Helical dichroism for hybridized quadrupole plasmon modes in twisted nanorods

Abstract: Helical dichroism (HD), originating from the interplay between chiral plasmonic structures and left and right vortex light carrying orbital angular momentum (OAM), has attracted significant attention across various disciplines owing to its implications in fundamental physics and applications. However, the precise relationship between HD and the excited plasmon modes remains elusive. Owing to the weak chiroptical response to OAM light, chiral structures have required dimensions larger than the incident light wavelength to obtain observable HD signals, resulting in complex superpositions of higher-order plasmon modes. In this work, we reveal that a simple twisted nanorod dimer with a size smaller than the incident light wavelength, exhibits remarkable HD due to the strong coupling between quadrupole plasmon modes excited in the nanorods, followed by the plasmon hybridization. Positive and negative HD responses were measured at different resonance wavelengths corresponding to two hybridized quadrupole modes, in good agreement with the calculated results. This spectral behavior of the HD is clearly different from that of the circular dichroism (CD) based on spin angular momentum (SAM) of light, indicating that the quadrupole HD arises from the OAM rather than the SAM. These findings pave the way for a deeper understanding of light-matter interactions concerning angular momentum.