Investigation Of Surface Plasmon Resonance-induced Enhancement Effect Of Third-order Optical Nonlinearity In Au Nanostructure

Surface plasmon has been studied extensively due to the local field enhancement and coupled efficiently energy transfer. Based on the properties of surface plasmons, metal nanostructures can be applied in the fields of surface enhanced raman scattering, ultrasensitive detection of single molecules, biological cancer therapy and nano-photonic devices. Noble metal composite nanostructures have large third-order optical nonlinearities due to the enhancement of local field and plasmon resonance absorption, which is always the key point of research due to their applications in the fields such as all-optical switches, optical information processors and optoelectronic devices, etc. While the ultrafast response property of metal nanostructures is one of the hot research topics for plasmonics. Metal surface plasmon-induced hot electronics have many important applications in photoelectric conversion, photoelectric detection, photochemical reaction and ultrafast devices. Therefore, it is a very important precondition for the application of hot electronics to understand deeply the generation of hot electrons and the physical mechanism of their relaxation processes, and then control the generation and relaxation timescale of them by manipulating nanostructural parameters and excitation conditions. In this paper, we have studied surface plamon resonance-enhanced the third-order optical nonlinearities of Au-Ni-Au composite nanorod arrays, Au nanobipyramids and Al nanofilms as well as the ultrafast response time of Au nanobipyramids, besides the set-up and theory of optical systems are introduced. Our work mainly includes:1. We have built optical Kerr effect, supercontinuum pump-probe and Z-scan measurement systems, and described the theory, and measured physical quantities as well as the intrinsic physical mechanism of each system. Then we utilized Z-scan and optical Kerr effect technique to investigate the third-order optical nonlinearity and response time of metal nanostructures.2. We fabricated ordered Au-Ni-Au composite nanorod arrays with rod diameter of18nm in anodic aluminum oxide (AAO) templates. Strong longitudinal surface plasmon resonance (LSPR) peak positioned around800nm was observed in the arrays. Z-scan technique was used to investigate the third-order optical nonlinearities of the sample. Large third-order nonlinear absorption coefficient of-2.65×106cm/GW was obtained at LSPR wavelength. The excitation wavelength and incident angle dependent nonlinear absorptions in the Au-Ni-Au composite nanorods show that strong coupling of the longitudinal plasmon between nanorods can result in a large enhancement of the third-order optical nonlinearity. Comparison between the Au-Ni-Au composite nanorods and pure Au nanorods demonstrates that further growing of an Au segment on the as-grown one separated by a Ni thin layer will lead to a large improvement of the third-order optical nonlinearities. It can be attributed to the combined effects of the nano-trapping levels induced by the nano-interfaces and the localized field enhancement of the intrarod and interred. The designing and measurement methods used here provide people a powerful way to manipulate the nonlinear optical properties of future plasmonic devices.3. The third-order optical nonlinearity and response time of Au nanobipyramids have been investigated by using optical Kerr effect technique. As the excitation laser wavelength varies from non-resonance wavelength of780nm to the LSPR wavelength of825nm, third-order nonlinear optical susceptibility (χ(3)) increases from7.4×10-14to3.9×10-13esu, The figure of merit (χ(3)/α) of Au nanobipyramids is on the order of～10-13esu-cm. The decay curve of Au nanobipyramids shows two different decay processes, a fast response time decreasing from141±23to83±8fs, and a slow one decreasing from3200±200to2310±158fs. The large enhancement of χ(3) is caused by the large local field enhancement of surface plasmon resonance, and the shortening of the response times are induced by the increased probability of the electron-electron and electron-phonon scatterings in the nanosystem. These fundamental studies give us a way to control the timescales of hot electrons. Through adjusting the surface plasmon resonance wavelength by designing and fabricating nanostructures of proper parameters, the relaxation time of the plasmon-induced hot electrons can be manipulated, making hot electrons applicable in photocatalysis, optical detection and optical trapping. This significant ultrafast optical property of Au nanobipyramids also holds great application in future ultrafast information processors.4. We presented research works of Al plasmon, and described the properties and applications of Al plasmon in detail. Based on the excellent properties of Al plasmon, we expect to achieve different structural types and sizes of Al nanofilms by adjusting the structural parameters and coating conditions, and then manipulate the surface plasmon resonance (SPR) wavelength of Al and simultaneously get a large third-order optical nonlinear response. Therefore, we fabricated two types of AAO templates by electrochemical template method, then evaporated Al on the AAO templates to form an Al nanoporous film, and finally measured the third-order optical nonlinear properties of it by Z-scan technology. The next step of work is to research the dependence of the SPR wavelength and third-order optical nonlinear properties on the AAO template structural parameters and coating conditions. The forth-coming research works will provide us a way to fabricate Al nanostructures with a simple and easy-controlled method with high feasibility for the application of Al plasmon.