRefractiveIndexMetamaterialsfortheTerahertzFrequencyRegimeNantakanWongkasem,Member,IEEE,AlkimAkyurtlu,Member,IEEE,KennethA.Marx,QiDong,JinLi,andWilliamD.Goodhue,SeniorMember,IEEEAbstract—Anovelnegativerefractiveindex(NRI)chiralmeta-material(MTM),basedontheYstructure,hasbeendesignedandtestedinthemicrowaveandterahertzfrequencies.Inadditiontoprovidingmagnetoelectriccoupling,thisMTMhasanegativeindexofrefractionpassbandthatcanbetunedinboththefre-quencyofoperationandbandwidthwithlowerlossescomparedtootherknownchiralstructures.GrouptheorywasusedtoanalyzethemagnetoelectriccouplingoftheY-shapedstructureandcircuitanalysiswasusedtoaidinthedesignoftheNRImaterialandfull-wavefinitedifferencetimedomain(FDTD)simulationswereconductedtodeterminethetransmissioncharacteristicsofthematerial.Wedge-andprism-shapedmodelscomprisedofthedesignedstructuresweresimulatedtovalidatetheNRIbehaviorandwerethencomparedtoexperimentalresultsinthemicrowaveregime.Furthermore,theY-shapeddesignwasfabricatedintheTHzregimeandtheco-andcross-polarizedtransmissioncoeffi-cientsweredeterminedfromexperimentsandwerecomparedtonumericalresults.IndexTerms—Chiral,metamaterials,microwave,negativerefractiveindex,terahertz.I.INTRODUCTIONARTIFICIALchiralmediacomposedofrandomlyorientedhelicesorotherleft-rightasymmetric(chiral)inclu-sions(withsizestypicallylessthanawavelength)embeddedwithinanachiralbackgroundmediumbelongtoaclassofopticalmetamaterials(MTM).Mostelectromagneticdesignapproacheswhichtrytoeliminatethecross-polarization,theformofhandedness,sensitivityofrotationtothepolarizationstate,andelliptizationofvisiblelightdiffractedfromthechiralManuscriptreceivedAugust15,2021;revisedJuly2,2021.ThisworkwassupportedinpartbytheUniversityofMassachusetts,Lowell,Chancellor’sSeedFunding.N.WongkasemwaswiththeDepartmentofElectricalandComputerEngi-neering,UniversityofMassachusetts,Lowell,MA01854USA.SheisnowwiththeDepartmentofElectricalEngineering,KhonKaenUniversity,KhonKaen40002,Thailand(e-mail:[e-mail]).A.AkyurtluiswiththeDepartmentofElectricalandComputerEngi-neering,UniversityofMassachusetts,Lowell,MA01854USA(e-mail:Alkim_Akyurtlu@).K.A.MarxiswiththeDepartmentofChemistry,UniversityofMassachu-setts,Lowell,MA01854USA(e-mail:Kenneth_Marx@).Q.DongwaswiththeDepartmentofPhysics,UniversityofMassachusetts,Lowell,MA01854USA.J.LiiswiththeDepartmentofPhysics,UniversityofMassachusetts,Lowell,MA01854USA(e-mail:Jin_Li@).W.D.GoodhueiswiththeDepartmentofPhysics,UniversityofMassachu-setts,Lowell,MA01854USAandalsowiththeLincolnLaboratory,Mass-achusettsInstituteofTechnology,Lexington,MA02420-9108USA(e-mail:William_Goodhue@).Colorversionsofoneormoreofthefiguresinthispaperareavailableonlineatfier10.1109/TAP.2021.909419structures.However,thesepropertiescanbeadvantageouswhenusedincertainoptoelectronicapplications[1],[2].Thepolarizationchangesinchiralmaterialsarestronglycorrelatedtothedegreeofchiralityofthematerialswhichisfoundasthedistinctionbetweenright-andleft-handedmaterials.Chiralresonancecanleadtonegativerefractionofonepolarization,resultinginimprovedandsimplifieddesignsofnegativelyrefractingmaterials[3].TheseNRIchiralMTMsmaygreatlybenefitnovelopticalapplicationsatvisibleandnearinfrared(IR)frequenciesinareassuchasdrugdiscovery[4],amongothers.Inthispaper,weproposeanewchiralstructurebasedonY-shapedresonatorswithanNRIbandwhichcaneasilybetunedinboththefrequencyofoperationandbandwidth.More-over,theNRIbandscanbegeneratedoverabroadfrequencyrangewithalowerlossthanisknownforotherchiralstructuresthathavebeenpreviouslyexamined(e.g.,omega[5]andgam-madion[6]–[8]).Thedesignmethodologyforthedevelopmentofthenovelchiralmetamaterialispresentedindetail.Specifi-cally,grouptheoryisusedtoderivematerialpropertiesofthesedesigns.Acomprehensivestudyusingrealisticmodelsforthematerialsviathefinite-differencetime-domain(FDTD)methodisconducted.Theproofofthenegativeindexofrefractionoftheproposedstructureispresentedbothcomputationallyandexperimentally.Fortheformer,awedgeshapedmodel[9]andaprismshapedmodel[10],[11]comprisedofYstructuresisusedtodemonstratethenegativerefractionusingFDTDsimulations.Forthelatter,measurementsonafabricatedwedgecomprisedoftheYstructurewereconductedinthemicrowaverangetovali-datetheexistenceofnegativerefraction.Moreover,planarmeta-materialsamplesbasedontheYstructuredesignwerefabri-catedintheTHzregimeandtheco-andcross-polarizedelectricfieldsfromexperimentswerecomparedwithnumericalresults.ThecomplementarysynthesisandmodelingtechniquesappliedtothenovelchiralNIRmetamaterialsprovidesaguidetotheoptimizationofmetamaterialdes