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

Abstract

Gold film (40-nm-thick) sputtered on the glass substrate was decorated by using the sol-gel copolymer templated mesoporous TiO2 thin film (275-nm-thick) to fabricate the plasmon waveguide (PW). The Raman spectroscopy based on the Au/TiO2 PW is studied theoretically and experimentally. The surface morphology of the mesoprous TiO2 thin film and the cross-section of the PW chip are obtained by scanning electron microscopy (SEM) and the porosity (P) of mesoporous TiO2 thin film is determined to be about 0.589 by fitting the calculated waveguide coupling dips to the measured resonance wavelengths based on Fresnel equations. The angular distributions of Raman power from the molecular dipole located in the core layer of the waveguide are theoretically investigated based on the optical reciprocity theorem. The calculated results suggest that the Raman light radiated into the substrate consists of the directional Raman signal propagating at the resonant angle and the non-directional Raman signal whose radiation angles are smaller than the critical angle of total reflection. The directional Raman signal could be detected with the aid of the prism coupler, while the non-directional Raman signal can be detected directly on the back of the sensor chip. Furthermore, the angular distribution of the backscattered Raman signal is divergent and it is unaffected by the use of the prism coupler. The highest power of the directional Raman signal is much larger than that of the non-directional Raman signal and the backscattered Raman signal. The Raman spectroscopy based on the PW is studied by experiment with CV molecules adsorbed into the mesoporous TiO2 thin film. The Raman spectrum is obtained with the 532 nm laser radiating directly onto the waveguide surface. The experimental results show that the Raman signal including the directional Raman signal, non-directional Raman signal and the backscattered Raman signal can be detected with the PW chip. Besides, the directional Raman signal can only be detected by using the prism coupler, while the non-directional Raman signal can be detected directly on the back of the chip. Then the results also show that the peak intensity of the directional Raman signal is twice higher than that of the non-directional Raman signal. The further measurements reveal that the backscattered Raman signal hardly changes under the condition with or without the prism coupler. The experimental results mentioned above are in accordance with the theoretical calculations. The Raman spectroscopy based on PW in this work has potential value in further developing the Raman sensing technique.