In a context of photonic integrated circuits, waveguide Bragg gratings are frequently used for purposes of spectral filtering. If these are build from material systems with high refractive index contrast and/or with large corrugation depths, wave propagation through finite devices tends to be accompanied by radiative losses, which are highly unwanted for certain applications such as optical signal-processing, sensors, or lasing reflectors.
We have recently explored the properties of gratings with simple rectangular corrugations and 1-D periodicity, as exemplified in the figure. The structures are excited by "semi-guided waves'' that are strictly confined in the direction perpendicular to the guiding slab, that have in-plane the form of a plane wave, and that come in at the grating at an oblique angle.
It turns out that the oblique excitation makes all the difference: For a sufficiently high angle of incidence, radiation losses, originating either from the corrugated region or from the interfaces between the grating and the original slab, are (mathematically) strictly suppressed. In the band-structure analysis, the concept of a "light line" does not apply. Further, by virtue of symmetry arguments, polarization conversion is strictly prohibited.
Our results cover a series of fully- and partly-etched finite gratings, for typical parameters from silicon photonics, and for TE-excitation at 45°. The devices generate a reasonably flat-top wavelength response; apodization can lead to even more box-shaped spectra. Transmittance and reflectance add up to one in all cases. Together with a narrow-band Fabry-Perot filter based on similar principles, these configurations exhibit reflection bands, or transmittance peaks, with widths that span three orders of magnitude.
Details of the study can be found in a recent publication in JOSA B, see doi.org/10.1364/JOSAB.485725.