Raman spectroscopy based on plasmon waveguide prepared with mesoporous TiO2 thin film

Plasmon waveguides (PW) were prepared by coating sol-gel copolymer templated mesoporous Ti₂ film on gold layer sputtered on glass substrate. The cross-sectional imaging with a scanning electron microscope indicates that the gold and mesoporous Ti₂ 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 Ti₂ 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 Ti₂ 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 Ti₂ 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.