Study On Coupling Field Characteristics Of Ultrafast SPP-excited Nanostructures

When the free electrons in the metal deviate from the initial equilibrium position by the external electric field and the collective oscillation occurs,an electromagnetic surface wave mode,that is,surface plasmons,is formed on the surface of the metal.It is mainly divided into two types: Surface Plasmon Polaritons(SPP)and Localized Surface Plasmon(LSP).In recent years,it has been found that the method of using SPP to stimulate nanostructures to generate LSPs can improve signal background noise and structural thermal effects,and improve the performance of plasmonic components and sensing detectors based on nanostructured LSPs.In-depth understanding of the characteristics of the coupling field of SPP excited nanostructures(including mode distribution,near-field intensity and de-phase time)and effective regulation of near-field intensity are important prerequisites for their application,but the current research in this area is seriously insufficient.In this paper,the Finite Difference Time Domain(FDTD)method was used to systematically study the coupling field characteristics of ultrafast SPP-excited nanorod composite structures.The specific research content is as follows:(1)The coupling field characteristics of the composite structure of nanorods excited by a single slit were studied.It is found that the coupling field resonance wavelength and near-field enhancement depend on the ratio of spacer layer thickness d to nanostructure size,and the smaller the ratio,the stronger the near-field enhancement combining with the gradually redshift of the resonance wavelength.In addition,by comparing the near-field spectra of the upper surface of the nanorod structure with the gap position,it is found that the gap mode energy comes from the upper surface of the nanorod.By changing the spacer layer thickness and nanorod length,the influence of LSP gap mode on the coupling effect of SPP and LSP was explored.The results show that the low-order mode has a better coupling effect with SPP in the visible range,and the high-order mode has a better effect on SPP in the near-infrared range.In addition,it is found that by changing the distance from a single slit to the nanostructure and the nanostructured material,the near-field strength and peak position of the coupling mode can be controlled.(2)Research on near-field strength regulation of SPP-inspired nanorod composite structures was carried out.The results show that by adjusting the time delay between SPP and direct excitation light,the near-field intensity control of the upper surface and gap position of the structure can be realized,which we attribute to the coherent superposition of SPP-excited LSP and light source excitation LSP.In addition,the near-field regulation mechanism of composite structures is attributed to the fact that the relative time delay regulates the near-field intensity of the light-dominated mode stronger.The regulation of near-field strength in the SPP-dominated mode is relatively weak.(3)Using the quasi-normal model combined with FDTD simulation,the coupling field dephasing time of ultrafast SPP-excited nanorod structure was investigated.The study found that the gap mode(～384 THz)dephasing time of light and SPP excitation can reach 6.1 fs.On the upper surface of the structure,the dephasing time of SPP-excited LSP(4.3 fs and 4.6 fs)at the same frequency is longer than that of photoexcited LSP(3.9 fs and 3.5 fs),while the gap position mode corresponds to a longer de-phase time than the upper surface mode.At different dual beam delay times,there is a difference in the dephasing time corresponding to the gap mode.This study provides a basis for the efficient excitation of nanoparticle composite structures by ultrafast SPP,and opens up a new way to further improve the near-field strength and enhance the dephasing time.This research will lay the foundation for the application of metal nanoparticles in the fields of surface-enhanced Raman scattering and nonlinear optics as well.