Studies On The Interaction Between Fluorescent Molecules And Localized Surface Plasmon At The Nanoscale

The interaction of matter with electromagnetic fields represents one of the most fundamental phenomena in physics.The exploration of electroluminescence at the nanoscale will not only help to develop electrically driven nano emitters,but also shed light on future nanoscale optoelectronic integration.One of recent hot topics in this field concerns about the interaction of quantum emitters with metallic plasmonic nanostructures.Collective electron oscillations in subwavelength metallic structures can give rise to localized surface plasmon resonances(LSP)with subwavelength confinement,which could enable strong interactions with other photonic elements nearby such as quantum emitters.The concept of weak or strong coupling regime has been introduced to describe to what extent the strength of the interaction is involved.For a weak coupling regime,the spontaneous emission rate of the emitter could be modified while the energy level and emission frequency remain unaltered.Yet if the coupling is strong enough,then the energy levels responsible for.the emission are also altered,producing new hybridized states and showing anti-crossing behaviors.This situation is known as the strong coupling regime and has received ever-growing interests in the last decade.Most of the studies to date focus on the interaction of multi-molecular systems with artificially fabricated metallic nanostructures,mainly on the macroscopic scale.On the other hand,the highly localized tunneling current in a scanning tunneling microscope(STM)can be employed as a source of low-energy electrons to locally excite photon emission from metals,semiconductors,and organic molecules.In recent years,STM-induced luminescence(STML)from fluorescent molecules supported on metals has gained much attention for its unique capability of combining the topographical,electronic,and optical information together with ultrahigh spatial resolution.In this molecular electroluminescence process,the localized frequency-matching plasmon stimulated by inelastic electron tunneling(IET)from metal surfaces has been found critical for the amplification of the spontaneous emission rate and thus enabling far-field detection.Such resonant enhancement still pertain to the weak coupling regime.However,many open questions still remain to be addressed,e.g.,can the interaction between the fluorescent molecule and the nanocavity plasmon(NCP)evolve into the strong coupling regime?Can STM help to provide more insights into this phenomenon by making use of its good control in both tunneling current and positioning?In this dissertation,we aim to carry out an in-depth study on the interaction of molecular excitons with plasmonic nanostructures by taking advantage of the STML technique.We start from the weak coupling between the NCP and the tunneling current excited excitons in porphyrin J-aggregates,and then proceed to the Fano resonance between single ZnPc molecules and NCP.We also present experimental studies on the modulation of the nanocavity plasmonic mode by introducing a two-dimensional plasmonic photonic crystal(PPC).Finally,we try to extend our study on the plasmon-exciton interaction beyond the STM configuration,through nanofabrication of a metallic plasmonic nanogap in a lateral configuration,aiming for the generation of electrically driven molecular light sources.The dissertation is composed of the following five chapters.Chapter one contains two parts.The first part addresses the physical concept and generation of surface plasmon,in particular its interactions with quantum emitters,which provides the theoretical foundation for the analysis in the following chapters.The second part presents a brief overview of the status of STML studies on metals,semiconductors and dye molecules.The chapter is ended with a brief introduction on the laboratory instruments related with the STML experiments in this work.In chapter two,we present the first observation of electroluminescence from self-decoupled individual porphyrin J-aggregates by STML,accompanied with the high-resolution STM images that offer direct evidence for the highly ordered porphyrin arrangement.The J-aggregate emission is found to originate from the delocalized excitons formed through the strong intermolecular couplings.Meanwhile a mechanism on J-aggregate light excitation is proposed based on the unipolar electroluminescence observed experimentally.The NCP enhanced STML from the molecule at low tunneling currents suggests a dominant weak plasmon-exciton interaction in the system.Nevertheless,the possibility and experimental limitations to observe a strong coupling between the J-aggregates and NCP are briefly discussed.In chapter three,we demonstrate the first observation of the Fano resonance between a single ZnPc molecule and the tip induced NCP.A reduced strength of the coupling with the decreasing plasmon intensity is confirmed.The double peaked emission spectra as a function of NCP modes exhibit an anti-crossing distribution curve of the hybrid states,with a splitting energy of several tens of meV.These observations suggest a much stronger coupling regime and the formation of a bistable system.Our results offer the feasibility to control light-matter interaction at the single-molecule scale and open wide horizons for new designs and applications in the area of quantum plasmonics.In chapter four,we investigate the influence of the resonant cavity of a two-dimensional plasmonic photonic crystal(PPC)on the STM induced NCP mode.We try to realize a super enhancement of the nanocavity field through the modulation of the plasmon resonance mode by combining the tip induced NCP with the PPC optical cavity.The chapter mainly presents the fabrication process of the well-defined PPC samples covered with suspended graphene as a spacer.The results lay the foundation for the future study of the interaction between NCP and quantum emitters in the cavity and the quality improvement of the PPC plasmonic nanolasers.In chapter five,a pair electrode constructing a plasmonic nanogap is designed and fabricated as a lateral analogue to the vertical STML setup,aiming for the realization of chip-applicable electrically driven molecular light sources.Different methods are applied to fabricate the nanogap at single-molecular scale and a preliminary exploration of the plasmon enhanced fluorescence is made.