Metamaterials

IntroductionofMetamaterialsManydesiredelectromagneticproperties,similartothecaseofmonopolemagnets,seemtobelackinginnatureeventhoughtherearenophysicallawspreventingtheexistenceofsuchphenomena.Metamaterials•Theprefix“meta”means“beyond,”andinthissensethename“metamaterials”signifiessystemsthatarebeyondconventionalmaterials.•Theword“metamaterial”firstappearedinliteraturein2000whenSmithetal.publishedtheirseminalpaperonastructuredmaterialwithsimultaneouslynegativepermeabilityandpermittivityatmicrowavefrequencies.•Metamaterialsaredefinedametamaterialas“anarrangementofartificialstructuralelements,designedtoachieveadvantageousandunusualelectromagneticproperties”•Suchadefinition,althoughitaddressesthe“artificial”natureofmetamaterials,isperhapsoverlyinclusiveandfailstorecognizetheimportantdifferencebetweenmetamaterialsandotherman-madestructuressuchasphotoniccrystals.•Manyleadingexpertsinthefieldprefertoputtermslike“propertiesunlikeanynaturallyoccurringsubstance”or“notobservedinnature”inthedefinitionofametamaterial.Thesedefinitionsusefullyemphasizethemajorpursuitofmetamaterialresearch–toachieveelectromagneticfeaturesnotfoundinconventionalmaterials.•However,itmightbetooglibtoexcludeanynaturallyoccurringpropertyfromthefocusofmetamaterialresearch.•Negativerefractionhasbeenastonishinglyobservedincompoundeyesofsomelobsters.•Ametamaterialisanartificiallystructuredmaterialwhichattainsitspropertiesfromtheunitstructureratherthantheconstituentmaterials.Ametamaterialhasaninhomogeneityscalethatismuchsmallerthanthewavelengthofinterest,anditselectromagneticresponseisexpressedintermsofhomogenizedmaterialparameters.•Thestructuralunitsofametamaterial,knownasmeta-atomsormeta-molecules,mustbesubstantiallysmallerthanthewavelengthbeingconsidered,andtheaveragedistancebetweenneighboringmeta-atomsisalsosubwavelengthinscale.•Thesubwavelengthscaleoftheinhomogeneitiesinametamaterialmakesthewholematerialmacroscopicallyuniform,andthisfactmakesametamaterialessentiallya“material”insteadofadevice.TheLycurgusCupviewed(a)inreflectedlightand(b)intransmittedlight.•Perhapsoneofthefirstmodernmetamaterialswithengineered,subwavelengthmeta-atomswasattributedtothe“twistedjute”materialproposedbyBosein1898toproduceanartificialchiraleffect.•Artificialdielectrics,whichareusuallyperiodicarraysofmetallicwires,spheresorplates,werestudiedextensivelybymicrowaveengineersmorethanhalfofacenturyago.•Otherexamplesofmetamaterialsortheirelementsdevelopedbeforethetermmetamaterialwascoinedincludethesplit-ringresonators,arrayedfrequencyfilters,bianisotropicandchiralmaterials,andothers.•Inouropinion,therearethreemilestonepapersthatshouldbementionedinthisregard.I.ThefirstisVeselago’spaperonlefthandedmaterials.Thispaperstudiedthestrikinglyunusualphenomenatobeexpectedinahypotheticalleft-handedsubstanceinwhichthefieldvectorsE,Handthewavevectorkformaleft-handedsystem.Thepaperalsoexplicitlypresentedtherequiredmaterialparameterstoachievethematerial–simultaneouslynegativevaluesofpermittivityandpermeability.II.ThefirstexperimentaldemonstrationofaVeselagomediumbySmithetal.,whichmakesthehugeleapfromatheoreticalpredictiontoexperimentalvalidation.III.ThethirdseminalpaperisPendry’sworkonaperfectlens[21],whichrepresentstheinitialattempttofillthegapbetweennovelmetamaterialsandexcitingapplications.•opticalmetamaterials•Lightistheultimatemeansofsendinginformationtoandfromtheinteriorstructureofmaterials–itpackagesdatainasignalofzeromassandunmatchedspeed.•Theburgeoningopticalmetamaterialresearchactivitiesarearesultofthecombinationofawealthofnanofabricationtechniqueswithadvancesinnanoscaleimagingandcomputationalelectromagneticdesignandsimulations.•Withintherapidlydevelopingandhighlymultidisciplinaryfieldofopticalmetamaterials,severalkeyresearchdirectionshavebeenemerging,includingopticalmagnetism,opticalnegativeindexmaterials,giantartificialchirality,nonlinearopticsinmetamaterials,superresolutionwithmetamaterials,andelectromagneticcloaksofinvisibility.•Opticalmetamaterialsrevolutionarilyalteredthewaythatpeopleproposeanddesignfunctionalopticaldevices.Materialsusedinconventionalopticaldesignsareusuallybothhomogeneousandisotropic;thereforethedesignofdevicesislargelyanissueofengineeringtheinterfacesbetweendifferentmedia.•Forinstance,inthelens-makingindustry,cascadedlensesofdifferentmaterialswithfinelycontrolledcurvaturesareoftenusedtominimizemultipletypesofaberrations.Theemergenceofphotonicmetamaterialsallowsustotailoropticalspaceandprovidenewresponsesthatareprecludedintheconstituentmaterials.•Thedesignstrategyofopticaldevicesisradicallychangedwhenmetamaterialsareinvolved–thedesiredfunctionalityisachievednotonlybyconfiguringtheinterfacesbetweendifferentmaterials,butbythecontrolofessentiallyeverysinglepointinopticalspace.Thisenablesapplicationsusingnovel,spatiallyvaryingarchitecturesofmetamaterialswheretheelectromagneticpropertiesofeverypositionarecarefullyprescribed.•Opticalmetamaterialscanhavenumerousandfar-reachingimplications.Thesematerialsbringthepromiseofcreatingentirelynewprospectsforcontrollingandma