Light Manipulation at the Nanoscale:All Dielectric Metasurfaces and Two-dimensional Material Polaritons

Author:Zhang Qing

Supervisor:cui xu dong

Database:Doctor

Degree Year:2019

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Pages:114

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Nanophotonic devices are largely dependent on the geometric resonances of nano-structures and the physical properties of host materials to achieve strong light-matter interaction at the nano-scale.For instance,nano-structures confine electromagnetic waves at the subwavelength space through local resonances;the quasi-particle excitations of half-light and half-matter(such as electrons,phonons and excitons)polaritons,enable slow light group velocity,which break through the diffraction limit and achieve light controlling at the sub-wavelength scale.Metasurface and plasmonics are two major branches of nanophotonics,the former focuses on the structure design,and the latter more relys on the physical properties of materials.In this thesis,by utilizing the cutting edge all-dielectric Ti02 metasurface platform and two-dimensional material van der Waals(vdW)polariton platform,we obtain novel controlling of visible free space light and in-plane infrared vdW polaritons,respectively.The thesis includes three major parts:The first part,all-dielectric TiO2 metasurface structure designs enable free-space light wavefront manipulation.We begin by establishing the generalized Snell’s law of phase-control refractive metasurface.Using this formalism,anomalous refraction beam deflectors,polarization insensitive metalens have been demonstrated,which can bend light propagation to any desired directions.Secondly.we realize polarization control of light wavefront by using non-centro symmetric TiO2 metasurface nano-brick antenna.Due to the in-plane anisotropic of TiO2 nano-brick,the resonance along the long axis direction of TM polarization component has different phase shift with the resonance along the short axis direction of TE polarization component,thus each TiO2 nano-brick performs birefringent property,and then leads to the design of the broadband 1/4 wave plate,half wave plate,and the polarization beam splitter,etc.Finally,we propose a birefringent TiO2 metasurface platform that could provide both phase and polarization control at visible wavelength.Within this platform,we successfully realize a polarization split metalens,which can replace the multiple functions of "wave plate+beam splitter+lens",without additional optical elements.Polarization properties of light are resolved,an arbitrarily polarized incident beam is split into two orthogonally polarized beams,in which the transverse magnetic(TM)component and the transverse electric(TE)component are focused on the right and left side of the central axis,respectively.These visible all-dielectric metasurfaces have powerful light wavefront controlling capability,combining with the technical advantages of functional customization and high transmission efficiency,may find potential applications in miniaturization and multi-functional integration of traditional optical components.The second part,unique physical properties of two-dimensional materials enable the in-plane control of vdW polariton.The rich types of 2D materials,and the "all-surface"passivated nature,allowing them to be stacked arbitrarily and support highly confined vdW polariton in the atomically-thin thickness,providing unique optical control beyond what can be achieved using traditional plasmonic counterparts in the range from visible light to terahertz.Current research works are limited to static excitation and detection of vdW polaritons,new approachs for propagation control are eagerly demanding for future vdW polariton systems,such as negative refraction,focusing,switching,topological transmission,which are potencial for on-chip optical circuits.We proposed a 2D/SiC heterojunction,utilizing the hybrid coupling between vdW polariton and SiC Surface Phonon Polariton(SPhP),we can achieve low-loss in-plane negative refraction without any need for nano-scale periodic pattering.1)h-BN-graphene/SiC heterojunction:within h-BN’s first "Reststrahlen" band,thin h-BN slab supports Hyperbolic Phonon Polariton(HP)with negative group velocity,when it transmitted to graphene Surface Plasmon Polariton(SPP)with positive group velocity,leads to the negative refraction at the interface.The incident HP and transmitted SPP propagate on the same side of the normal line,and then inspired a tunable superlens design by conrtolling graphene’s fermi level.2)MoS2-graphene/SiC heterojunction:thin MoS2 layer with high refractive index can flip the dispersion of SiC’s SPhP to obtain negative group velocity.Such a mode transmitted to the graphene SPP with positive group velocity,the negative refraction transmission also occures at the hetero-interface.Due to the superior spatial confinement of vdW polaritons,above two types of superlens have deep subwavelength resolution(FWHM<1/80λ0).Further,the focal length and the resolution of the superlens can be electric tuned via the graphene Fermi-level.3)We proposed a new approach for phase control of planar polariton refractive optics,by constructing a curve lateral heterojunction-one side hosts positive neff polariton and another side hosts negative neff polariton.The same absolute value of ±neff keeps the momentum matching,and the sign difference of±neff results the phase accumulation at the curved interface.As a proof of concept,we example highly confined van der Waals polariton lenses in a heterojunction of h-BN-graphene on SiC,the lens resolutions are 100 times smaller than the illumination wavelengths.By electric controlling graphene’s Fermi level,actively tuning the resolution from 50nm to 180nm.Varying the geometry of the heterojunction interface,it would be possible to realized arbitrary phase control of polariton waves,with important implications for nano-imaging and novel guiding devices.The third part,we began to nanopattern multi-layer TMDs to realize novel local polariton resonances.TMDs have abundant physical properties(exciton,spin,valley,etc.),and also show extremely high optical refractive index,it can realize strong local resonance in subwavelength TMDs antennas,provides great research interest for nanophotonics.For instance,we proposed a graphene/MoS2-grating/SiC heterostructure,high-index MoS2(n-4)grating can directly excite the SiC local surface phonon resonance(d-LSPhR).The collective resonance behaviors of d-LSPhR and graphene plasmons,enable the hybrid resonance to inherit the high quality factor(Q-85)of LSPhR concurrently with the electric tunability of graphene plasmons,resulting in superior electrostatic modulation of light from the overall structure,the modulation depth is over 95%.In addition,subwavelength WS2(n-4.5)nano-grating support distinct Mie resonances at visiable,and the the resonant photon strong coupling with WSe2 exciton can excite the so-called excite exciton with large rabbi splitting.Overall,combining metasurface structure with new materials,exploring new effect,and developing new applications would be important research trends for future nanophotonics.Here,we put metasurface and 2D vdW polariton together for the first time,open up a new way for nanophotonic by comobining the metasurface structure design with the unique properties of two-dimensional material.Our results suggest that nanopatterned TMDs are promising materials for high-index nanophotonics applications with enriched functionalities and superior prospects.We predict a couple more complex gradient metasurface devices based on TMDs thin-layer in near future.In addition,the valley degrees of TMDs exciton are related to the circular polarization of light,it is expect to control circular polarized light through TMDs geometric phase metasurface,so as to spatially control the transport of valley excitons.