Near-field asymmetries in plasmonic resonators.

Surface-enhanced infrared absorption (SEIRA) spectroscopy exploits the locally enhanced field surrounding plasmonic metamaterials to increase the sensitivity of infrared spectroscopy. The light polarization and incidence angle are important factors for exciting plasmonic nanostructures; however, such angle dependence is often ignored in SEIRA experiments, typically carried out with Cassegrain objectives. Here, the photothermal induced resonance technique and numerical simulations are used to map the distribution and intensity of SEIRA hot-spots surrounding gold asymmetric split ring resonators (ASRRs) as a function of light polarization and incidence angle. The results show asymmetric near-field SEIRA enhancements as a function of the incident illumination direction which, in analogy with the symmetry-breaking occurring in asymmetric transmission, we refer to as symmetry-breaking absorption. Numerical calculations reveal that the symmetry-breaking absorption in ASRRs originates in the angle-dependent interference between the electric and magnetic excitation channels of the resonators' dark-mode. Consequently, to maximize the SEIRA intensity, ASRRs should be illuminated from the dielectric side at an angle that maximizes the constructive interference of the two excitation channels, (35° for the structures studied here), in place of the Cassegrain objectives. These results can be generalized to all structures characterized by plasmonic excitations that give rise to a surface-normal magnetic moment and that possess an electric dipole.