Low-loss hollow-core waveguide using high-contrast sub-wavelength grating

We present a novel form of hollow-core waveguiding that enables chip-scale integration. Light propagates in air along a zig-zag path between very highly-reflective Si metastructures comprised of a single layer of sub-wavelength high-contrast gratings (HCGs) without the aid of sidewalls. Top and bottom subwavelength HCGs separated by 9um of air and with periodicity perpendicular to the propagation of light reflect light at shallow angles with extremely low loss. The HCGs are patterned on SOI wafers with 340 nm-thick Si device layers engraved in a single etch step, and have been measured to have a 0.37 dB/cm propagation loss. Our work demonstrates the light-guiding properties of HCG hollow-core waveguides with a novel form of lateral beam confinement that uses subtle reflection phase changes between core and cladding HCG regions capable of bending light around 30 mm radius-of-curvature tracks.