基于介孔TiO_2薄膜等离子体波导的拉曼光谱技术研究

Raman spectroscopy based on plasmon waveguide prepared with mesoporous TiO_2 thin film

利用溶胶-凝胶分子模板法在表面覆金的玻璃基底上制备275 nm厚的介孔二氧化钛(TiO_2)薄膜,形成等离子体波导(PW)传感芯片.利用菲涅耳公式拟合实验测得的导模共振波长,得出TiO_2薄膜的多孔度约为0.589.基于光学互易定理仿真分析了置于介孔导波层中的电偶极子的拉曼辐射角分布,结果表明:辐射到衬底中的拉曼光包含沿导模共振角辐射的定向信号和辐射角小于全反射角的非定向信号;前者需借助棱镜耦合器才能被探测到,后者可从芯片背面直接被探测到;从导波层直接辐射到空气中的拉曼光称为背向信号,它的角分布呈发散式,几乎不受棱镜耦合器影响;定向信号的最大功率值远大于非定向信号和背向信号的相应值.实验研究了吸附于介孔导波层中的结晶紫分子的拉曼光谱,采用体光束激发方式探测到了定向、非定向和背向拉曼信号,定向信号强度的最大值是非定向信号的2倍多,在使用棱镜耦合器前后测得的背向信号强度几乎不变.

Plasmon waveguides（PW） were prepared by coating sol-gel copolymer templated mesoporous TiO_2 film on gold layer sputtered on glass substrate. The cross-sectional imaging with a scanning electron microscope indicates that the gold and mesoporous TiO_2 films are 40 nm and 275 nm thick, respectively. The PW resonance wavelengths at different incident angles were measured with the Kretschmann configuration and also calculated based on Fresnel equations. The porosity of TiO_2 film was determined to be 0.589 by fitting the calculated values to the corresponding experimental data.For optimizing Raman spectroscopy application of the PW, angular distribution of the power density from molecular dipole located within the mesoporous TiO_2 film was calculated based on optical reciprocity theorem. The results show that Raman light emitting into the substrate includes the directional signal propagating at the guided-mode-resonance angle and the non-directional signal with the emission angles being smaller than the critical angle at the gold/glass interface. The non-directional signal can be directly detected from the backside of the substrate while a prism coupler is needed to detect the directional Raman signal. The backscattered Raman signal is divergent and is almost unaffected by the prism coupler. The highest power of the directional Raman signal is much larger than those of the non-directional and the backscattered Raman signals. The PW-based Raman spectroscopy for crystal violet molecules adsorbed in the mesoporous TiO_2 film was experimentally studied using a 532-nm laser beam at normal incidence. As theoretically expected, the directional and non-directional and backscattered Raman signals were detected, and the peak intensity of the directional Raman signal is 2 times higher than that of the non-directional Raman signal, and the backscattered Raman signal detected in the presence of the prism coupler is as same as that in the absence of the prism coupler. The work is believed to be helpful for understanding and design of PW-based surface enhanced Raman scattering substrates.