  In this paper, we demonstrate a radiation-strength-chirped metasurface
centimeter-long grating antenna, whose beam quality is approaching the
diffraction limit in its footprint with 90 percent efficiency.Instead of
chirping the duty cycle to tune radiation strength, which calls for
non-scalable fabrication resolution, we design a 2D fish-bone surface grating
with apodized width of the bone.The meta surface is made by silicon nitride on
top of silicon waveguide to weakly perturb the mode. Moreover, a Monte Carlo
Markov Chain(MCMC) algorithm is implemented to design the radiation strength
profile, making the optimized trade off between efficiency, beam quality and
fabrication challenge. A metal mirror is designed to deposit on the top of the
cladding for unidirectional upside-down radiation and uniform lateral gain. For
the 1cm grating, our design is for a 0.01 degree beam with 37000 1D directional
gain and 90% efficiency, with 200nm feature size. Finally, we fabricated the
antenna and measured the beam quality with Fourier imaging system, which gives
result of 0.02 degree -10dB beam divergence. To the best of our knowledge, this
is the state-of-the-art on-chip antenna with CMOS- foundry-compatible
technology.
