Integrated W-type structure for spectral and spatial filtering of optical radiation propagating in a slab waveguide

We present a simple and compact integrated narrowband filter that enables spectral or spatial filtering of optical radiation propagating in a slab waveguide. The filter consists of two grooves on the waveguide surface and constitutes an integrated analogue of the so-called W-type structure. The filtering is performed in transmission at oblique incidence due to the excitation of a leaky eigenmode of the structure localized at a ridge separating the grooves. For the considered parameters, neither "parasitic" out-of-plane scattering nor polarization conversion occurs, and strictly zero reflectance and unity transmittance are achieved at resonant conditions. The quality factor and location of the resonance can be controlled by changing the widths of the grooves and of the ridge between them, respectively. The proposed filter may find applications in novel devices of integrated nanophotonics. (paper)