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Directional light flow is fundamental to the development of photonic information processors. One all optical way of generating such nonreciprocal transport involves exploiting the nonlinear Kerr effect within an asymmetric arrangement of high Q resonators. However, current demonstrations involve optical paths that are tens to hundreds of microns in length.
Here, we show that Kerr based nonreciprocal devices can be further miniaturized to the nanoscale by working with Silicon nanoantenna-based metasurfaces. In the subwavelength regime structural asymmetry alone isn’t enough to generate directionally-dependent field amplification. We overcome this limitation by overlapping a sub-radiant electric dipolar mode with a perpendicular super-radiant magnetic dipole. In this case, breaking out-of-plane inversion symmetry leads to nearfield coupling between the two excitations. Because of interference between nearfield and far field magneto-electric coupling, the electric dipole is suppressed (enhanced) for a normally incident plane wave propagating in the backward (forward) direction. When the metasurface is illuminated with powers of a few 100kW/cm2, the electric field strength within the Si becomes sufficient to change its refractive index, red-shifting the narrow transmission dip. For forward excitation the resonance is shifted by a significant portion of the FWHM, making the metasurface transparent. For backward excitation the much smaller shift renders the transmission very low.
We show, for the first time, that bianisotropy provides a means to achieve optical nonreciprocity at the nanoscale. Relying simply on collocated dipolar excitations, our scheme has, in principle, no lower size limitation and could be miniaturised further by making use of gain assisted plasmonics.
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