  Mode-division-multiplexing (MDM) is attractive as a means to increase the
link capacity of a single wavelength for optical interconnects via the use of
multiple mode-channels in multimode bus waveguide.In order to route the data in
MDM systems, waveguide bends are typically needed.In particular, ultra-sharp
bends are desirable for reducing the chip area occupied by high-density
photonic intergrated circuits (PICs). In this work, we propose and demonstrate
a novel meta-surfaced waveguide structure on silicon, which enables
ultral-sharp multimode waveguide bends to have low loss and low crosstalk among
different mode-channels.An ultra-sharp S-bend with a bending radius of
R=10{\mu}m is realized for an MDM optical interconnect with three
mode-channels. The numerical simulation shows that the proposed ultal-sharp
meta-surfaced multimode waveguide bend (MMWB) has low excess losses (0.2~0.5dB)
and low inter-mode crosstalk (<-30dB) over a broad wavelength-band(>100nm) for
all the three mode-channels. This is verified by experimental results, which
show that the fabricated S-bends have excess losses of 0.5~0.8dB and inter-mode
crosstalk of <-20 dB over a wavelength of >60 nm. The experimental results were
partially limited by the measurement setup, particularly the on-chip mode
(de)multiplexers which had inter-mode crosstalk of about -20dB. The proposed
MMWB can be extended for the transmission of more than three mode-channels.Our
work paves the way to use meta-surfaced silicon photonic waveguide structures
for on-chip multimode optical interconnect.
