Research On Near-field Optical Properties Of Polaritons In Low-dimensional Materials

The research of polaritons is the novel electromagnetic formed by the coupling between the photon of incident light and the various quasiparticle in the materials.The study of polaritons is essentially the process of solving Maxwell's equations.It is to calculate the induced electromagnetic field under the negative real part of permittivity of materials caused by the response of free electron,optical phonon,and other responses.Polaritons are electromagnetic fields propagating at the interface of materials,which can break through the traditional diffraction limit,realize the electromagnetic field control on the sub wavelength scale,and enhance the interaction between light and matter.They have important research significance and application in information,physics,chemistry,biology,energy and other fields.In this dissertation,we present the general methods for the study of polaritons in nanostructure,one-dimensional nanowires and two-dimensional crystal.We study the localized polaritons in the silver nano-triangle,gold clusters and hexagonal boron nitride nanodisk;the propagating polaritons in indium arsenide semiconductor nanowires,graphene and hexagonal boron nitride.To the study of localized surface plasmons in silver nano-triangle,we first give a general method to study the extinction response of silver nano triangle under optical excitation by using boundary element method.Then,we get the extinction spectrum of DNA molecules in the etched silver nano triangle.We reveal the specific circuit of DNA molecules in the etching process and the role of different DNA molecules in the etching of silver nano-triangle.Using the boundary element method,we numerical calculate the electron loss energy spectrum of gold nanoclusters.Combining with the experimental electron loss energy spectra of different diameter of the gold nanoclusters,we study the redshift of absorption the gold nanocluster electron loss spectrum in the range of 4 to 15 nm.The results show that the absorption peak of the gold nanocluster redshift is originated from the absorption of the localized surface plasmons.And the modified permittivity caused by size of nanocluster results to the redshift of absorption peak in the electron loss energy spectra.We applied the finite element method to calculate the extinction spectrum of single hyperbolic Hexagonal boron nitride(h BN)nanodisk.We show that the hyperbolic h BN nanodisk exhibits two extinction mechanisms in the mid-infrared region.The volume confined phonon polaritons propagating in the nanodisk induce the Fabry-Perot resonances then give rise to a series of weak extinction peaks.The extinction crosssection originated from this dipolar resonance extinction is three orders of magnitude stronger than the volume confined phonon polaritons extinction.The localized surface phonon polaritons lead to a robust dipolar extinction,and the extinction peak position is tunable by varying the size of the h BN nanodisk.The quality factor of the LSPh P resonant extinction peak is about 193,and the electromagnetic field enhancement is about 40,which make the h BN nanodisk a promising candidate for applications on midinfrared antenna,molecular sensing and enhanced spectroscopy.For the one-dimensional propagating surface plasmons in the indium arsenide semiconductor nanowires,we theoretically proved that the metallic indium arsenide(In As)nanowires can excite and support the propagation of surface plasmons.Then,we firstly realize the real-space imaging of surface plasmons in In As nanowires in experiment by the means of s-SNOM,and confirmed them with the finite element calculation results.The research results show that the surface plasmons in In As nanowire have high confining ability and low propagation loss.The wavelength of the surface plasmon can be tuned by the doping level,substrate medium and the diameter of nanowire.For the surface plasmon in graphene,we give the analytical solution of the dispersion and electric field distribution of the graphene surface plasmon.At the same time,we give the numerical simulation of the surface plasmon by the finite element method.We systematically studied surface plasmons reflection by graphene wrinkles with different heights on Si C substrate.Combined with numerical simulation,we found the geometry corrugation of a few nanometer height of wrinkle alone does not causes a reflection of graphene plasmons.Instead,the separated wrinkle from substrate exhibits a localized spatial Fermi energy distribution along the wrinkle causes the reflection of graphene surface plasmons.We study the propagation of phonon polaritons excited by the wrinkle in the hexagonal boron nitride(h BN)crystal by the scattering scanning near-field optical microscopy(s-SNOM).Combining the numerical simulation,we find that the wrinkle in h BN can directly excite the phonon polaritons and obtain an interference-free near field imaging.The polaritons excited by the wrinkle propagate without the extra geometrical damping factor can reflect the intrinsic property of the phonon polaritons.