Plasmonics of Au nano-objects by photoemission electron microscopy - A polarization study

Localised surface plasmon resonances (LSPRs) are coherent and collective oscillations of the delocalised electrons in metallic nanoparticles (NPs) under light excitation. These resonances depend on the environment, NP and... [ view full abstract ]

Localised surface plasmon resonances (LSPRs) are coherent and collective oscillations of the delocalised electrons in metallic nanoparticles (NPs) under light excitation. These resonances depend on the environment, NP and illumination parameters. Here we investigate experimentally and theoretically the light polarisation effect on the low order LSPRs of Au nano- cubes and prisms.

The plasmonic response of Au nano-objects of subwavelength sizes are investigated by photoemission electron microscopy (PEEM). The LSPRs are excited by a femtosecond pulsed laser source operating in the visible and near IR wavelength ranges. The polarisation is adjusted with a half-wave plate. The experimental results are predicted and interpreted within a group theory approach. Further theoretical support is obtained by boundary element method (BEM) numerical simulations.

To illustrate our investigation, figures (a) and (b) display the experimental and numerical dipolar signatures of a subwavelength sized nanocube under grazing incidence in p polarisation. These results show a good agreement between experiment and group theory. Taking into account the object symmetry, the near-field distribution can be interpreted as a combination of two dipolar modes, one excited by an electric field component normal to the cube upper face and a second one excited by the in-plane electric field component parallel to the same face (figure (c)). Any change in the light polarisation modifies the near-field optic distribution, so we can selectively address NP plasmon modes by changing the polarisation of the incident light.

To sum up, the plasmonic response of nano-objects depends strongly to the polarisation of the incident light and the object symmetry. Near field distribution of NPs of subwavelength size can be predicted within a group theory approach. This approach can be generalized to other objects of finite dimensions (prisms, hexagons…).