| Confinement,enhancement and manipulation of electromagnetic field in localized surface plasmon(LSP)excitations have received extensive attention for applications in subwavelength optics,surface-enhanced Raman scattering and optical sensing.Resonances with easy coupling to incident light are named superradiant modes,while the difficult way are called subradiant modes.With the disappearance of its dipole moment,the subradiant plasmon(dark modes)exhibits strong electromagnetic enhancement in the near-field region.Due to the scattering effect of metal nanoparticles on electromagnetic waves and their own radiation loss,it is difficult engineering to maintain a high excitation efficiency while changing the size,shape and components of the structure to achieve manipulation of subradiant modes.Combining material properties with new optical methods for generating,localized and controlling subradiant modes has important potential applications.In this dissertation,we propose to achieve subradiant effective excitation and its directional manipulation using the unique polarization characteristics of the cylindrical vector beam(CVB).Owing to the spiral phase of azimuthally polarized(AP)and the radially polarized(RP)light exhibits a radioactive distribution along the incident axis as well as its unique focusing characteristics,this beam can modulate the electromagnetic response of the plasmon.Numerical calculations and theoretical analysis are carried out based on finite-difference time-domain simulations(FDTD)and Mie’s theory.Firstly,the electromagnetic response of the bulk Dirac semimetal(BDS)spherical particles at different polarized light incidence is numerically analyzed and theoretically investigated.The simulation results for a single spherical particle illuminated by linearly polarized light are in perfect agreement with Mie’s theory.The interaction between CVB and BDS dimer is investigated by the finite-difference time-domain method.It is shown that the unique polarization and focusing properties of CVB alter the spectral response of the BDS dimers,particularly through the direct excitation of dark modes by RP and AP beams that are inaccessible to coupled linearly polarized light.In the meantime,we prove that the subradiant mode which is dynamically tunable in the mid-infrared by using the Fermi energy variation property has a large absorption and an abnormally high field enhancement for the AP beam.Secondly,a vector diffraction theoretical model of focused CVB under high numerical aperture conditions was developed using Richards-Wolf theory,and the field distribution near the focal plane was investigated in detail and specifically.Successful excitation of subradiant modes in the transverse dimer structure based on the dispersive distributed polarization characteristics of RP illumination.The connection between the longitudinal dimer and the focusing characteristics of the RP beam is further investigated,and the excitation of subradiant modes with high quality factors is achieved while the superradiant modes is excited in the longer wavelength.Furthermore,we demonstrate the existence of phase shift on both sides of the focal plane can support the emergence of two different modes,as well as the degree of coupling between those modes and the incident light can be quantitatively depicted by the simple harmonic oscillator model.Finally,the subradiant mode has been investigated in each anisotropic borophene discs.Rich spectra responses are obtained by coupling between different modes.By solving the eigenvalues,the mode distribution of the individual discs is successfully predicted,which have a good match with the numerical calculation.The hybridization of the nanodisk modes enables the excitation of resonances that would be invisible or dark in a single disk.By designing the disk size and material carrier density,a dark-bright coupling resonance dependent on the polarization direction of the incident electromagnetic field is successfully excited in the communication band. |
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