Unique mode properties of optical metacavities

Unusual optical properties of a cavity that contains a layered structure with an alternating refractive index are considered. The layers with the negative refraction and the negative phase velocity provide for drastic changes to the properties of the longitudinal and transverse modes. The transverse-mode structure depends on the effective diffraction length whereas the spectrum of longitudinal modes is determined by the effective optical length of the cavity. The effective lengths can be positive, negative, or zero. In the last case, the transverse distribution of the amplitude can be arbitrary. The conditions for the stability of the metacavity are derived. It is demonstrated that an unstable empty cavity becomes stable when a metamaterial is introduced inside it.