Broadband nonreciprocal quadrupolarization-induced asymmetric transmission (Q-AT) in plasmonic nanoparticle aggregates.

Non-reciprocal asymmetric transmission, i.e., the dependence of optical transmittance on the direction of light propagation in the material, can be used in optical isolators or photonic circuits. Broadband asymmetric transmission is observed in near-field coupled aggregates of small plasmonic nanoparticles, even for unpolarized light. Non-reciprocity is demonstrated and, using a phenomenological model, induced electric quadrupole moments are identified as the root cause of the effect.