Extrinsic chirality and circular dichroism at visible frequencies enabled by birefringent $\upalpha$-{MoO}3 nanoscale-thick films: implications for chiro-optical control

Nanophotonics community has shown great interest in 2D materials because of their unique properties of electromagnetic field manipulation. Many of these materials exhibit strong natural anisotropy, which further opens possibilities of polarization manipulation. Here, we show that α-MoO3, an emerging natural hyperbolic 2D material, can be combined with plasmonic nanostructures to provide strong extrinsic chirality in the visible range. A combination of biaxial anisotropy in α-MoO3 and Fabry–Perot cavities with nanoscale features leads to different absorption of left and right circularly polarized photons, hence exhibiting circular dichroism (CD). Our simulation results predict that multilayer nanoscale-thick films including α-MoO3 are potential candidates for achieving extrinsic chirality across the visible range. Furthermore, we show a significant CD increase when the α-MoO3 layer is coupled with plasmonic nanohole arrays or plasmonic nanocubes. Such designs are achiral in geometry and therefore easier to fabricate. Moreover, we optimize the CD dissymmetry factor gCD for the nanocube-based design at 780 nm, obtaining 84%. We believe that utilizing biaxially anisotropic α-MoO3 films to control and engineer chiro-optical properties in the visible frequency range will open research directions and enable enhanced functionalities in chiro-optical control at the nanoscale, further leading to applications in chiral sensing and CD.