  Multimode interference couplers are a fundamental building block in many
integrated photonic systems, ranging from high-speed coherent receivers to
quantum splitters. However, their basic structure has remained fundamentally
unchanged for almost four decades, limiting their size and operation bandwidth.
Using sub-wavelength metamaterials, photonic devices with break-through size
and performance have been recently reported. Leveraging the inherent anisotropy
of these structures, here we derive a semi-analytic expression that enables the
design of compact and ultra broadband multimode interference couplers. We
experimentally demonstrate virtually perfect operation over a bandwidth in
excess of 300nm (500nm in simulation), for a device three times shorter than
its conventional counterpart, making this the most broadband multimode
interference coupler reported to date. These results will enable ultra
broadband integrated systems for applications in communications and sensing.
