1、Metasurfaces: Physics & Applications - II超材料的前沿技術(shù)研究及應(yīng)用AcknowledgementsKey collaboratorsYuanbo Zhang, Qiong WuKey contributorsShaojie Ma, Ziqi Miao, He-Xiu Xu, Tong Cai, Shiwei Tang, Shiyi Xiao, Shulin Sun, Qiong HeNSFC Shanghai Sci. Tech. MOSTMinistry of Education MIM metasurfaces and graphene metas

2、urfaces (THz)Tunable microwave metasurfaces Multifunctional metasurfacesWave-diffusion metasurfacesAngular dispersions ConclusionsWhat is MIM Metasurface?MIM metasurface can control reflectance and phase on resonanceChange geometry/materialReflectance modulationPhase modulationmetal/insulator/metalM

3、etalSpacerPatternApplications 1：Reflectance ModulationMIM metasurface triggers many applications through reflectance modulationPerfect absorption N. I. Landy, et. al. PRB 78, 241103(2008)Active modulationYu Yao, et. al. Nano Lett. 14, 6526 (2014)Perfect absorptionJM Hao et. al. APL 96 251104 (2010)N

4、a Liu, et. al. NL 10, 2342(2010)Color PrintingA.S. Roberts, et. al. Nano Lett. 14, 783 (2014)Application 2: Phase ModulationMIM metasurface triggers many applications through phase control J.M. Hao, et. al. PRL. 99, 063908 (2007)polarization controlanomalous reflectionS.L. Sun, et. al. Nat Mater 11,

5、 426-431(2012)flat-lens focusingAnders Pors, et. al. Nano Lett. 13, 829 (2013)hologramsWei Ting Chen, et. al. Nano Lett. 14, 225 (2014)Do we really understand the system physically ?So many interesting applications of MIM systemsI. Conflicting models for MIM systemsJ.M. Hao, et. al. APA 87, 281 (200

6、7)APL (2003) H.T. Chen, et. al. OE 20, 7165 (2012); PRL (2010)The system treated as a wholeMetallic layers treated separately1. Magnetic resonance model2. Interference modelWhich one is correct ?II. Why diversified functionalities ? Why and when certain functionality ? Guidelines for designing MIM m

7、etasurfaces with desired functionalitiesChangegeometry/materialMetalSpacerPatternReflectance modulationNa Liu, et. al. Nano Lett. 10, 2342(2010)Phase modulationS.L. Sun, et. al. Nat Mater 11, 426-431(2012)OutlineEigen resonant modes in MIM(PRB 2016)Complete functionality phase diagram for MIM (PRL 2

8、015)Applications: Graphene MIM for wide-range phase modulation (PRX 2015) Mode-expansion method for simplest MIM Scattering properties can be analytically studiedGood agreements between MEM, FTDT and experimentsCoupling parametersConditions:SubwavelengthThin-slitSimplifications under single-mode app

9、roximationEffective admittancePhysics well captured by the analytical expressionOriginal admittanceModification by couplingsResonanceThin-spacer limit: Lateral resonanceThick-spacer limit: Vertical FP modehd 1hd 1Analytical expressions for f and Q A transition from magnetic resonance to FP resonance

10、 occurs with increasing hThe intriguing resonance behaviors are universal !Picture valid for general metasurfacesGeneral definition for S parameters:Fundamental-mode distribution: II. Why diversified functionalities ? Why and when certain functionality ? Guidelines for designing MIM metasurfaces wit

11、h desired functionalitiesChangegeometry/materialMetalSpacerPatternReflectance modulationNa Liu, et. al. Nano Lett. 10, 2342(2010)Phase modulationS.L. Sun, et. al. Nat Mater 11, 426-431(2012)Coupled-mode theory for MIM metasurfaceS. Fan et. al., J. Opt. Soc. Am. A 20, 569(2003)W. Suh et. al., IEEE J.

12、 Quantum Electron.,QE 40,1511(2004)Response:CMT model describes response through lumped Qr and Qametal/insulator/metalCoupled mode theory:MetalSpacerPatternThe competitions between two Q factors generate a variety of physical effectsReflectancemodulationPhasemodulation = 0Coupled-mode analyses: Gene

13、ric phase diagramQuestion: what determines Q factors? Coupled-mode analysis on MIM metasurfaceQr factor:Concrete expressions of Q factors are obtained Analytical expressions for Q factorsQa factor: (perturbation)Total EnergyLossin spacerLoss in metalMa et al, PRB (2016)Tailor MIM Metasurface Through

14、 Structural/Material Tuning Response:Qr factor:Qa factor:MIM propertiesTwo Q factorsStructure & MaterialsTailor MIM Metasurfaces via tuning hTuning h can dramatically change the property of a metasurfaceQr factor:Qa factor:Critical parameterTailor MIM metasurfaces via tuning material lossQr factor:Q

15、a factor:Tune dielectric loss：Tuning material can dramatically change the property of a metasurfaceGate-controlled graphene ：Miao, PRX 5, 041027 (2015)Our structure Graphene + MIM Combining Graphene with reflective meta-surface Working at THz domain The transmission port is blocked - crucial ! Top-g

16、ating graphene using Ion-GelMetallic sheetExperimental results As voltage increases, reflection amplitudes first decreases to 0, then increases to 1 Phase behaviors change from a magnetic (360 variation) to electric (180) resonance Phase modulation covering +/- 180 degrees Complete phase diagrams (E

17、xpt.) - different spacers85um 60um 40um Critical point determined by structure geometry Some system dose not support critical transitionRole of graphene upon gating(Fitting FDTD with CMT)Effects of gating graphene:increasinghas little effect on drives the system to transit from an under-damped to an

18、 over-damped resonatorGating graphene beaks the subtle balance between intrinsic and radiation losses!Full-range active phase control (Expt.) Two slightly different pixels lead to full-range phase modulation at a frequency interval Previous mechanism can never work, due to intrinsic limitationsEABRe

19、-interpreting previous works with our phase diagramPhase-modulationPolarization controlGradient MSPerfect absorbers MIM metasurfaces and graphene metasurfaces (THz)Tunable microwave metasurfaces Multifunctional metasurfacesWave-diffusion metasurfacesAngular dispersions ConclusionsIssues with passive

20、 metasurfacesFunctionality locked in passive metasurface Dispersion issue limits the performance and bandwidthDistorted phase gradient Undesired reflection modesLimited efficiency and bandwidthPassive metasurface has narrow bandwidth Why performance always deteriorated even in “broad-band” sample?Ph

21、ase distribution cannot maintain at other frequencies Tunable meta-atoms can help With varactor diode incorporated, phase of our meta-atom can be controlled precisely by external biasing voltageCan realize any phase distribution controlled by external voltages Single-mode reflection; Very high effic

22、iency; Truly broad bandActive metasurface with dispersion compensated Xu, Sci. Rep. 6: 38255 (2016)On-state: A SPP couplerOff-state: A specular reflectorActive metasurface for functionality switching Xu, Sci. Rep. 6: 38255 (2016)Active meta-lenses Precisely control the local phase of each “meta-atom

23、”Make the dispersion-induced aberrations corrected; make focal length the same for different frequencies.Make the focal point actively tunable at a single frequency H.X. Xu et. al, Appl. Phys. Lett. 109 193506 (2016)MIM metasurfaces and graphene metasurfaces (THz)Tunable microwave metasurfaces Multi

24、functional metasurfacesWave-diffusion metasurfacesAngular dispersions ConclusionsWhy multifunctional metasurfaces ?Integration systemsMultifunctional meta-devicesMetasurfacesDevice miniaturization Functionality diversified ACS Photonics 4, 1906 (2017)SR 6, 27628 (2016) (X. Chen)Issues with existing

25、approaches 1) Functionality cross-talking2) Low efficiencies 1. Merge two structures Exhibiting similar functionalities LSA 3, e197 (2014)(Bolzhevolni)SR 5,9605 (2015) (T. J. Cui)2. Single anisotropic meta-atomOur motivations Meta-devices with distinct functionalities with high efficiency and low cr

26、oss-talking Meta-atoms with polarization-controlled responsesReflection-typeTransmission-typeFull-space1) Reective multifunctional metasurfaces Focusing lens for E | y PW-SW convertor E | xAdv. Optical Mater. 5, 1600506 (2017)Structural tuning High-efficiencyreflective meta-atom2) Transmissive multi

27、functional metasurfaces4-layer meta-atomCoupling between different layers forms a wide transparency bandTransmission-phase covers 360range Adv. Optical Mater. 5, 1600506 (2017)High-efficiencytransmissive meta-atom Working efficiency (72%)Transmissive bifunctional meta-deviceAdv. Optical Mater. 5, 16

28、00506 (2017) Focusing lens for E | y; Beam deflector for E | x3) Full-space multifunctional devicesPhys. Rev. Applied 8, 034033 (2017) (Editors suggestion)Special meta-atomContinuous stripes on bottom block x-polarized waveFP-resonance enhances transmission of y-polarized waveMeta-device 1: Deflecto

29、rPhys. Rev. Applied 8, 034033 (2017) (Editors suggestion)Anomalous reflectorAnomalous refractorMeta-device 2: LensPhys. Rev. Applied 8, 034033 (2017)(Editors suggestion)Transmissive lensReflective lensMeta-device 3: A bifunctional devicePhys. Rev. Applied 8, 034033 (2017) (Editors suggestion)Anomalo

30、us reflectorTransmissive lensArbitrary full-space devices realizable via designing appropriate phasesMIM metasurfaces and graphene metasurfaces (THz)Tunable microwave metasurfaces Multifunctional metasurfacesWave-diffusion metasurfacesAngular dispersions ConclusionsWhy we need diffusion?Importance o

31、f diffusion in stealth/cloaking applications1. Objects made by metals suffer from strong specular reflections2. Being easily detected Signals dispersed to all directionsas uniform as possible Scatterings are too small to be detected Issues with available cloaking approachesTransformation-optics cloa

32、ks bulky, extreme parameters Absorbers narrow bandwidth, heat generation(2006)(2010)Cui, LSA, 2016Coding metasurfaces: Phase “bits” arranged in certain “coding sequence” Merits: ultrathin, compatible to complex shapes Issues: Complicated optimization to determine the “coding sequence”O(jiān)ur motivations

33、Polarization independent - working for arbitrary incident polarizationDeterministic design approach - no need to do optimizations Robust diffusion to all angles - not only suppress specular reflectionsMeta-atom: Resonance (LP)Bit: Uniform phaseSequence: Optimized random Meta-atom: PB meta-atom (CP)Bit: Parabolic phaseSequence: Moderately random Our design approach I: PB meta-atomPB mechanism release the polarization dependence , wide-band, high-efficiencyII: “Bit” with parabolic phase profileUniform/gradient phase converts incident K to a single-