Gradient-index antireflective subwavelength structures for planar waveguide facets

We demonstrate the use of subwavelength gratings etched into the facets of silicon-on-insulator ridge waveguides as a means of reducing facet reflectivity by the gradient-index effect. Reflectivities as low as 2.0% and 2.4% for the fundamental TE and TM modes, respectively, are demonstrated experimentally for light of $1.55\mu \mathrm{m}$ wavelength, in agreement with both effective medium theory and finite-difference time domain calculations. Simulations show that facet reflectivites can be further reduced to less than 1% by increasing the grating modulation depth.