Observation of Magnetic Quadrupole Endowed Helical Dichroism in Artificial Propeller Meta-Molecules

This study demonstrates that the magnetic quadrupole (MQ) can make an indispensable contribution to helical dichroism (HD), which is distinct from HD caused by electric quadrupole (EQ). Specifically, the established multipole expansion theory of the light–matter interaction reveals that EQ-induced HD belongs to the pure electric dipole–quadrupole (E1-E2) excitation, while newly discovered MQ-induced HD belongs to the pure magnetic dipole-quadrupole (M1-M2) excitation. Metallic propeller meta-molecules are used to elaborate on this novel physical mechanism and demonstrate that HD can be significantly enhanced and flexibly tuned by increasing stereo twist of propeller meta-molecules or slightly focusing the linearly polarized vortex beam to improve the orbital angular momentum number. Importantly, the aforementioned intriguing phenomena are validated by fabricating stereo twisted propeller meta-molecules via nano-kirigami method. The findings reveal a paradigm-shift approach to manipulate the chiroptical response of a single metallic nanostructure via photonic orbital angular moment, opening prospects for optical encryption, communication, next-generation chiroptical spectroscopy, and chiral vortex optical devices.