An epsilon-near-zero effective medium is a composite in which the averaged diagonal permittivity is approximately zero. In the source digest it appears as the mechanism by which nonreciprocity is amplified in gyrotropic heterostructures.
Mechanism in gyrotropic heterostructures
Katsantonis, Tasolamprou, Koschny, Economou, Kafesaki and Valagiannopoulos (Scientific Reports 13, 21986, 2023) alternate gyrotropic and metallic or plasmonic films. The consecutive layers have diagonal permittivities of opposite sign; averaging them drives the effective diagonal permittivity toward zero. The homogenization process leaves the nonzero off-diagonal permittivities of the original gyrotropic substance unaltered. Because the diagonal susceptibilities are suppressed in the collective response, the off-diagonal terms — which are responsible for nonreciprocity — become dominant and ignite a strong nonreciprocal response.
Practical considerations
The paper provides realistic material examples implementable in the mid-infrared spectrum. It discusses the robustness of the enhanced nonreciprocity in the presence of actual media losses and elaborates bandwidth limitations due to unavoidable frequency dispersion. The concept is proposed for optical devices serving signal isolation, wave circulation, unidirectional propagation and asymmetric power amplification.
Context
The approach addresses a known weakness of conventional Gyrotropy: gyrotropic materials have very weak time-reversal-symmetry-breaking effects, requiring large, bulky setups and very strong magnetic biases for technologically useful Nonreciprocity.
Source notes & attribution
- https://rexresearch.com/CISSMetamaterial/CISSMetamaterial.html