Direct Observation of Phase-Free Propagation in a Silicon Waveguide

We demonstrate silicon waveguides that support phase-free propagation in the telecom regime. These waveguides have smaller footprints and exhibit improved energy transfer capabilities as compared to previous optical waveguides that support phase-advance-free modes. We measure the effective refractive index using on-chip interferometry and observe a zero-crossing around a wavelength of λ = 1625 nm. At this wavelength, we observe the coherent oscillations of waves that span the entire length of the waveguide. These waveguides are based on a 220-nm-thick silicon-on-insulator platform and, thus, are inherently compatible with established silicon photonic technologies.