Study Of Enhanced Fluorescence By Plasmonic Nanocavities

Surface plasmons（SPs）have gained wide attention due to their subwavelength spatial profile of modal field,which can remarkably enhance the interaction strength between photons and materials,spurring the fast-growing developments of plasmon-enhanced fluorescence,Raman spectroscopy,heat generation,photoacoustics,photocatalysis,nonlinear optics,solar energy conversion,and so on.While common plasmonic nanocavities only have the ability to confine light down to a deep-subwavelength scale（e.g.,10 nm）,achieving more tightly restricted optical fields（e.g.,sub-5 nm）is challenging.Recently,nanoparticle-on-film（NPo F）plasmonic nanocavities,formed by placing a metal nanoparticle on a metal film separated with a nanometer-thick dielectric layer,have triggered state-of-the-art progress in nanophotonics due to their capability of extreme optical confinement and enhanced capabilities.However,the optical quality factor and thus performance of these nanoconstructs are undermined by the granular polycrystalline metal films used as a mirror.Therefore,in this thesis,we design a low-loss mirror nanocavity structure composed of single-crystal gold microplate and gold nanorods（NPo M）.Crystal violet dye and single layer WS₂ are used as research objects.The fluorescence characteristics of two kinds of luminescent materials in nanocavity are discussed and compared.In addition,the field distribution of NPo M structure is simulated theoretically based on FDTD（Finite difference time domain）.The main findings are as follows:（1）A hybrid system of confocal microscopy and atomic force microscope（AFM）was used to scan the surface morphology of the nanocavity structure.At the same time,the fluorescence characteristics of crystal violet dye molecules were tailored by changing the thickness of spacer layer under 532 nm continuous wave laser.The results show that when the gap size of the NPo M cavity is 13 nm,the fluorescence enhancement effect dominated the competition with the quenching effect,and the fluorescence enhancement of crystal violet molecules up to 143 fold was achieved.The fluorescence lifetime was shortened by 5.4 fold.Under the same conditions,the corresponding fluorescence enhancement factor in the NPo F nanocavity based on gold film was only 23.8,and the lifetime was reduced by 1.8 times.In addition,the crystal violet coupled to the nanocavity shows good linear polarization characteristic.（2）The fluorescence characteristics of excitons in the strong coupling between nanocavity and WS₂ were studied.The experimental results show that the fluorescence of single layer WS₂in the NPo M nanocavity based on the single crystal microplate was significantly enhanced by204 times,and the fluorescence saturation power in the nanocavity was only 2%of the free space through power-dependent measurement.However,under the same conditions,the fluorescence of single layer WS₂ in the NPo F nanocavity based on polycrystalline gold film was only enhanced by 37 times.In addition,two obvious peaks in the photoluminescence spectrum of WS₂ was observed.That is,under the strong interaction,the neutral exciton captures the photoionization carrier and converts it into a charged trion,and the trion-dominated fluorescence emission is realized at room temperature.（3）Electromagnetic induced transparency（EIT）phenomenon caused by mode coupling in the nanocavity structure of silicon plasmons is studied.The influence of gap distance between bright mode resonator and dark-mode resonator on EIT transmission spectrum and mode field enhancement inπ-type all-dielectric silicon metamaterials are theoretically simulated.The results show that with a gap width of 20 nm,a characteristic EIT peak with a Q-factor of 2436and a transmittance of 99%is obtained.At the same time,the strongest electric field enhancement up to 2×10⁴ inside the nanogaps is observed at the EIT peak（974.4 nm）.In conclusion,the influence of NPo M nanocavity on the fluorescence dynamics of quantum emitters under weak and strong coupling is preliminarily explored,and the induction of strong trions emission by a low power at room temperature is realized.These results not only provide ideas for the realization of highly sensitive fluorescence probes,but also have important significance for the application of charged trions in photoelectric devices.