Impact of local-field effects on the plasmonic enhancement of vibrational signals by infrared nanoantennas.

We have developed an analytical model that provides a mechanistic description of the plasmonic enhancement of vibrational signals by infrared nanoantennas. Our treatment is based on a coupled-point-dipole model which considers the interaction between a point-like nanoantenna and a single vibrational dipole moment. This idealized model is refined in two consecutive steps. The first step generalizes the model to make the treatment of non-point-like nanoantennas possible. The second step deals with local-field effects originating from the mutual interaction of the molecular vibrations. We have compared the results of our model with finite-difference time-domain simulations, and we find that our model predicts both the lineshapes and the amplitudes of the vibrational signals in a quantitative manner. Our analysis shows that the local-field effects play a surprisingly dominant role in the plasmonic enhancement, and we discuss possibilities of engineering this local field in order to further boost the plasmonic amplification.