Artificial Periodic Structures Research Articles

Surface waves of printed antennas on planar artificial periodic dielectric structures

The characteristics of surface waves from a Hertzian (elementary) dipole on a multilayer structure with planar periodic material elements are investigated. An integral-equation moment method in conjunction with an analytical array scanning scheme is applied to the boundary-value problem associated with source interaction with infinite periodic structures. A pole-extraction technique and the saddle-point method are applied to find the far-zone periodic surface waves due to a current source. The investigation provides a fundamental study of surface wave properties of printed circuit antennas on planar artificial periodic (photonic bandgap) structures. It is found from the power patterns that surface waves are suppressed in the directions with wave bandgap and greatly enhanced in the directions just outside the bandgap zones. The surface wave pattern may be highly directive and the beam angle varies with frequencies. The finding suggests possible frequency-space selection devices. Experiments are carried out to validate the surface wave bandgap phenomenon and the beam angle frequency-selection property.

Josephson-like effects in coherent motion of vortices in a periodic potential

Resonant quasi-Josephson effects induced by coherent vortex motion in artificial reversible periodic potential structures have been investigated. Periodic potential has been imposed by application of a magnetic tape containing a prerecorded harmonic signal to the surface of a high- T c thin film strip. Motion of current driven vortices in the film with applied tape is highly coherent and leads to the appearance of a series of self-resonant DC current steps on the I– V characteristic of the sample. The presented model attributes these steps to locking of the vortex nucleation frequency to the internal self-frequencies of the system; the latter being set by the time of flight of vortex bundles across the sample width and across the spatial period of the applied potential. Voltages of the current steps have been found to scale consistently with the proposed model. The velocity of vortex motion inferred from the voltages of the first quasi-Josephson steps was found to be extremely large. The possible explanation of this fact in terms of the huge size of vortex bundles is discussed and confronted with independent experimental evaluations of the bundle size from the flux-flow noise spectra and vortex induced random telegraph voltages in high- T c superconductors.

Current-driven vortex dynamics in a periodic potential

Quasi-Josephson effects due to coherent vortex motion in artificial reversible periodic potential structures in high-${T}_{c}$ superconducting thin films have been investigated. Periodic pinning conditions have been created by applying a magnetic tape containing a prerecorded harmonic signal to the surface of high quality ${\mathrm{Y}}_{1}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ thin films. Application of the periodic pinning enforces coherence in current-driven motion of Abrikosov vortices in wide and short macrobridges and leads to the appearance of Josephson-like effects manifesting themselves in series of self-induced current steps on the current-voltage characteristics. The equation of motion for vortices flowing across periodic potential structures is analogous to the phase equation for low-capacitance classical Josephson junctions. The perturbation solutions of this equation contain resonant Shapiro-like self-steps resulting from the locking of the frequency at which vortices are created at the sample borders to the resonant frequencies of the vortex system. Self-resonant frequencies are set by the characteristic time of flight across the sample width and across the period of the applied potential. Voltages of the self-induced current steps have been found to scale with inverse of the characteristic length corresponding to the magnetic period and/or to the sample half-width, consistently with the theoretically derived relations. Experimental data indicate that vortices move in large bundles containing several thousands of flux quanta. The temperature dependence of the step voltages can be ascribed to changes in vortex velocity due to the temperature-dependent viscosity factor.

Organic/semiconductor superlattices as an attractive perspective

Organic/amorphous semiconductor superlattices are built up by means of alternating application of two different methods for thin film formation. Organic sublayers of cadmium arachidate are deposited by means of the Langmuir-Blodgett technique, and inorganic sublayers of amorphous CdSe are grown via the thermal vacuum evaporation method. The details of the superlattice preparation providing deposition of smooth and homogeneous sublayers are described. The periodicity of the structures obtained is evidenced by means of the small-angle X-ray diffraction method. The experimental realization of this new type artificial periodic structures opens a number of possibilities for the combination of the advantages of superthin ordered organic and the semiconductor films. The areas of application of such superlattices are outlined.

The diffraction optics of very cold neutrons

Neutron diffraction processes caused by artificial periodic structures are considered. The method of analysis of the considerable wavefront distortions that take place both under diffraction and the gravitational interactions of very cold neutrons (VCN) spreading out in an interferometer is developed. It is shown that a three-grating neutron interferometer is completely free of aberrations and is workable with a non-collimated and non-monochromatized illuminating beam. The method of aberration analysis of a VCN interferometer scheme in the case of non-ideal adjustment is developed. This allows us to evaluate the requirements concerning the parallelism of the grating planes and the symmetry of the interferometer.

Intense electromagnetic radiation from relativistic particles

Several mechanisms for emission of electromagnetic radiation by relativistic electrons in various media which are of interest for generating intense ultraviolet, x-ray, and γ radiation are examined. Theoretical and experimental results on ultraviolet and x-ray Cherenkov radiation, quasi-Cherenkov radiation in artificial periodic structures, radiation from crystals which results from the diffraction of virtual photons, and radiation accompanying channeling are discussed.

Phase matching in second-harmonic generation using artificial periodic structures

The use of artificial periodic structures, consisting of spatial modulations of the linear and nonlinear susceptibilities of a nonlinear optical medium, to achieve phase matching in second-harmonic generation is analyzed. Dispersion relations and approximate formulas for the second-harmonic fields generated under various conditions are obtained and used to evaluate the experimental situation.