Ultra-Narrowband Anisotropic Perfect Absorber Based on α-MoO3 Metamaterials in the Visible Light Region 2022 Gui Jin.pdf

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Article
Ultra-NarrowbandAnisotropicPerfectAbsorberBasedon
α-MoO3MetamaterialsintheVisibleLightRegion
GuiJin1,TianleZhou2andBinTang2,*
1DepartmentofElectronicInformationandElectronicEngineering,XiangnanUniversity,
Chenzhou423000,China;******@
2SchoolofMicroelectronicsandControlEngineering,ChangzhouUniversity,Changzhou213163,China;
******@
*Correspondence:******@
Abstract:Opticallyanisotropicmaterialsshowimportantadvantagesinconstructingpolarization-
,anewtypeoftwo-dimensionalvanderWaals(vdW)
material,knownasα-phasemolybdenumtrioxide(α-MoO3),hassparkedconsiderableinterest
,wetheoreticallypresentananisotropic
metamaterialabsorbercomposedofα-MoO3ringsanddielectriclayerstackingonametallicmirror.
Thedesignedabsorbercanexhibitultra-narrowbandperfectabsorptionforpolarizationsalong[100]
and[001],theinfluencesofsomegeometricpa-
,theproposedultra-narrowband
anisotropicperfectabsorberhasanexcellentangulartoleranceforthecaseofobliqueincidence.
Interestingly,thesingle-bandperfectabsorptioninourproposedmetamaterialscanbearbitrarily
extendedtomulti-
mechanismcanbeexplainedbytheinterferencetheoryinFabry–Pérotcavity,whichisconsistent





anisotropicopticaldeviceswithtunablespectrumandselectivepolarizationinthevisiblelightregion.
Citation:Jin,G.;Zhou,T.;Tang,B.
Ultra-NarrowbandAnisotropicKeywords:perfectabsorber;α-phasemolybdenumtrioxide(α-MoO3);metamaterials;polarization
PerfectAbsorberBasedonα-MoO3
MetamaterialsintheVisibleLight
,12,1375.
/
AcademicEditors:FilippoGiannazzoTwo-dimensional(2D)materialswithatomic-scalethicknesses,.,graphene[1],black
andIvanShtepliukphosphorus(BP)[2],hexagonalboronnitride(h-BN)[3],andtransitionmetaldichalco-
genides(TMDs)[4],havebeenmuchconcernedduetotheirdistinctiveopticalandelectrical
Received:21March2022
Accepted:15April2022propertiesoverthepastfewyears[5].Differentfromtheoptoelectronicdevicesmade
Published:17April2022ofconventionalbulkmaterials,theselayeredmaterialsmayprovideexcitingopportu-
,anewtypeof2D
Publisher’sNote:MDPIstaysneutral
vanderWaals(vdW)material,knownasα-phasemolybdenumtrioxide(α-MoO3),has
withregardtojurisdictionalclaimsinbeenexperimentallydemonstratedandsparkedconsiderableinterestduetoitshighly
publishedmapsandinstitutionalaffil-anisotropiccharacteristicsstemmingfromtheuniquecrystallinestructure[6–8].Infact,
iations.
mostofthevanderWaalsmaterials,.,molybdenumdisulphide(MoS2),inwhichthe
centralMoatominMoS2issandwichedbetweentwoSulphuratoms,areuniaxialcrystals.
However,α-MoO3isactuallyatypeofnaturalbiaxialhyperboliccrystal,anditexhibits
α
Copyright:©--MoO3isconstructedbylayers
LicenseeMDPI,Basel,[9],inwhichMoatomsareseparatelylinkedwiththree
Thisarticleisanopenaccessarticledifferentoxygenatoms,.,symmetricallybridgingOs,terminalOt,andasymmetricOa.
distributedunderthetermsandEachα-MoO3layerconsistsoftwosub-layers,whicharecreatedbycorner-sharingrows
conditionsoftheCreativeCommonsandedge-,α-MoO3crystalmaterialscanbecombinedinto
Attribution(CCBY)license(https://metamaterialstoachievemoredegreeoffreedomformanipulatinglight-matterinteraction
,thestronganisotropyofα-MoO3materialscouldbeusefulfor
/).awealthofapplicationsrangingfromcolorfilter[10],polarizationconverter[11],and
Nanomaterials2022,12,:///nano12081375
Nanomaterials2022,12,13752of10
molecularsensorstoin-planeimaging[12].Besides,someotherexoticphysicalphenomena
wereobservedbyexploitingphononpolaritonsexcitedinα-,Quetal.
reportedatunableplanarfocusingnanophotonicdeviceworkinginthemid-infraredre-
gion[13].
angleintwistedbilayeredα-MoO3flakes[14].
Metamaterials,knownasartificialcompositematerialscomposedofperiodicalsub-
wavelengthscalenanostructure,possesssomeexoticelectromagneticcharacteristicsthatare
notfoundinnaturalmaterials[15,16].Todate,tremendousinteresthasbeenattractedfor
theirextensiveapplications[17,18].Asanimportantbranchofmetamaterials,metamaterial
absorbermanifestsintriguingstrategiesforitsrelativelyflexibledesignincomparisonwith

experimentaldemonstration[19],manytypesofmetamaterialabsorberswithnarrow-band
absorption[20,21],broadbandabsorption[22,23],andevenmulti-bandabsorption[24,25],
havebeenproposedowingtotheirwideapplications,suchassolarcells[26],plasmonic
sensors[27],moleculardetectors[28],andselectivethermalemitters[29].Byreasonably
designingthegeometricstructure,theoperatingfrequenciesofmetamaterialabsorberscan
,variousof2Dmaterials-based
metamaterialshavealsobeenproposedforobtainingtunableperfectabsorptionoroptical
enhancementabsorbance[30–37].Forexample,
completeopticalabsorptioninperiodicallypatternedgraphenesheet[38].Sangetal.
proposedatwo-bandabsorberutilizingthepatternedMoS2[39].
tunablewide-angleandultra-broadbandperfectabsorbersbyusingBP-dielectricmulti-
layerstackingstructureandBP-dielectric-metallichybridarchitecture[40,41],respectively.
However,toourknowledge,theunderstandingoftheinteractionoflightwithα-MoO3
materialsisstillinitsinfancy,andfewworkshavebeenreportedontheelectromagnetic
absorbersbasedonα-MoO3[42–44],especiallyforultra-narrowbandanisotropicperfect
absorption,whichareofsignificanceforsomeapplications,suchasphotodetectors,spectral
imaging,andsensors.
Inthiswork,wetheoreticallyproposeandnumericallydemonstrateanultra-narrowband
anisotropicmetamaterialabsorbercomposedbytopα-MoO3ringandadielectriclayer
,ultra-narrowband
perfectabsorptioncanbeachievedinvisiblefrequencyforpolarizationalongbothx-and
y-,theproposedanisotropicmetamaterialabsorberhasanexcellent
,thesingle-bandperfect
absorptioninourproposedmetamaterialscanbearbitrarilyextendedtomulti-bandperfect

explainedbytheinterferencetheoryinFabry–Pérotcavity,whichisconsistentwiththenu-
,theelectromagneticsimulationsperformedbyfinite-difference
time-domain(FDTD)-
gationshowspromisingpotentialinsensing,multispectraldetection,filtersandmultiplexing
bindingbio-moleculardetection,etc.

Figure1ashowsschematicallytheunitcellofproposedanisotropicperfectabsorber,
whichconsistsoftopα-

processoffabrication,themultilayerstructurecanbefabricatedbyusingphysicalvapor
depositiontechniques,whichhasbeenawell-knownmethodforscalableandrepeatable
,thecomplexdielectricfunctionofα-MoO3canbedescribedas
follows[10]:
2
ωpi
ε(ω)=ε+∑(1)
∞2+2−
iωoiωiγiω
Nanomaterials2022,12,xFORPEERREVIEW3of10
2
ωpi
ε(ω)=ε+
(1)
∞ω2+ω2-iγω
ioii
whereiindicatesthenumberofLorentzoscillators,ε∞,ωpi,ωoi,andγireferstothehigh
frequencydielectricconstant,theplasmafrequency,theeigenfrequency,andthescatter-
ingrateoftheithLorentzoscillator,(1)to
calculatethepermittivitytensorsofα-MoO3arelistedinTable1[10].
(1)toobtainthepermittivitytensorsofα-MoO3inthevisible
range.
Polarizationε∞ωpj[cm−1]ω0[cm−1]γj[cm−1]
,67227,
,07832,
Figure1billustratestherealandimaginarypartsofα-MoO3permittivityalong[100]
and[001]directionsextractedfromRef.[10].Fullfieldelectromagneticcalculationswere
-dimensional
FDTDsimulationsweremadeinaunitcellarea,andthenon-uniformmeshischosen,and
themeshsizegraduallyincreasesoutsidetheα--
graphicdirectionconventions,thex-,y-,andz-directionsrepresentthe[100],[001],and
[010]directions,,theplanewaveswereilluminatedalongthe
negativez-direction,andperiodicboundaryconditionswereusedinx-andy-directions.
Ingeneral,theopticalabsorptioncanbeexpressedasA=1−R−T,whereRandTindicate
thereflectionandtransmission,
mirrorhasexceededtheskindepthofthelight,-
fore,theabsorptioncoefficientAcanbeabbreviatedasA=1−,thedielectric
Nanomaterials2022,12,,andthepermittivityofgold3ofis10
describedbyDrudemodel:
𝜔
𝜀
𝜔
𝜀
(2)
whereiindicatesthenumberofLorentzoscillators,𝜔
ε∞𝑖𝜔𝛾,ωpi,ωoi,andγireferstothehigh
frequencydielectricconstant,theplasmafrequency,theeigenfrequency,andthescattering
whereωistheangularfrequency,×1016rad/s,the
rateoftheithLorentzoscillator,(1)to
scatteringrateγ=×1013rad/s,andε∞=1.
calculatethepermittivitytensorsofα-MoO3arelistedinTable1[10].
3
.((aa))TheThestructurestructureunitunitcellcelldiagramdiagramofofthetheproposedproposedabsorberabsorberconsistingoftopα-MoO3ringring
,thegeometricalparametersarelistedas
,thegeometricalparametersarelistedas
follows:H=200nm,h=300nm,w=40nm,D=360nm,p=500nm,andthethicknessofα-MoO3
follows:H=200nm,h=300nm,w=40nm,D=360nm,p=500nm,andthethicknessofα-MoO3
ringt=105nm.(b)Therealpartandimaginarypartoftheα-MoO3permittivityinthevisibleregion.
ringt=105nm.(b)Therealpartandimaginarypartoftheα-MoO3permittivityinthevisibleregion.
Theinsetistheschematicoftheα-
α
representTheinsetismolybdenumtheschematicandoftheoxygen-MoOatoms,
representmolybdenumandoxygenatoms,respectively.

(1)toobtainthepermittivitytensorsofα-MoO3inthevisiblerange.
−1−1−1
Polarizationε∞ωpj[cm]ω0[cm]γj[cm]
,67227,
,07832,
Figure1billustratestherealandimaginarypartsofα-MoO3permittivityalong[100]
and[001]directionsextractedfromRef.[10].Fullfieldelectromagneticcalculationswere
-dimensional
FDTDsimulationsweremadeinaunitcellarea,andthenon-uniformmeshischosen,and
themeshsizegraduallyincreasesoutsidetheα--
graphicdirectionconventions,thex-,y-,andz-directionsrepresentthe[100],[001],and
[010]directions,,theplanewaveswereilluminatedalongthe
negativez-direction,andperiodicboundaryconditionswereusedinx-andy-directions.
Ingeneral,theopticalabsorptioncanbeexpressedasA=1−R−T,whereRandT
indicatethereflectionandtransmission,
goldmirrorhasexceededtheskindepthofthelight,thetransmissionTisequaltozero.
Therefore,theabsorptioncoefficientAcanbeabbreviatedasA=1−,the
,andthepermittivityof
goldisdes