Multi-order surface lattice resonances and dark mode activation in metallic nanoantenna arrays

Abstract

Surface lattice resonances (SLRs) in arrays of metallic nanoantennas are formed via hybridization of their localized surface plasmon resonances with the Rayleigh Anomaly. In this paper, we study the impact of inter-nanoantenna plasmonic coupling on such resonances, demonstrating a significant departure from the cases where the nanoantennas are only coupled to the diffraction modes of the arrays. For this, we study SLRs in a series of metallic nanoantenna arrays wherein their inter-nanoantenna spacings (lattice constants) along their short axes are varied between the limits where transverse modes of the nanoantennas are efficiently coupled to each other to the case where they are well separated. Our results show that for the latter case, when the incident light is polarized along the short axes of the nanoantennas, SLRs are formed via first order parallel coupling. As we reach the limit of inter-nanoantenna plasmonic coupling, however, the nature of SLRs is changed, becoming a second order orthogonal coupling. Our results also show that in the presence of inter-nanoantenna plasmonic coupling, the forbidden quadrupole state of nanoantennas can strongly couple to light, becoming the dominant optical feature of the arrays. For light polarized along the long axes of the nanoantennas, we demonstrate the Rayleigh Anomaly splitting of first order infrared and second order near-infrared SLRs under oblique incident angles. Wavelength multiplex optical filter application of such diffraction orders is discussed.