End-fire all-anisotropic transition metal dichalcogenide nanoantennas

Abstract

We demonstrate that all-anisotropic transition metal dichalcogenide (TMDC) nanostructures can support advanced optical nanoantenna functionalities, such as high end-fire directivity and directionality inversion. The designed chain structures, driven by a dipole emitter, are composed of negative uniaxial anisotropic tungsten disulfide $({\mathrm{WS}}_{2})$ TMDC nanoparticles featuring ordinary/extraordinary refractive indices. The ordinary index, whose values are much higher than those of Si (Si is typically used in all-dielectric nanostructures), is also much higher than the extraordinary index, thus allowing for a strong material birefringence. We find that anisotropy of this type makes it possible to achieve high end-fire directivity, and also offers an extra degree of freedom that allows for an orientation-based coupling between a free-to-rotate dipole emitter and fixed ${\mathrm{WS}}_{2}$ elements. Furthermore, appropriately rotating half the array elements achieves end-fire directional inversion (at a fixed operating wavelength) simply by adjusting the emitter orientation. This unique feature is characteristic of the high anisotropy exhibited by ${\mathrm{WS}}_{2}$ in the optical-near infrared regime, and cannot be readily realized with isotropic all-dielectric or plasmonic spherical structures due to their material symmetry. The presented material-anisotropy-based designs comprise a nanophotonics platform that allows for unique functionalities at the nanoscale, as well as for the implementation of tunable nanophotonic devices.