  Optical phase shifters are extensively used in integrated optics not only for
telecom and datacom applications, but also for sensors and quantum computing.
While various active solutions have been demonstrated, progress in passive
phase shifters is still lacking. Here, we present a new type of ultra-broadband
90{\deg} phase shifter, which exploits the anisotropy and dispersion
engineering in subwavelength metamaterial waveguides. Our Floquet-Bloch
calculations predict a phase shift error below $\pm$1.7{\deg} over an
unprecedented operation range from 1.35 $\mu$m to 1.75 $\mu$m, i.e. 400 nm
bandwidth covering the E, S, C, L and U telecommunication bands. The flat
spectral response of our phase shifter is maintained even in the presence of
fabrication errors up to $\pm$20 nm, showing greater robustness than
conventional structures. Our device was experimentally demonstrated using
standard 220-nm-thick SOI wafers, showing a fourfold reduction in the phase
variation compared to conventional phase shifters within the 145 nm wavelength
range of our measurement setup. The proposed subwavelength engineered phase
shifter paves the way for novel photonic integrated circuits with an
ultra-broadband performance.
