The Research And Applications Of Plasmonic Metamaterials

The light–matter interaction and the manipulation of light have always been essential research topics in the fundamental science.Surface plasmons,the coherent oscillations of free electrons on the metal surface excited by the incident light,lead to strong absorption or scattering of the incident photons at a given frequency,which can focus the electromagnetic fields at the subwavelength scale,resulting in strong localization of near-field and enhanced light-matter interaction.With the development and upgrade of the micro-nano manufacturing techniques,people at present can design and assemble periodical arrays of subwavelength structures at will.These newly emerged metamaterials can break through the limitation of natural materials and achieve arbitrary electromagnetic properties.Plasmonic metamaterials,combined by surface plasmons and metamaterials,endow people with the unprecedented abilities for manipulating light.People can precisely design and assemble plasmonic metamaterials using metals or metallic materials to overcome the diffraction limit,and realize the generation,transmission,modulation and detection of light at the subwavelength scale.Therefore,plasmonic metamaterials have brought a revolution for the development of micro-nano photonic devices and become a forefront of modern optics.In this dissertation,through theoretical analyses and numerical simulations,we investigate three different kinds of plasmonic metamaterials including metal plasmonic metamaterials,graphene plasmonic metamaterials and metal-graphene hybrid plasmonic metamaterials and apply them to refractive index sensing,light absorption enhancement and engineering,and light modulation of electromagnetically induced transparency analogue.The main achievements are as follows:1.We design a multi-wavelength and polarization-insensitive refractive index plasmonic sensor based on combined split ring metamaterials.According to the intrinsic connection between the physical symmetry and the plasmonic resonance modes,we integrate four identical metal split ring resonators into one unit cell.By mutually rotating and combining,the simultaneous excitations of odd and even resonance modes are realized.Due to the rotational symmetry,the whole structure is free from polarization dependence.We in detail analyze the effects of the geometric parameters on the plasmonic resonance modes in the proposed metamaterials,and shift the operating wavelength across the whole near infrared spectrum.The numerical simulations verify that the redshifts of odd and even resonance modes can be clearly observed when the environmental refractive index is changed,and ultrahigh sensitivities ? 1000 nm/RIU and ? 500 nm/RIU are obtained,respectively.2.We design a series of active devices for plasmon-enhanced light absorption based on graphene metamaterials.(1)We design a hybrid periodic array composed of a graphene ring on the top of the light-absorbing materials(a bulky semiconductor or 2D material).The excitations of graphene plasmons trap a sizable part of the incident light in the near field and significantly enhance the light-matter interaction,thus the absorption in these light-absorbing materials is improved with more than one order of magnitude.Compared with previous metal plasmon-enhanced light absorption devices,graphene plasmons possess active tunability.With the manipulation of the Fermi level of graphene,the operating wavelength and absorption performance could be modulated.As an extension,we introduce more graphene concentric rings into the unit cell,and consequently enable dynamically tunable multi-wavelength light trapping and absorption enhancement.(2)Based on above,we design a graphene plasmon-enhanced photosensor model with an ultrathin semiconductor region.We integrate cross-shaped graphene metamaterials on the top of the semiconductor materials and obtain even higher absorption performance.The strong electric fields due to graphene surface plasmons amplify the photoresponse to the incidence light at the resonance wavelength and provide a comsiderably high photogeneration rate,showing a comparable performance with those in metal plasmon-enhanced photosensor model.(3)We further explore graphene complementary metamaterials.We integrate graphene circular nanohole metamaterials with the semiconductor materials,and enhance the absorption by more than one order of magnitude.By transforming the isotropic circular nanohole to anisotropic elliptical nanohole,the absorption enhancement in the semiconductor materials shows polarization selectivity,in which either TM or TE plane wave at the specific wavelength can be efficiently absorbed by simply manipulating the Fermi level of graphene.3.We design a series of electromagnetically induced transparency structures for terahertz light modulation based on metal-graphene hybrid metamaterials.(1)We place a monolayer graphene layer on the top of the two-gap split ring metamaterials,and with the high conductivity of graphene the opposite type of charges at the ends of the two gaps can be neutralized and the absolute strength of the dark Fano resonance is strongly suppressed.With the manipulation of graphene conductivity via altering its Fermi level or layer number,the amplitude of the dark Fano resonance can be actively modulated.In addition,the ?sensitivity‘ to the graphene layer of the dark Fano resonance is also highly appreciated,which can be employed in sensing biomolecules with the high conductivity.(2)Based on above,we integrate a monolayer graphene into the cut wire-split ring coupled terahertz metamaterials,and the conductive graphene neutralizes the opposite type of charges at the ends of the split gap and enhances the losses in the dark mode resonator,thus weakens the destructive interference between the bright and dark resonance modes.With manipulating the Fermi level of graphene,the resonance strength of the electromagnetically induced transparency analogue and the accompanying slow light effect can be actively modulated.(3)We propose multi-split ring mirror-like symmetric terahertz metamaterials,in which the bright-dark-dark-bright coupled modes can lead to steeper electromagnetically induced transparency analogue.With the monolayer graphene placed under the dark split rings,the losses in these dark mode resonators will be enhanced and the destructive interference between them and those bright mode resonators can be strongly suppressed.We complete an on-to-off modulation of the resonant strength of electromagnetically induced transparency analogue and the group delay in the terahertz metamaterials via actively manipulating the Fermi level of graphene.Compared with previous studies,the active modulation of the EIT analogue in our proposed hybrid plasmonic metamaterials shows much better efficiency and feasibility.