Transmission filtering of a waveguide coupled to a stub microresonator

Using a finite difference time domain method, the authors study the electromagnetic transmission coefficient of a microwaveguide coupled to a lateral stub. It is shown that if the stub is covered with a thin metallic layer, the transmission spectrum contains several very narrow and deep depressions despite the small size of the resonator. The frequencies of the zeros of transmission are studied as a function of the geometrical parameters of the stub. The quality factor of the dips is increased when the waveguide is separated from the stub by a small air gap.