  In the present paper we investigate the restriction conditions for three
kinds of cavity resonators ({\it i.e.}, the rectangular, cylindrical, spherical
resonators). It is shown that the layer of materials with negative optical
refractive indices can act as a phase compensator/conjugator, and thus by
combining such a layer with another layer made of the regular medium one can
obtain a so-called compact thin subwavelength cavity resonator.
