On-Chip Metasurface-on-Facets for Ultra-High Transmission through Waveguides in Near-Infrared

Nature has long inspired scientists and engineers to develop transparent surfaces via constructing anti-reflective surfaces. In absence of anti-reflection (AR) coating, silicon reflects about 35% of light for a single interface air-silicon. Here, inspired by jellyfish anti-reflective eyes, a man-made anti-reflective surface on the facet of the waveguide is proposed and demonstrated for waveguides transparency in near-infrared. The optimized metamaterial with unit cells of 560 x 560 nm shows transparency of 2.6 times better as compared to the waveguide with blank facet. Metasurfaces are milled on the waveguides facets with a focused ion beam. Silicon-on-insulator waveguides are tested with an inline set-up. Far-field scattering diagrams reveal that it is the special geometry of the unit cells of the engraved metamaterial, which can be associated with the directional scattering resulting in combined effect: on one hand the ultra-high transparency of the device and on the other hand the efficient coupling to the low-order modes due to the focusing dielectric nano-antennas effect. Reported here waveguide facets as AR metamaterials on a chip, opens up opportunities to engineer transparent on-chip devices with high coupling efficiency for diverse applications from sensing to quantum technologies.