Surface Enhanced Raman Scattering from an Ag Nanorod Array Substrate: The Site Dependent Enhancement and Layer Absorbance Effect
We have designed experiments to put Raman probe molecules on different locations of Ag nanorod array substrates and found that the apparent enhancement factor for surface enhanced Raman scattering (SERS) is about 50 200 times from the surface of the nanorods than that from the Ag thin film surface under an Ag nanorods array. When excited from the top of Ag nanorod array substrates, the ratio of SERS intensity excited by s polarization to that excited by p polarization is 1.8 to 2.2, while when excited from the bottom of Ag nanorod array substrates, this ratio becomes 0.77. These effects cannot be interpreted directly by the local electric field enhancement effect. By combining the local electric field effect calculated by the three dimensional finite difference time domain method and the anisotropic optical absorbance of the SERS signal from the anisotropic Ag nanorod array, the numerical calculations semiquantitatively agree with the experimental results.