The Study Of Third-Order Optical Nonlinearities In Noble Metal Nanoparticle Arrays

In the 21st century, in the development of information society, the widely application of the fibre communication fits the requirement of a high speed transmission and high signal noise ratio. However, the optical-electronic conversation devices in the fibre communication system lead to a slow response time and a high noise, which significantly limits the development of the optical communications. Thus, people try to control and process the optical signal by light directly. Presently, it often uses an intensive laser to induce the third-optical nonlinearities in the medium of the signal, resulting in the changing of the refraction, absorption or the polarization properties of the medium to realize the all-optical switching. Such demands of the all-optical devices have driven people to search for a nonlinear material with a high nonlinear refraction index but a low nonlinear absorption. Furthermore, in the development of micromation and integration of the modern optical devices, kinds of the light resource in the micro-nano meter scales have drawn a lot of attentions. Especially, the optical pumped nanowire laser based on the optical nonlinearity of the materials demands for a high nonlinear absorption coefficient. Therefore, the study of the fabrication and the optical nonlinearity in nanostructures is one of the most interesting areas in studies of optical materials.The measurement of the third-order optical nonlinearity in optical materials is an important method in the investigation of the optical nonlinearity. It is not only able to estimate the value and style of the nonlinear materials but also indicate the original mechanisms of the nonlinearity. It provides important experimental supports in order to find the nonlinear materials, meeting the requirement of new micro-nano optical devices with a high performance. In this dissertation, systematical investigations on the optical nonlinearity of the noble metals and ZnO in micro-nano scales are presented:(1) Combined pulsed laser deposition technique with nanosphere lithography method, a serial of noble metals (Au and Ag) nanoparticle arrays with different size have been fabricated on quartz substrates. From scanning electron microscopy images, the well closed-packed nanosphere mask and periodical array of noble metals nanoprisms are observed. Besides, ZnO nanowires are synthesized by a high temperature thermal evaporation process, which has a hexagonal cross section in scanning electron microscopy images. And then, an individual ZnO nanowire is transferred to a quartz substrate for optical measurements.(2) By measuring UV-visible absorption spectroscopy, the strong absorption band in the visible range caused by surface plasmon resonance of Ag and Au particles can be observed. The intensity-dependent optical nonlinearities in an Au nanoparticle array with a size of 60 nm are investigated. The results exhibit a sign reversal process in the nonlinear absorption and the nonlinear refraction properties as the intensity increases.(3) The size effects on the optical nonlinearities of the Au nanoparticles are also investigated in a serial of Au nanoparticle arrays with four sizes (37 nm,70 nm,140 nm and 190 nm). It shows a reversal process from the two-photon absorption to the saturated absorption as the size of the nanoparticles increase. The competition of these two different mechanisms is also observed.(4) The optical nonlinearities of an Ag nanoparticle array are investigated using the selected wavelengths (400 nm,600 nm,650 nm and 800 nm):as the intensities increase, the nonlinear absorption change over from the saturated absorption to the reversed saturated absorption process in the resonant region (400 nm); In the off-resonant region (800 nm), it is observed that optical nonlinearities change over from the two-photon absorption to the saturated absorption as the intensity increases; In the edge of the absorption band (600 nm and 650 nm), low nonlinear absorptions are observed even under high intensities. Furthermore, the nonlinear refraction index in the resonant region exhibits a large enhancement of the 2 orders larger than that in the off-resonant region.(5) The anisotropy of the third-order optical nonlinearity of an individual ZnO nanowire is studied systematically. The two-photon absorption coefficient and the nonlinear refraction index exhibit oscillations with a period of n/2, the influence of the different incident crystal planes is also presented. The results indicate a highly polarized optical nonlinearity of the ZnO nanowire that a high nonlinear absorption property can be obtained under excitation laser with a proper polarization property.