  We propose a simple integrated narrowband filter consisting of two grooves on
the surface of a slab waveguide. Spectral filtering is performed in
transmission at oblique incidence due to excitation of an eigenmode of the
structure localized at a ridge cavity between the grooves. For the considered
parameters, strictly zero reflectance and unity transmittance are achieved at
resonant conditions. Width and location of the transmittance peak can be
controlled by changing the widths of the grooves and of the ridge,
respectively. The proposed filter may find application in waveguide-integrated
spectrometers.
