The Depolarisation Behaviour of Surface-Enhanced Raman Scattering Photons in a Metal Nanodimer Structure

The metal–molecule interface provides highly attractive fields that interact with each other, such as in charge transfer. However, little is known about the materials localised at the interface. Polarised Raman measurement represents an attractive research field related to metal surfaces. The polarised SERS signal has a highly polarised character because of the anisotropic EM feature on the metal surface. Moreover, such signals are considered to have high sensitivity to the electronic state at the interface. Polarised SERS measurements give information about both the incident and Raman scattering polarisations, and several interesting polarised SERS studies have been carried out [1–13]. If one assumes a symmetrical single nanodimer structure in which only the localised EM field at the gap contributes to the enhancement, SERS depolarisation is not observed because the polarisations for excitation and scattering are the same. Linearly polarised scattered light was observed in a metal nanodimer system by Haran et al., who reported that an asymmetric metal nanoparticle trimer generates elliptically polarised scattered light; thus, these researchers showed that the polarisation of light scattered from molecules can be manipulated on the nanometre scale [7]. A well-accepted model of the twofold EM enhancement theory for excitation and scattering for the SERS process cannot explain the present observations, because the theory predicts that the polarisation of the scattering photons is controlled in the same manner as that of the localised field [3, 14]. In contrast, depolarisation behaviour has been also reported. Polarisation rotation induced by the plasmon resonance of the SERS signal with respect to the incident polarisation was shown in arrays of uniform Au nanostructures [11]. By using an elliptic nanostructure that exhibits two LSP resonances each associated with one of the two principle axes, it was found that scattered light is resonant with another axis to one axis of the excitation process.