Study On Several Propagation Properties Of Surface Plasmon Waveguides

Traditional optical devices are limited by the diffraction,and the minimum size of optical device is about half wavelength,hindering the miniaturization and density of integrated optical circuits to a large extent.To address this issue,we need to break the limit,realizing high efficient optical devices with subwavelength scale.Surface plasmons(SP),one of the hottest research topics in nanophtonics,are surface waves(mainly in infrared and visible band)that propagate along metaldielectric interface with properties that electric field intensity on the interface of metal has a maximum value and evanescently decays away from the surface.For another,the equivalent wavelength of SP is shorter than free-space wavelength,meaning that SP can realize subwavelength confinement of electromagnetic energy,that is,break the diffraction limit.Owing to its properties of local field enhancement,subwavelength confinement,long-range propagation,surface plasmon has been widely used in the fields of communication,biology,chemistry,energy,including nanowaveguide,nanolaser,surface enhanced Raman scattering,sensing,single molecular fluorescence,super resolution imaging,photolithography,solar cell,cancer treatment etc.Among them,the plasmonic waveguide is one of the key research branches.Up to now,many kinds of plasmonic waveguides have been proposed and investigated theoretically and experimentally.Analytical method and finite element method(FEM)are generally used to calculate the effective refractive index and modal field of surface plasmon mode,and then one can study the propagation properties.This dissertation is based on the analysis of the existing surface plasmon waveguides.Then we propose new waveguiding structures and pay emphasis on the study of the propagation properties and the impact of nonlocal effects on them.The frequency lies in terahertz and telecommunication band.We have obtained some meaningful results: 1.A dielectric coated two-wire waveguide for terahertz waves is proposed.Terahertz two-wire waveguides have the properties of low propagation loss,efficient coupling,etc.The TEM(Transverse electric and magnetic)mode supported by this type of waveguide has small bend loss with energy mainly confined in the area between the two wires.However,the weak surface plasmon effects lead to a weakly confined modal field.As the optical fields tend to be confined in regions with higher refractive index,we propose a dielectric coated two-wire waveguide(DCTWG)for terahertz waves,and investigate the impact of dielectric and its thickness on propagation properties by the widely used finite element method.This work has the following innovations:(a)The DCTWG can achieve propagation length about meter scale with modal field area smaller than that of terahertz two-wire waveguide with same sizes.By choosing different dielectric thickness and permittivity,one can tune the propagation length and modal field area easily.(b)As we add a dielectric layer,the gain medium can be easily involved in,which offers a method to realize active terahertz metal wire devices.By adding gain material,the propagation loss can be reduced to some extent.2.An approximate three-wave model is suggested for describing the modal field inside the high-index dielectric rod of a hybrid plasmonic waveguide.The hybrid plasmonic waveguide consists of a dielectric waveguide and a plasmon waveguide.The photonic and SP modes couple to form a hybrid mode,achieving subwavelength confinement and relative long propagation length simultaneously.Most previous studies focused on the properties of the modal field inside the low-index dielectric region of the hybrid plasmonic waveguide.Here the photonic mode(HE11)strongly couples with the SP mode,leading to that the modal field inside the high-index dielectric rod is no longer the pure HE11 modal field.Besides,the hybrid structure is very complicated,and it is very difficult to establish the eigen equation.In this work,by using the property that the Ey component of the hybrid mode is absolutely dominant,we establish a three-wave model to describe the modal field inside the high-index dielectric rod.We use nonlinear fitting method to obtain the undetermined parameters.The result is in good agreement with the numerical simulation.This work has the following innovations:(a)A three-wave model is suggested to describe the modal field inside the highindex dielectric rod.From the field expressions one can deduce that the modal field inside the high-index dielectric consists of evanescent waves and propagating waves.(b)The nonlinear fitting method is used for determining the coefficients in the field expressions.3.Nonlocal effects in nanoscale plasmonic waveguides.Generally,we suppose that the materials are homogeneous(without spatial dispersion)when studing the plasmonic waveguides.Under this circumstance,the permittivity of metal can be obtained by the Drude or Lorentz model.However,as the sizes of plasmonic devices shrink down to the size of about 10 nm or smaller,the quantum nature of the electrons and the nonlocal effects associated with them significantly change the plasmonic response,thus the classical model can no longer describe the electromagnetic properties accurately.At this time,one needs to consider the impact of nonlocal effects on electron motion.In this work,we investigate the nonlocal effects on waveguiding properties of a metal nanowire waveguide,nanowire dimer,and an elliptical hybrid plasmonic waveguide based on the Hydrodynamic Drude(HD)and Generalized nonlocal response(GNL)model.This work has the following innovations:(a)The HD model introduces only the quantum pressure term while the GNL model introduces the quantum pressure term and the induced charge diffusion in the equation of motion for electrons.We show that,compared with the local description,the imaginary part of the effective refractive index is enlarged when using the GNL response model.This result is quite different from that of the Hydrodynamic Drude(HD)model,where the imaginary part becomes smaller compared with that of the local model.Results also show that the real parts and field profiles based on the HD and GNL model have no significant difference.Further,we investigate the influence of geometry parameters on propagation properties and find that the nonlocal effects are much more remarkable for smaller gap and sharper tip.Under certain circumstance,the normal component of electric field is continuous across the metal boundaries.Different to the classical electromagnetic theory assumption of a Dirac delta function distribution of the induced charges on the metal surface,the induced charges spread into the metal when taking the nonlocal effects into account.This means that the nonlocal effects result in an ‘effective' modification of the metal interface boundaries,reducing the ‘effective' radius of the metal wire.We can obtain that,under the same condition,the plasmonic mode of a plasmon waveguide with smaller size has larger loss,suggesting the smaller imaginary part in the HD model is unphysical.The enlargement of loss based on the GNL model could be qualitatively explained by the superposition of surface modification effect as well as the induced charge diffusion.(b)Previous studies on nonlocal effects need to solve a partial differential equation(PDE)and Maxwell's equations simultaneously.The finite element method is widely used,however,one needs to give the weak form of the PDE.The variational method is used to get the weak form expressions of the PDE,and this process is rather complicated.Here,we add an External current density to the metal domain,without adopting the weak forms,and this method is easier and more transparent than using weak form.(c)This work offers a certain reference value to the applications of nanoscale plasmonic waveguides.At the nanoscale,nonlocal effects result in the decrease of propagation distance.Therefore,one needs to carefully choose the plasmon waveguide size when using it for all-optical integrated circuits or other applications.This dissertation includes our studies on surface plasmon waveguides in terahertz and telecommunication bands.We have laid emphasis on the propagation properties of terahertz dielectric coated two-wire waveguides,modal properties of hybrid plasmonic waveguides,and the impact of nonlocal effects on propagation properties.The sizes of these structures are ranging from microscale to nanoscale.These results may have a certain applying prospect in waveguiding devices,optical integrated circuits,superresolution imaging,nanolasers,sensing,etc.