Large beam size grating coupler in silicon-on-insulator using fully etched subwavelength gratings

Several emerging applications of silicon photonics, including sensing, ranging and optical trapping, require fixed, well-collimated beams that enable interaction with targets placed centimeters away from the chip. Generating such beams without using bulk-optic lenses entails radiating beams with diameters of hundreds of microns directly from the chip. Gratings with sufficiently low strength have so far only been shown in the silicon nitride platform using specialized shallow etch steps; in silicon-on-insulator the implementation becomes much more challenging due to the increased index contrast. Here we report the first fully etched silicon-on-insulator grating capable of radiating such large beams. Using a judiciously designed double-period subwavelength structure, we experimentally demonstrate a beam diameter in excess of 350 μm, with feature sizes compatible with deep-ultraviolet lithography.