Discovered in the 19th century, circular dichroism (CD) – absorption difference of left circularly polarized (LCP) and right circularly polarized (RCP) lights – has found diverse applications in various technological fields [[1], [2], [3]]. Typically, a structure can exhibit CD under normal incidence of circularly polarized light (CPL) only if it is geometrically chiral [4,5]. Achiral plasmonic nanostructures can also exhibit CD but under oblique incidence of CPL [[6], [7], [8]]. Non-zero CD exhibited by achiral nanostructures surrounded with chiral media has also been reported [9,10]. Limited by the mismatch between chiral nanostructure size and incident wavelength, the chiroptical responses are very weak [11]. From application point of view, the chiroptical response of a nanostructure needs modulation and controlling by some external factor [12]. Different strategies to modulate chiroptical properties of plasmonic nanostructures include mechanical controlling, use of phase change materials, and application of electric or magnetic fields. For example, plasmonic CD in a VO2-integrated chiral plasmonic system was observed to decrease with phase transition of VO2 from metallic to insulator form [13]. Chiral nearfields of an achiral plasmonic nanostructure were reported to exhibit 100% enhancement in the presence of magnetic field as compared to those in the absence of magnetic field [14]. Au nanorod and DNA hybrid system showed reversible CD caused by temperature dependent assembling and de-assembling of the system units [15].

Out of these strategies, use of magnetic fields, due to the advantages of non-contact manipulation and ultrafast modulation, has attracted fascinating attention. Recent advances in magnetically modulated chiral plasmonics have shown notable progress in dynamic control of circular dichroism. For example, in experiments carried out by Cao et al., achiral elliptical nanohole arrays in planar magnetic metamaterial (ENA-MIM), and in gold thin film (ENA-Au) evinced CD under obliquely incident light. Due to enhancement of polarization effects caused by local magnetic dipole moments, ENA-MIM arrays were observed to exhibit larger CD as compared to that exhibited by ENA-Au [16]. Jeong et al. used helical magnetic field to self-assemble Ag@Fe3O4 nanoparticles into helical superstructures, and observed the dynamic switching of helically chirality at millisecond level; about 6000 times faster than the typical template-assisted approaches [17]. Qin et al. reported switchable optical chirality of Au nanoholes in presence of magnetic field of Ce:YIG [18]. In similar experiments reported by Zubritskaya et al., 25% tunability in chiroptical characteristics of Ni-Au-Au trimer nanoantenna was obtained in presence of external magnetic field [19]. Armelles et al. reported successful magnetic field driven controlling of CD of gold gammadion nanostructure [20]. In their work, Ikram et al. reported magnetic field caused inducement of CD in achiral plasmonic nanostructure under normal incidence of CPL [21]. The work of Han et al. presented geometry dependent modulation of magnetic circular dichroism [22]. Li et al. reported exhibition of polarization-independent non-reciprocal transmission by a magneto-optical chiral metasurface [23]. The performance arose from the combination of chiral and magnetic components.

CD has diverse applications such as in chiral discrimination [24], biosensing [25] and circular polarizers [26]. Since for each magnetic material, there is an easy axis of magnetization along which smaller applied magnetic fields are required for acquiring saturation magnetization, the modulation of CD with magnetic field along different axes would be anisotropic. Taking it the opposite way, this means that CD of plasmonic nanostructures can be used to detect variations in magnetic field. Although, magnetic modulation of plasmonic CD is reported in literature, anisotropic sensitivity towards magnetic fields has not been reported. In recent decades, a number of various types of magnetic sensors such as fluxgate magnetic sensors [27], magneto-resistive sensors [28], Hall effect sensors [29] and SQUID magnetic sensors [30] have been introduced. None of these magnetic sensors is chirality-based.

In this article, variation in CD of an achiral plasmonic metasurface is proposed for magnetic field sensing. The proposed chiro-magnetic sensor has the advantage of contactless and remote sensing. The model nanostructure is epsilon shaped nanostructure supported on magnetic material Ce:YIG. Intended theoretical studies were carried out using COMSOL Multiphysics. In addition to calculating CD of the nanostructure as functions of magnetic field as well as angle of incidence of CPL, effects of structural parameters on CD were also investigated. In section 2, a brief introduction of the structure model and of computational method is given. Section 3 presents the obtained theoretical results along with detailed discussion. Finally, we propose the utilization of extrinsic CD for magnetic field sensing, highlighting its advantages in enabling contactless and remote sensing capabilities. Conclusions are made in section 4.