Polarization-independent helical dichroism via surface lattice resonance of plasmonic helices

In the context of orbital angular momentum (OAM) associated optical chirality, the involvement of photonic spin angular momentum and higher-order transition moments has long been regarded as essential. To date, the existence of helical dichroism that is exclusively determined by OAM remains elusive. Through the excitation of plasmonic surface lattice resonances (SLRs), we report a polarization-independent helical dichroism without the engagement of electric quadrupole moments, a scenario previously deemed implausible. Using a modified dipole sum under rotational symmetry, the theoretical framework of SLRs is successfully generalized to plasmonic helices. Our results reveal that the arbitrary plasmonic helix can be structurally optimized to exhibit both narrowband SLR peaks and polarization-independent helical dichroism. Notably, SLRs excitation can simultaneously generate tightly confined near-fields and collimated far-field radiation, which facilitate the OAM transfer in the chiral interaction and directional OAM projection, respectively. This work may clarify the intrinsic physics governing OAM-mediated chirality, potentially enabling advances in enantioselective detection and vortex-enhanced spectroscopy.