The Surface Plasmon Resonance And Enhanced Nonlinear Optical Properties In Gold Nanorod Arrays

Metal surface plasmonic is one of the important branches of nonlinear optics.Its research objects mainly include the preparation and characterization of metal nanostructures and their applications in nonlinear optics,ultrafast optics and enhancement media.Compared with the single metal nanostructure,the metal nanostructure array is one of the focuses of optical research in recent years because of its advantages of large area preparation,functional module design,and multi-segment coupling enhancement.Recent research results show that when the metal nanostructures are arranged in array structures,they can show new nonlinear effects such as nonlinear photoluminescence(PL),multi-mode resonance coupling enhancement and so on,which are difficult to be possessed by the monomer structure.In order to study the internal relationship and physical mechanism between these array structures and nonlinear optical effects,this dissertation will focus on the internal relationship between the gold nanorod array and all kinds of nonlinear optical effects(second harmonic generation(SHG),third-order optical nonlinearity,ultrafast response,PL and so on)it displays.The main research contents include the following aspects:1.The PL characteristics of Au-Al₂O₃-Al and Au-Al₂O₃nanorod array samples were studied and compared.Supercontinuum generation was observed in Au-Al₂O₃-Al sample.The alumina template was prepared by a two-step anodization technique.On this basis,the Au-Al₂O₃-Al array sample was fabricated by electrochemical deposition method.Two different shapes of the PL spectra were observed in Au-Al₂O₃-Al and Au-Al₂O₃samples with the oblique incidence PL measurement setup.The PL characteristics were studied and compared through PL lifetime measurement,varying incident laser intensity and polarization.It was confirmed that the broadband(450-1050 nm)PL of Au-Al₂O₃-Al sample originates from the localized surface plasmon coupling enhanced supercontinuum(SC)in the Al₂O₃ gap layer.The sub 10 nm gap layer improves the thermal damage threshold.Optimization of the Au nanorod geometries would leads to a further SC enhancement which was simulated in FDTD and realized in the experiment.FDTD calculations also revealed that the SC could be further enhanced by reducing the Al₂O₃ gap size.These observations are very important for obtaining SC light source on nanoscale and make the sample have potential applications in the minimization and integration of light sources in photonic devices.2.The nonlinear optical properties of Au,Ag and Au-Ag-Au hybrid array structures were studied and compared,and the optimal nonlinear quality factor was obtained in the Au-Ag-Au sample.The surface roughness of the array sample was improved through high temperature annealing.The interface effect and multimode matching conditions were used to fabricate a Au–Ag–Au nanorod hybrid array structure together with the corresponding Au and Ag hybrid structures.The surface plasmon enhanced SHG,SC,thermal PL and the third-order nonlinear optical properties of the samples were systematically studied and compared by using oblique incidence SHG/PL and polarization dependent Z-scan measurements.It was found that the Au–Ag–Au hybrid sample had excellent and comprehensive nonlinear optical properties.The figure of merit and thermal PL emission are more than several times stronger than single component samples.The SHG is comparable to the Ag sample but much stronger than the Au sample,while the SC is slightly weaker than the Au sample but much stronger than the Ag sample.FDTD calculated the plasmon characteristics and the origin of SHG.The lifetime of SC is 1.0-1.7 picoseconds which was measured with the monochromatic pump probe PL spectrum system.These results provide guidance for the design of composite structures to enhance optical nonlinearity.3.The nonlinear optical properties of random-distributed gold nanorod arrays(R-GNRA)deposited on the glass were studied.Large polarization independent third-order optical nonlinearity was obtained under low density power excitation.The plasmon resonance peaks of the R-GNRA deposited on the glass have slightly blue-shift compared to that of gold nanorods in the water,which is the result of the change of dielectric environment.The surface plasmon peak around 780 nm appears in R-GNRA is formed by the aggregation and coupling of Au nanorods.The former results were confirmed by FDTD calculation.Through Z-scan measurement,it was found that the imaginary and real part of third-order nonlinearity near the new surface plasmon resonance peak have a crossing zero behavior.The large third-order nonlinearity is originated from the surface plasmon resonance and high density,well dispersion and flat-spread layout characteristics of nanorods.At the same time,well dispersion characteristic leads to the third-order nonlinearity and SHG are independent of the excitation polarization.The dichromatic pump-probe setup measured that the electron-phonon coupling time of Au nanorods is 2-4picoseconds.These nonlinear optical properties make R-GNRA have potential applications in low-power,high-integration-density and large-sized nonlinear nanophotonic devices.