Nonreciprocal lensing and backscattering suppression via magneto-optical nonlocality

We introduce a special kind of nonreciprocal electromagnetic response which gives rise to backscattering suppression in the bulk, a long-sought feature in topological photonics, as well as nonreciprocal lensing - an effect when the same structure focuses light incident from one direction and defocuses light propagating in the opposite way. We predict this response in spin spirals and in specially designed metamaterials, validating the key predictions.

💡 Research Summary

In this paper the authors introduce a novel form of nonreciprocal electromagnetic response that stems from magneto‑optical nonlocality. By expanding the permittivity tensor εij(ω,k) to first order in the wave vector k and separating even and odd contributions with respect to a static bias magnetic field H0, they derive a set of symmetry constraints based on Onsager‑Kasimir relations and loss‑less reciprocity. The key result is the identification of a tensor α(o)ijℓ that is real, symmetric in its first two indices, odd under spatial inversion and time reversal, yet even under the combined PT operation. This tensor contains 18 independent components, ten of which lie outside the conventional bianisotropic framework (χ, κ, and Tellegen tensors).

The authors focus on a specific “quasi‑moving” form of nonlocality, described by the permittivity correction
ε(ω,k)=ε(ω,0)+χqm q