Enhanced terahertz magneto-plasmonic effect enabled by epsilon-near-zero iron slot antennas

Terahertz magneto-plasmonics plays a crucial role in platforms for isolation and sensing applications, operating at terahertz frequencies. In spite of recent efforts to enhance magneto-optic effects using metasurfaces, the mechanism for optimizing these effects remains unclear in the terahertz regime. Here we investigate terahertz magneto-optic effects using 100 nm-thick iron slot antennas with varying widths, ranging from 20 µm to 300 nm. Interestingly, as the width of slot antenna decreases, this enhancement peaks around 1 µm, after which the effect diminishes for smaller widths. Based on the effective medium theory, the slot antennas exhibit a maximum Faraday rotation angle near the epsilon-near-zero region. Although the field enhancements in the slot become stronger with the sub-micron widths, the magneto-optic effect may decrease with increasing effective dielectric constant due to gap plasmon effects in the sub-micron region. Our findings provide essential criteria for designing ferromagnetic metasurfaces with enhanced Faraday rotations at terahertz frequencies.