It is shown that the violation of periodicity in a low-dimensional waveguide microwave photonic crystal, in which the periodic structure is formed by dielectric layers and adjacent metal plates partly overlapping the waveguide cross section and forming capacitive gaps between the plate edge and wide wall of the waveguide, leads to the appearance of a defect (impurity) mode. It is established that the defect mode position on the frequency scale significantly depends on both the thickness of "disturbed" dielectric layer and the capacitive gap width of diaphragms.The current interest in studying microwave photonic crystals (PCs) is related to their possible practical application in filters, resonators, reflectors, slowwave systems, and antennas [1][2][3][4]. Laws governing the behavior of microwave photonic crystals can also be used to create analogous devices operating in the optical range, in which a significant role can be played by technological factors that are difficult to take into account.A low-dimensional waveguide microwave photonic crystal can be created using a periodic structure consisting of elements representing dielectric layers and adjacent thin metallic plates partly overlapping the waveguide section and forming capacitive gaps between the plate edge and wide wall of the waveguide. The capacitive gap acts as a source of high-order wave modes. If a metal reflector is arranged in the vicinity of the gap, this system can feature a resonance of these modes [5]. The high-order wave modes arising near the hole in the diaphragm form the so-called near field. Resonance characteristics of the "diaphragmmetal reflector" element are highly sensitive to changes in the capacitive gap dimensions and distance to the reflector. Studying the properties of such systems can be of interest for improving characteristics of near-field scanning microwave microscopes [6].In particular, a low-dimensional PC can be constructed so that every next element would have its capacitive gap alternatively at the opposite wide wall of the waveguide [7]. The distance from the diaphragm to reflector can be selected to be small so that resonance interaction with a metal plate of the neighboring element will take place. Under the Bragg condition of (where a is the period of a structure with photonic bandgap, β = is the phase constant, λ is the central wavelength of the bandgap, and ε is the dielectric permittivity), this relationship will concern a highorder wave mode with a wavelength much shorter than that of the fundamental mode. A violation of periodicity in this low-dimensional PC must lead to the appearance of a defect (impurity) mode. The present work was aimed at theoretical description of the conditions of defect mode formation and experimental verification of these conditions. In order to obtain a "bandgap" in the amplitudefrequency characteristic (AFC) of the PC, we used capacitive diaphragms as its terminal elements. Then, a "disturbance" of the PC periodicity can be introduced by a different size of the central dielectric la...
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