Tunable frequency filtering by using dielectric waveguides

We show that for a transverse-electric (TE) mode, a specific frequency of incident light can be selected by using dielectric slab waveguides. The ray of light incident upon the top of the waveguide travels inside the waveguide and interferes with light scattered at the boundary of the waveguide. A rigorous calculation reveals that if the incident angle satisfies some conditions, the interference spectrally narrows the outgoing wave so that selection of a specific frequency is possible. We also show that this frequency is tunable by changing both the refractive index and the thickness of the dielectric waveguide. To explore the nature of the spectral narrowing, we studied the leaky waveguide mode and are used the finite difference time domain (FDTD) method to confirm our study.