Leaky Mode Resonance Of Polyimide Waveguide Couples Metal Plasmon Resonance For SERS

The essence of surface plasmon is the electronic collective oscillation mode produced by the free electrons on the surface of the material(usually metal) under the excitation of the external electromagnetic field, which is the result generated by the resonance of the free electrons oscillation and the electromagnetic field of the incident light. Since being found, it has attracted the attention of many researchers because of its special optical properties. With the development of modern science and technology, especially the improvement and innovation of nanotechnology, surface plasmon has become the hotspot in scientific research. Its mechanism, effect, characteristic and development application are intensively studied by researchers, so it has been widely used in many scientific fields. The surface plasmon technology being applied in surface enhanced Raman spectroscopy is one of the most important parts in these scientific fields, which is mainly because the resonance effect of surface plasmon could improve the detection sensitivity of surface enhanced Raman spectrum. In this thesis, we have designed and fabricated a kind of SERS substrates according to the characteristics of the surface plasma combining with several characteristics of optical waveguide, then detected its SERS spectrum and characterized its properties. The main features of the design are as follows: first, it could enhance electromagnetic field basing on the characteristics of surface plasmons resonance; second, it could enhance electromagnetic field further basing on the characteristics of optical waveguide; third, it could excite the leaky resonance mode to make the strongest electromagnetic field enhancement distribution in the waveguide layer / air interface based on the properties of optical waveguide structure; last, it could couple the local plasmons resonance of the Ag particles absorbed on the surface of the waveguide layer with the leaky mode resonance to achieve the stronggest enhancement of lectromagnetic field.The detailed research content of this thesis include the following two aspects:1. Fabrication and characterization of optical waveguide substrate. First, a layer of Ag film was deposited on clean glass by vacuum evaporation as the cover layer(or gap layer) in optical waveguide structures; second, liquid polyimide material was coated onto the Ag modified slide by a spincoater; third, cured the polyimide film at high temperature, which is used as the waveguide layer in optical waveguide structures. In the process of preparation, we adjusted several parameters of the experiment in order to achieve the purpose of regulating the thickness of waveguide layer. In addition, an ellipsometry and a bench were used to characterize the thickness and the refractive index of waveguide layer in this process, an atomic force microscope and a UV- visible- near infrared spectroscopy were used to characterized the roughness and the transmittance of waveguide layer, which proves that Polyimide film has outstanding advantages in physical strength, thermal stability and optical transmittance, so it is an ideal material for the fabrication of optical waveguides. Finally, the electromagnetic field in the waveguide layer / air interface of the optical waveguide structure is simulated by FDTD, through the analysis and comparison of the electromagnetic field in different thickness of the waveguide layer, we chose the polyimide optical waveguide substrate with the thickness of 300 nm as the best one.2. Fabrication and characterization of SERS substrate. An Ag colloid with a nanoparticle size of～50 nm was assembled onto the surface of polyimide waveguide layer by self-assembly method. Then we combined it with a semi cylindrical prism to form a kind of structure of Kretschmann prism coupling. As a result, a more than 450 times enhancement local electromagnetic field around the silver nanoparticles was produced in the effect of leaky mode resonance of polyimide waveguide coupling metal plasmon resonance. After that, a transmission electron microscopy and a scanning electron microscopy were used to characterize the morphology of substrate. Finally, we detected the SERS signal by a self-developed microspectrometer in order to characterize the properties of the SERS substrate in enhancing ability, the lowest detection capability and basal repeatability, and we used FDTD simulation to verify several parts of the experimental results during the process. This study is a proof of the concept on the SERS excitation light paths and coupling elements for integration, this configuration can be extended to a function system that integrates the SERS analysis with all the SERS substrate, excitation and collection light paths.