  Metamaterials with artificial optical properties have attracted significant
research interest. In particular, artificial magnetic resonances in non-unity
permeability tensor at optical frequencies in metamaterials have been reported.
However, only non-unity diagonal elements of the permeability tensor have been
demonstrated to date. A gyromagnetic permeability tensor with non-zero
off-diagonal elements has not been observed at the optical frequencies. Here we
report the observation of gyromagnetic properties in the near-infrared
wavelength range in a magneto-plasmonic metamaterial. The non-zero off-diagonal
permeability tensor element causes the transverse magneto-optical Kerr effect
(TMOKE) under s-polarized incidence that otherwise vanishes if the permeability
tensor is not gyromagnetic. By retrieving the permeability tensor elements from
reflection, transmission, and TMOKE spectra, we show that the effective
off-diagonal permeability tensor elements reach the 10-3 level at the resonance
wavelength (~900 nm) of the split-ring resonators that is at least two orders
of magnitude higher than that of magneto-optical materials at the same
wavelength. The artificial gyromagnetic permeability is attributed to the
change in the local electric field direction modulated by the split-ring
resonators. Our study demonstrates the possibility of engineering the
permeability and permittivity tensors in metamaterials at arbitrary
frequencies, thereby promising a variety of applications of next-generation
nonreciprocal photonic devices, magneto-plasmonic sensors, and active
metamaterials.
