Nonlinear Optical Properties And Applications Of Plasmon-exciton Hybird Nanostructures

The advantages of nanomaterials are not limited to controllable optical properties of single components but extend to the unique possibilities to combine different nano materials into composite structures.The surface plasmons of metallic nanostructures can induce large local electromagnetic fields near the metal surfaces upon resonant excitation Such enhanced local fields can interact strongly with adjacent semiconductors or organic molecules.The optical behaviors of such hybrid systems can be distinctly different from those of either of their constituents and are of substantial fundamental and practical importance in understanding light-matter interactions.In this thesis,we investigate plasmon-exciton interaction induced ultrafast energy transfer and third-order nonlinear enhancement in metal-dye and metal-semiconductor hybrid system.Firstly,we use complementary experimental approaches to study both the linear and nonlinear optical responses of strongly coupled hybrid Fano systems consisting of Au nanorod(AuNR)cores decorated with near-infrared(NIR)dye molecular shells.We show that in the weak external field regime the absorption of the hybrids at the Fano dip decreases as the concentration of the dye molecules increases.The severely suppressed linear absorption around the Fano dip significantly enhances the unidirectional energy transfer from the plasmons to the excitons and further allows one-photon nonlinearity to be drastically and reversibly tuned,providing its prominent advantages toward a low-power(<1 μJ/cm2)excitation and an ultrafast response time(<1 ps).These observations are interpreted within a microscopic model stressing on two competing processes:saturated plasmonic absorption and weakened destructive Fano interference from the bleached excitonic absorption.Besides,we investigate chlorophyll-a molecules strongly coupled to Au nano structured films via Fano resonance in solar cells.Through an efficient channel of coherent energy transfer from metallic plasmons to molecular excitons,the output power of the plexciton-sensitized solar cells(SSCs)is 62%larger than the sum of those of the individual plasmon-and exciton-SSCs.Our observations provide a practical approach to monitor energy and electron transfer in plasmon-exciton hybrids at a strong coupling regime and also offer a new strategy to design photovoltaic nanodevices.Secondly,we investigate the third-order nonlinear optical response in Au/CdS,Au/Ag2S core-shell as well as Au/AuAg/Ag2S/PbS core-mutlishell nanorod.We find that the nonlinear optical behaviors of the hybrids can be greatly tailored by the semiconductor shell with high refraction index.Compared with the initial gold nanorods,the strong plasmon-exciton interaction and great local field enhancement in Au/CdS core-shell nanorods induce a nonlinear absorption responses transformed from saturable absorption to reverse saturable absorption,and effective nonlinear refraction index is increased by 45%.The FDTD calculations reveal that the plasmon resonance and local field confinements could be varied with the shell morphology in three different kinds of Au/Ag2S core-shell nanorods,leading the nonlinear refraction and saturable intensity of the Au NRs increasing 750%and 424%in corner-opened and end-opened core-shell nanostructures.The Au/AuAg/Ag2S/PbS core-multishell nanorod possesses an air gap between the Au core and the AuAg shell.Together with the Ag2S shell,the air gap can efficiently trap light,causing strong field confinement and nonlinear enhancement.The one-photon figure of merit(FOM)of the core-mutishell nanostructure is enhanced by 2.4 times,while the two photon FOM is decreased by 89%,making it a suitable candidate for all optical switching.At last,we investigate the exciton emission and transient absorption properties in size-dependent WSe2 monolayers through photo luminescence and pump-probe microscopy system.We find that,the monolayer exhibits a bright concentric fluorescence triangle in the PL mapping.What’s more,as the size of monolayer WSe2 increase,the exciton emission intensity is increased,at the same time,the saturable absorption of WSe2 is enhanced and there exists a much longer exciton recombination process at a larger size.These novel optical properties are attributed to chemical heterogeneity in the growth of WSe2 monolayer.