超材料的前沿技术研究及应用-元表面：物理与应用

1、Metasurfaces: Physics & Applications -I 超材料的前沿技术研究及应用元表面：物理与应用BackgroundsPeriodic/homogeneous metasurfacesGradient metasurfaces Metasurfaces for spin-polarized lightConclusions自由调控电磁(光)波意义重大 自由调控电磁（光）波不仅具有重要基础理论 意义，并且在信息/国防/能源等方面可能产生 重要应用 原子和其排列方式均选择有限 变化范围有限对电磁波调控能力受限 自然材料调控电磁波能力有限 “超材料”与“超表面” “人工原

2、子”- 亚波长电磁微结构 排列方式任意设计 均匀性体材料非均匀超表面“人工原子”及其“排列方式”可任意设计 超材料/超表面对电磁波具有强大调控能力！ Whats metamaterial? 原则上，除了真空以外，没有物质是均匀的。 然而当结构单元 电磁波波长时，物质的非均匀性被抹平，具有 有效的介电常数及磁导率。自然界存在的物质对光波来说就是均匀 介质 基于同样的原理，我们可以将结构单元由原子换成人工亚波长微结 构，这就是特异介质（Meta-material)超构材料（Metamaterials）的定义特异介质，结构单元 80%), Polarization conservedCollabor

3、ated with Dinpin TsaiApplications: 1) Meta-holograms High-efficiency Broad-band Polarization-controlled dual hologramsCollaborated with Dinpin TsaiFlat metasurface with parabolic phase distributionFocusing EM waves with very high efficiencyFlat, ultra-thin, highly efficientApplication: 2) Flat lens

4、focusingLi et al., Opt. Lett. 37 (2012) 4940 Similar works by Bolzhevolni, Capasso, etcIssues arisingWhat happens here?Perfect agreement between expt. & full-wave simulationReflection beams are steered to large angles for large Radiation fields almost disappear when k0.Near-field (NF) characterizati

5、onsIlluminate the gradient surface with incidence EM waveDirectly map the local field distribution on the surfaceClear evidence on the SW generation with Direct proof on the dispersion relationBoth FF & NF experiments confirm the dispersion As far field radiations disappear, local field strongly enh

6、ancedSW-SPP: Expt & FDTDIlluminating the meta-surface onlyStrong SPP signals appear on mushroom, only coming from the “driven” SW.Non-resonant scheme, no momentum matching requestHas the issue been solved completely? No！Decouplings in conventional couplersAvailable mechanisms all have intrinsic limi

7、tations, low excitation efficiency Problems of the side-coupler “Driven SW” flows on the “inhomogeneous” meta-surface Decouplings due to scatterings are inevitable Only works for small beam width Q. Che et. al., EPL 101, 54002 (2013) SPPOutput PortMetasurfacePlasmonic MetalOur motivationCan we desig

8、n a SPP meta-coupler to yield high excitation efficiency ?Meta-coupler for LP lightTransparent gradient meta-surface: No reflectionNo x -x symmetry, decoupling is forbiddenNew coupling config.: More efficient to extract SPPHighest efficiency achievedBalancing two effects at appropriate dAn ideal eff

9、iciency 94% achieved Design/fabrication of the metasurfaceFive microwave meta-atoms chosen in different phasesExperiments agree with FDTD simulationsNear-field characterizations: SPP dispersionMicrowave plasmonic metal: Mush-room structure Measured SPP dispersions agree well with FDTD Near-field sca

10、nning Varying incident angle NF characterizations: Comparison with other couplersMeta-coupler is the best, several times higher than others;Working bandwidth: 8.8-9.5 GHz;Far-field characterizations: Absolute SPP excitation efficiency Integrate all scattered energy to obtain R Estimate absorption A

11、by FDTD Estimate the ratio between left/right SPP Final expression to estimate SPP excitation efficiency SourceDetectorSPP excitation efficiency: Measured vs FDTD 73% efficiency (Expt.)Match well with FDTD (75%).W. Sun et al., Light: Science & Applications 5, 16003 (2016).Latest development: Directi

12、on-controllable high-efficiency SPP meta-couplerHigh-efficiency PB metasurface;Correct design of the plasmonic metalDuan, et al, Scientific Reports 7 1354 (2017).BackgroundsPeriodic/homogeneous metasurfacesGradient metasurfaces Metasurfaces for spin-polarized light: 100%-efficiency PSHEConclusionsCi

13、rcularly polarized light: Spin momentum Circularly polarized light carries spin angular moment Spin-polarized light has important applications in manipulating chiral objects (e.g., chiral molecules)Can we use metasurfaces to control spin-polarized lights at will? PSEH: Controlling spin-polarized lig

14、ht1. Intrinsic photonic spin-Hall effect (PSHE) A direct analogy of electron SHE SOC term is crucial Effect very weakNagaosa PRL (04); Bliokh PRL (06); O. Hosten, Science (08): Xiang Zhang, Science (13)2. Extrinsic PSHESpin-dependent scatterings at meta-surfaces Effect very significant, can even lea

15、d to PW-SPP conversion Efficiency is low (3-5 %) ! - multi-mode generation; - normal modes exist Shuang Zhang, Light Science & Appl. (12), Capasso, Science (13), etc. Low EfficiencySpin DependentOur motivationCan we realize a giant photonic SHE with almost 100% efficiency?To realize PSHE with 100% e

16、fficiency the Jones matrix of a “meta-atom”? How to realize PSHE with 100% efficiency ?Generic structure of a Berry SlabConsider R-matrix (in CP basis) onlyCriterion to realized 100% efficiency PSHENormal modesAnomalous modesLuo et. al., Adv. Opt. Mater.,3, 1102 (2015)Half-wavelength wave-plateMeta-

17、surfaces go to spin-dependentCase 1: SymmetricalCase 2: AsymmetricalAchieve 100% efficiency Photonic Spin Hall effect based on Meta-surfacesHalf-wavelength wave-plateBroad-band and high-efficiency SHE of Light (experiments)Single anomalous modeAlmost NO normal mode Symmetrical AsymmetricalLuo et. al

18、., Adv. Opt. Mater.3,1102 (2015)QuestionCan we realize 100%-efficiency PSHE in transmission mode?Challenges in transmissive PB metasurface73Extension to transmission case is highly nontrivial4 modes exist generally New physics and new design Criterion in transmission geometryIdeal half wave-plate in

19、 transmission geometry Symmetrical case, Interchange r and t25% efficiency limit in ultrathin metasurfacesTheory, arXiv:1411.2537ExperimentWhy 25% limit in transmissive PB metasurface?jeOnly electric response both R and TJeSolution: Electric + magnetic responsesCut off R T=1jejm+Out-phaseIn-phase77c

20、hoosing appropriate e/m responses, we can realize the desired 100-efficiency meta-atom Design of the 100%-efficiency PB meta-atomStill deeply subwavelength in thickness ( )Magnetic responses introduced through couplings between adjacent layers ABA structure - PRL (2005)Experimental characterization

21、on PSHE Three undesired modes Ra, Rn, Tn are suppressed Measured PSHE efficiency: 91%Transmission sideReflection sideFDTD studies on PSHE Three undesired modes Ra, Rn, Tn, deeply suppressed Simulated PSHE efficiency: 94%Transmission sideReflection sideApplications I: vortex generation In the working

22、 band, vortex beam is of high efficiency and pure. Otherwise, vortex beam is of low efficiency and blurred q=1q=2q=310.5G14.5GApplications II: Bessel beam generationVery high efficiencies, without normal-mode interferenceSelf-healing after being scatteredWang, Appl. Phys. Lett, 12 191902 (2018)High-efficiency PSHE in the THz regimeIn collaboration with Yan ZhangUnit-cell Measurements on PB MSPSHE Efficiency & Generalized Snells law Relative efficiency reaches 92%Satisfying generalized Snells law High-efficiency, background-free THz BBMin Jia. et. al, (under review) Clean, no normal-field back