| Surface plasmon polaritons(SPPs)are the excited-state surface evanescent wave generated by the external electromagnetic field coupled with the collective oscillation of free electrons in a good conductor material,with sub-wavelength localization(breaking the diffraction limit of conventional optical devices)and surface field enhancement.As a natural material that can generate SPPs,metals first appeared in research more than100 years.The range of SPPs excited on metals is in nanometer scale which cannot be used in applications.Recently,due to the development of micro-nano processing technology,surface plasmons have once again become a research hotspot,and are widely used in near-field imaging,chemical and biological sensing,surface Raman scattering enhancement,optical transmission enhancement,coherent radiation enhancement and other fields.In addition,new artificial materials such as graphene,topological insulators,and Vale semi-metals have also been introduced into research,which broadens the application range of surface plasmons.Therefore,studying and understanding the excitation characteristics and radiation transmission characteristics of surface plasmons has important guiding significance for its application.This thesis is based on the study of the excitation characteristics of SPPs stimulated by free electrons in metal structures and its application in radiation structures.The theoretical research is carried out by solving the electromagnetic field equations of the structure through the longitudinal field method and boundary matching,and finite difference time domain(FDTD)is employed in simulation,aiming to comprehend the interaction process of SPPs excitation,especially extreme conditions(low confinemet factor)stimulation characteristics of SPPs.In radiation structure,the radiation influence of excited SPPs is in-deep analyzed,which provides theoretical basis for the application of SPPs in high-sensitivity biochemical sensing and new radiation devices.The results are presented below.SPPs frequency blue shift is observed in FDTD simulation of parallel electrons excitation Au bulk structure.Comparing with cold dispersion of SPPs,an obvious frequency blue shift is obtained in low confinement region excitation simulation results.Then,according to SPPs transverse attenuation characteristics,the excited frequency mode instead of cold dispersion corresponding frequency mode matches it.Thence,this excited mode is confirmed as SPPs mode.It is well known that the lower the frequency,the smaller the confinement factor,the lower the excitation efficiency,and the wider the bandwidth of excited SPPs.And considering the attenuation in whole structure,the excited surface field contains attenuation signal,which possesses broader spectrum information.In low confinement factor region,the higher the SPPs frequency,the higher the excitation efficiency,while broadband frequency information obtained in attenuation signal provides high frequency information.Thence,in beam-wave interaction,as the signal oscillation time increases,the frequency of the oscillation field gradually increases.Compared with cold dispersion,the frequency of excited SPPs is blue shift.This hypothesis is verified by monitoring and comparing the time domain signal of excited field in low and high confinement factor region.Then,this frequency blue shift is confirmed as commonality of SPPs,which is independent of SPPs materials and structures.Finally,this frequency blue shift is confirmed in attenuated total reflection(ATR)experiment.Due to frequency dependence of most SPPs devices,such as coherent enhancement radiation and enhancement transmission,the frequency blue shift presented here is of great influence in the SPPs applications.The interaction of excited SPPs in metal film and propagating field in slab dielectric waveguide was studied.The results show that the asymmetric mode of SPPs excited in metal film is converted into coherent Cherenkov radiation in the waveguide,and it can be coupled with the waveguide mode,which greatly enhances the radiation intensity.The radiation intensity is enhanced about 30 times.Due to the reflection effect of the waveguide boundary,in non-coupling conditions,the radiation intensity is also enhanced about 5 times.Compared with traditional dielectric waveguides,due to the influence of SPPs,the main propagation mode is the waveguide mode near the surface plasmon frequency.It achieves higher order mode selection and radiation enhancement.Smith-Purcell radiation of PEC grating excited by parallel free electrons with different energy has been optimized.The results show that the optimized grating is a large groove aspect ratio grating,and the lower the excitation electron energy,the greater the optimized aspect ratio.Due to the strong locality of surface plasmons,the introduction of surface plasmon materials into the grating substrate greatly reduces the optimal depth of the grating,thereby obtaining better radiation.Then,the excitation of SPPs and localized surface plasmons(LSP)in several different grating structures was studied.The results demonstrate that SPPs are excited on the bottom surface of the grating groove in the metal substrate with dielectric ribbons grating and about 40 times radiation intensity enhancement is obtained in it.When the depth of the grating increases,the distance between the stimulation electrons and the metal surface increases,and the excitation efficiency of SPPs decreases,while diminutive groove depth leads to low diffraction radiation efficiency.Therefore,this method has a weaker ability to enhance Smith-Purcell radiation.In PEC substrate with metal ribbons grating and metal reflection grating,the LSP excited on the upper surface overcomes the contradiction between the excimer excitation efficiency and the diffraction radiation efficiency in gratings,thence,hundreds of times enhancement of radiation intensity is obtained.The results presented here provide a way of developing miniature,integratable,tunable,high-power-density radiation sources from visible light to ultra-violet rays at room temperature. |
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