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Three-dimensionally periodic dielectric structures, photonic crystals, possessing a forbidden gap for electromagnetic wave propagation, a photonic band gap, are now known. If the perfect 3D periodicity is broken by a local defect, local electromagnetic modes can occur within the forbidden band gap. Addition of extra dielectric material locally, inside the photonic crystal, produces ``donor'' modes. Conversely, local removal of dielectric material from the crystal produces ``acceptor'' modes. It is now possible to make high-Q electromagnetic cavities of 1 cubic wavelength, for short wavelengths at which metallic cavities are useless. These new dielectric cavities can cover the range from mm waves to UV wavelengths.
Yablonovitch et al. (Mon,) studied this question.