Subwavelength grating structures in planar waveguide facets for modified reflectivity

We demonstrate experimentally and by simulations the use of subwavelength grating patterns on the facets of planar waveguides as a means of modifying facet reflectivity over a wide range of values, from antireflective to highly reflective. An antireflective structure can be obtained from a gradient index effect with triangular gratings. Square gratings can be used to obtain either antireflective or highly reflective facets by an interference effect. Finite difference time domain simulations and calculations based on effective medium theory show that reflectivities well below 1% can be achieved with triangular gratings. Experimentally, facet reflectivities as low as 2.0% and 2.4% for the fundamental TE and TM waveguide modes, respectively, are demonstrated for light of 1.55 μm wavelength in silicon-on-insulator ridge waveguides. The experimental results are in good agreement with both effective medium theory and finite difference time domain simulations. The polarization dependence of the effects is also discussed in detail.