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In this paper, we demonstrate a radiation-strength-chirped metasurfacecentimeter-long grating antenna, whose beam quality is approaching thediffraction limit in its footprint with 90 percent efficiency.Instead ofchirping the duty cycle to tune radiation strength, which calls fornon-scalable fabrication resolution, we design a 2D fish-bone surface gratingwith apodized width of the bone.The meta surface is made by silicon nitride ontop of silicon waveguide to weakly perturb the mode. Moreover, a Monte CarloMarkov Chain(MCMC) algorithm is implemented to design the radiation strengthprofile, making the optimized trade off between efficiency, beam quality andfabrication challenge. A metal mirror is designed to deposit on the top of thecladding for unidirectional upside-down radiation and uniform lateral gain. Forthe 1cm grating, our design is for a 0.01 degree beam with 37000 1D directionalgain and 90% efficiency, with 200nm feature size. Finally, we fabricated theantenna and measured the beam quality with Fourier imaging system, which givesresult of 0.02 degree -10dB beam divergence. To the best of our knowledge, thisis the state-of-the-art on-chip antenna with CMOS- foundry-compatibletechnology.

Diffraction limited centimeter scale radiator: metasurface grating antenna for phased array LiDAR