Study Of Surface Plasmon Polaritons In Metal-nanofilm Hybrid Structures

Surface plasmon polaritons?SPPs?are usually referred to the oscillated quasiparticles formed by the interaction of free electrons on the metal surface with external excitation light.Graphene,a typical two-dimensional?2D?material,has metal-like optical properties in the mid-infrared to terahertz frequencies,and can also excite SPPs,which named as graphene plasmons?GPs?.Since the wavelength of GPs is much smaller than the wavelength of the incident light,and the confiment of the electromagnetic field of the GPs is stronger than that of the SPPs in the metal surface,it is extremely difficult to directly characterize the GPs.In addition,due to the local field enhancement effect of the SPPs of the metallic nanorods,the gold nanorod is conbined with the two-dimensional semiconductor material,and the interaction between the light and the nano quantum system can be enhanced by the SPPs.Conversely,the incorporation of a nanofilm into a metal nanostructure can also affect and regulate the SPPs of the metal.The hybrid structures of metal and two-dimensional materials or nanofilms exhibit different optical properties from metal or two-dimensional material.The study for the SPPs of the metal-nanofilm hybrid structures can not only deepen the understanding of the light-matter interaction,but also promote the development of nanoscale optical components.In this thesis,the excitation and characterization of GPs,the strong coupling between excitons of 2D semiconductor and SPPs,and the dynamic tunning of SPPs were studied by preparing the metal-nanofilm hybrid structures.The optical properties such as near-field distribution of the GPs,scattering cross section of the hybrid structures were analyzed by numerical simulation techniques such as Finite-Difference Time-Domain?FDTD?and Rigorous Coupled Wave Analysis?RCWA?methods.The main work is included as follows:Firstly,we prepared a graphene/grating hybrid structure and characterize the near-field properties of the GPs.The GPs are excited by a mid-infrared laser beam obliquely incident on graphene suspended over a metallic grating with a dielectric spacer.The photo-induced force microscopy?PiFM?records the optical force yielded by the interaction between the electric field of GPs and the metallic tip.By detecting the interference pattern of GPs on the grating trenches,the wavelength and propagation distance of GPs can be obtained.The PiFM technique demonstrated here provides a powerful tool to characterize GPs in a deeply subwavelength scale and aid in the design of nanoscale optical devices based on graphene.Secondly,we demonstrated the optical characteristic in the chemically treated monolayer sulfur-based transition metal dichalcogenides?S-TMDs?.The chemical treatment using bis?trifluoromethane?sulfonamide?TFSI?to repair the defects in the monolayer S-TMDs.We have performed the photoluminescence?PL?enhancement for the chemically treated S-TMDs.29 and twice time's PL enhancement was achieved in MoS₂and WS₂,respectively.Moreover,the large Rabi splitting was demonstrated in a single nanorod coupled chemically treated S-TMDs for both MoS₂ and WS₂ cases at room temperature.Rabi splitting energies of MoS₂ and WS₂ can achieve to 140 meV and 147meV,respectively.The results have revealed a simple chemical process that optimizes the optical prosperity of S-TMDs,which is of potential applications of the development of the optoelectronic devices based on metal-2D material hybrid structures.Finally,we demonstrated the polarization-dependent tuning of plasmon resonances by incorporating photoresponsive nanofilm of polymer into a plasmonic resonance nanocavity,which consists of a gold nanosphere on an ultrasmooth gold film separated by a very thin azobenzene?SD1?molecular layer.Upon polarized ultraviolet?UV?light exposure,the SD1 molecules re-orientate to a preferred direction that is perpendicular to the polarization direction of the illuminating light.As a result,the refractive index of the polymer layer is modified dramatically by the polarization orientation.Therefore,the anisotropic spacer between nanosphere and gold film can be obtained and reserved simply by adjusting the polarization angle of the illumination UV light,leading to the remarkable shifting of the plasmon resonant frequencies.Moreover,the physical mechanisms underlying the spectral shifts are elucidated by studying series gap distances with fundamental plasmon resonances,revealing good agreement with FDTD simulations.