Influence of angular momentum of light on excitation of near field hot spots

Realization of plasmonic nanostructure which show hot spots with a strong enhancement of the local near field has been a strong field of research in the last decades, for either fundamental interest as well as for applications... [ view full abstract ]

Realization of plasmonic nanostructure which show hot spots with a strong enhancement of the local near field has been a strong field of research in the last decades, for either fundamental interest as well as for applications such as Surface Enhance Raman Scattering (SERS). To this purpose, we have used FDTD simulation methods to analyze the field profiles in different concentric structures, upon excitation with radially polarized light sources having different orders of Orbital Angular Momentum (OAM), and compared with the response to normal linearly polarized and circularly polarized light. Among the structure used there are mainly circular arrangement of radially distributed plasmonic antennas and concentric tapered plasmonic structures. Our interest has been mainly devoted to the structure and intensity of the near field hot spots in the small metallic gaps in the center of the different structures. Our simulations show a strong dependence on the OAM order of the field profile at the gaps in the center of the structure. In particular, due to the phase mismatch in the oscillation of the different plasmonic components in the structure, the near field hot spots in the structure shift, with a strong intensity dependence on the OAM of excitation source. Comparison with other type of excitation are also interesting. In particular, excitation with circularly polarized light lead to results which are much closer to those obtained with OAM>0, a result which is in accordance with literature results that support the evidence of a mixing of spin and orbital angular momentum of light in plasmonic structures. These results can lead to an important optimization of the intensity of the hot spots used in SERS by exploiting the angular momentum of the excitation source as an alternative degree of freedom to control the excitation of nanostructured samples, and they might have application in optimization of specific high-sensitivity detection of bio-molecules on nanopatterned samples. Further work is currently ongoing in our lab to simulate other structures as well as to experimentally realize and test for Raman and SERS applications some of the devices we have simulated.