Low-loss optical magnetic metamaterials on Ag–Au bimetallic fishnets

We have designed a low-loss magnetic metamaterial with double-fishnet structures of silver and gold operating at near-infrared wavelength. A Particle swarm optimization algorithm was employed to optimize the geometry dimensions of Ag–dielectric–Au unit cell. The Ag–Au bimetallic fishnet produces a maximum figure of merit value of 21, transmission as high as 62%, moderate negative permeability Re(μ) of −0.5, and Re(n) of −1 at 1463 nm. The negative permeability Re(μ) reaches minimum value of −5 at 1573 nm, where the antisymmetric charge distribution in fishnet metamaterials was observed. The antisymmetric plasmonic resonance at 1573 nm is a mixed mode of asymmetric magnetic dipole and symmetric electrical quadrupole in Ag–MgF2–Au fishnet structures.

► The antisymmetric surface charge distribution is revealed for the magnetic resonance. ► The magnetic metamaterial is made of silver and gold bimetallic fishnet structures. ► The magnetic response has a low loss with a maximum FOM value of 21 in near-IR wavelength. ► The magnetic resonance wavelength λm and the minimum of Re(μ) are tunable.