3-dimensional photonic band structure

Three-dimensionally periodic dielectric structures, (photonic crystals), possessing a forbidden gap for electromagnetic wave propagation, (a photonic bandgap), are now known. If the perfect 3-dimensional periodicity is broken by a local defect, then local electromagnetic modes can occur within the forbidden bandgap. The addition of extra dielectric material locally, inside the photonic crystal, produces 'donor' modes. Conversely, the local removal of dielectric material from the photonic crystal produces 'acceptor' modes. It will now be possible to make high-Q electromagnetic cavities of volume ∼ 1 cubic wavelength, for short wavelengths at which metallic cavities are useless. These new dielectric cavities can cover the range all the way from millimeter waves, down to ultraviolet wavelengths.