  Silicon photonic integrated circuits routinely require 3-dB optical power
dividers with minimal losses, small footprints, ultra-wide bandwidths, and
relaxed manufacturing tolerances to distribute light across the chip and as a
key building block to form more complex devices. Symmetric Y-junctions stand
out among other power splitting devices owing to their wavelength-independent
response and a straightforward design. Yet, the limited resolution of current
fabrication methods results in a minimum feature size (MFS) at the tip between
the two Y-junction arms that leads to significant losses for the fundamental
mode. Here we propose to circumvent this limitation by leveraging subwavelength
metamaterials in a new type of ultra-broadband and fabrication-tolerant
Y-junction. An exhaustive experimental study over a 260 nm bandwidth (1420-1680
nm) shows excess loss below 0.3 dB for the fundamental transverse-electric mode
(TE0) for a high-resolution lithographic process (MFS about 50 nm) and less
than 0.5 dB for a fabrication resolution of 100 nm. Subwavelength Y-junctions
with deterministically induced errors of plus-minus 10 nm further demonstrated
robust fabrication tolerances. Moreover, the splitter exhibits excess loss
lower than 1 dB for the first-order transverse-electric mode (TE1) within a 100
nm bandwidth (1475-1575 nm), using high-resolution lithography.
