Nonreciprocity

Nonreciprocity is asymmetric propagation: a wave or current passes in one direction but not the other. In photonics it is a highly desirable feature because it allows control over traveling electromagnetic waves beyond ordinary filtering. The source digest treats it in two settings — photonic media and superconducting transport.

Photonic nonreciprocity

One of the most conventional ways to achieve nonreciprocity is via gyrotropic materials, but their time-reversal-symmetry-breaking effects are very weak, so large, bulky setups combined with very strong magnetic biases are required for technologically useful devices. Magneto-optic effects break Lorentz reciprocity, which is the necessary condition to construct devices such as optical isolators.

Katsantonis et al. (Sci. Rep. 13, 21986, 2023) introduce artificial heterostructures to enhance effective nonreciprocal behavior by reducing the contribution of the diagonal susceptibilities in the collective response, so that the off-diagonal terms responsible for nonreciprocity appear bigger. Alternating gyrotropic and metallic or plasmonic films make an Epsilon-Near-Zero (ENZ) Effective Medium by averaging diagonal permittivities of opposite sign; homogenization leaves the nonzero off-diagonal permittivities of the original gyrotropic substance unaltered, so they become dominant and ignite a strong nonreciprocal response. Realistic mid-infrared material examples are provided, and the robustness of the enhancement under actual media losses and bandwidth limitations from frequency dispersion is discussed. Applications cited include signal isolation, wave circulation, unidirectional propagation and asymmetric power amplification.

Superconducting nonreciprocity

Kokkeler, Tokatl and Bergeret (SciPost Phys. 16, 055) study nonreciprocal transport in superconducting systems, specifically the Superconducting Diode Effect. A common feature of such systems is gyrotropic symmetry, characterized by a polar vector; alongside time-reversal symmetry breaking, the polar axis allows magnetoelectric effects which, combined with proximity-induced superconductivity, produce spontaneous non-dissipative currents underpinning the diode effect. Their model is a lateral Josephson junction composed of a normal metal supporting the spin Hall effect, attached to a ferromagnetic insulator, so that magnetoelectric effects arise without external magnetic fields. The theory unifies nonreciprocal superconducting transport with the spin Hall effect, spin galvanic effect and spin Hall magnetoresistance, and the authors propose an experiment measuring normal-state magnetoresistance together with superconducting-state nonreciprocal transport.

Magnet-less routes

Boshgazi, Mehrany and Memarian (Opt. Express 33, 24370) propose emulating gyrotropy — and hence nonreciprocity — by time-modulating the permittivity tensor of a static achiral, non-gyrotropic crystal, avoiding magnetic materials and external bias fields. Time-varying media more generally have unlocked non-reciprocity, frequency conversion and parametric amplification.