Terahertz chiral sensing and magneto-optical enhancement for ferromagnetic nanofluids in the chiral metasurface

The highly sensitive detection and magnetic field sensing of magnetic nanomaterials have received extensive attention, and its weak magneto-optical effect in the terahertz (THz) band limits its application. In this study, we investigated a chiral metasurface sensor filled with ferromagnetic nanofluids. Based on its artificial chiral resonance, the nanoparticle concentration and magneto-optical chiral response of the ferromagnetic nanofluids have both been detected using the THz time-domain polarization spectroscopy. The results show that the detection sensitivity of the concentration of the magnetic nanoparticles can reach 5.5 GHz %^-1 by chiral sensing, needing only a trace amount of the nanofluid. More importantly, in this hybrid device, the magneto-optical chiral response of the ferromagnetic nanoparticles can be greatly enhanced by the chiral metasurface, which results in higher sensitivity to the external magnetic field. The Verdet constant of the ferromagnetic nanofluid in the metasurface is 15 times stronger than that without the chiral microstructure. This THz chiral sensing for nanoparticles and the chirality enhancement mechanism will promote a new sensing method and chiral manipulation device, especially for the highly sensitive magneto-optical device in the THz band.