Surface and bulk waves in superconductor/dielectric guiding structures

Waveguide properties of two- and three-layer planar structures on the basis of a high-temperature superconductor (HTS) and a dielectric is considered with allowance for the time dispersion of their permittivity. The existence conditions and dispersion dependences of surface and bulk waves in the region of temperatures T ≪ T _c are obtained. The frequency dependence of the depth at which surface modes penetrate into each medium are found and the polarization structure of such modes is determined. It is shown that TE and TM bulk waves can propagate in an HTS waveguide in two frequency intervals. Variation in the mode index of bulk TM modes by unity at the frequency of the longitudinal optical phonon is found.     

Measurements on nonlinear space-charge waves

Results of measurements on the nonlinear ac currents in a velocity modulated confined electron beam are reported. The test vehicle is a two cavity klystron type device with an axially movable demodulator. The ac beam currents at the fundamental and at the two lowest harmonic frequencies have been measured as functions of the distance along the beam for various modulation levels and dc beam currents. The work provides experimental data in the transition range between linear space-charge wave-type and ballistic-type behavior of the ac beam current. The experimental results are in good agreement with the nonlinear space-charge wave theory.     

Radiation-induced space-charge buildup in MOS structures

An analysis of a simple model for radiation-induced space-charge buildup in the SiO2layers of MOS structures has been carried out. The model assumes that hole-electron pairs are created in the SiO2by the radiation and that some of the electrons thus created drift out of the SiO2layer under the action of an applied potential across the oxide, VG, while the corresponding holes become trapped. The diffusion of electrons is assumed to be negligible. The analysis predicts 1) a dependence of charge buildup on radiation doseD, approximately of the form (1 - e^{-beta D}); 2) a linear dependence of the charge buildup on VG, for both polarities of VG; and 3) the dependence of the charge buildup on the total dose absorbed and not on the rate at which the dose was received. Experimental observations on the SiO2layers found in commercial MOS-FET's show good general agreement with the predictions of the analysis. To obtain quantitative agreement, however, it was necessary to assume that the mobility-lifetime product for electrons in the oxide is much lower at the SiO2-Si interface than at the SiO2- metal interface. Other discrepancies were observed but they can be explained as the result of oversimplifications employed in the analysis. In particular, it was necessary to postulate that under some circumstances diffusion of electrons out of the oxide was important and that, in addition to holes, a small number of electrons may be trapped in the oxide in some cases. The space charge was found to accumulate within ≤ 200 Å of the cathode-oxide interface.     

Leaky space-charge waves II: Smith-Purcell radiation

This paper contains a new theoretical approach for analyzing Smith-Purcell radiation. The new theory is self-consistent, i.e., it takes into account the effect of the radiation back on the beam. This approach is not only more precise than the previously available one, but it brings to light a number of new effects. It shows that two kinds of waves contribute to radiation, namely, slow and fast space-charge waves, and demonstrates that these waves are of leaky nature. The numerical results show in detail that there is an optimum beam thickness and an optimum beam velocity.     

Propagation of space-charge waves in diodes and drift spaces

It has generally been found that propagation of space-charge waves may be described 1) in space-charge-limited gaps by Gauss' hypergeometric functions, 2) in gaps with no space charge but with an accelerating homogeneous field by Bessel functions with real argument; and 3) in spaces free of dc fields by simple trigonometric functions. The equations derived become simpler through the selection of the dc velocity u, rather than the time t, as the independent variable.     

Electrodynamic analysis of the interaction between electromagnetic waves and nonlinear semiconductor inclusions in guiding structures

A method is developed for simulation of nonlinear propagation and diffraction of electromagnetic waves in guiding structures containing highly nonlinear distributed semiconductor inclusions. A planar Gunn effect diode located in a strip-slot guiding structure is considered, and the nonlinear effect of frequency multiplication in such a diode is simulated. The simulation is performed with the use of a computational algorithm based on universal autonomous blocks with Floquet channels.     

A symmetry property of space-charge waves in accelerated electron beams

A symmetry property is deduced for an electron beam modulated and demodulated by gaps of cavities. This property results from a combination of the theory of space-charge wave propagation in an accelerated beam and of the theory of gaps. It states that in a region bounded by two cavities, the power detected by the second cavity is unaltered by an interchange of potentials on the two cavities, provided the widths of the gaps are small. Experimental results are given for both linear and nonlinear acceleration and deceleration regions. In all cases, this property is found to be true. Most of the experimental results are also tested vs theoretical solutions of the electronic equation for regions of acceleration. The reduction factor is included in the theory.     

Cerenkov radiation in slow-wave guiding structures

Cerenkov radiation of a prebunched beam in a dielectric tube waveguide is here shown to be about an order of magnitude higher than that in a comparable slab of dielectric material. Furthermore, two distinct types of interaction are analyzed: multibunch, and bunch-by-bunch radiation, wherein integral transform methods are used. Multibunch Cerenkov radiation in a helix waveguide is used to show the identity with TWT interaction. It is subsequently shown that for open boundary inhomogeneous waveguides a TWT type analysis will give the same results as multibunch Cerenkov radiation based on integral transform methods. For single bunch or a bunch-by-bunch type of interaction in such structures where energy orthogonality and completeness of the eigenfunctions do not hold, an integral transform method is used. The underlying difference between bunch-by-bunch and multibunch radiation is discussed. The utility of Cerenkov radiation in a dielectric tube waveguide for the generation of coherent electromagnetic power in the millimeter range is pointed out. Numerical results are given to show the variation of power with respect to various parameters. Experimental results from a 0.8-Mev beam are shown to agree satisfactorily with theory.     

Collinear interaction of light with space-charge waves in a semiconductor waveguide

The dispersion equation for space-charge waves (SCWs) that couples the wave vector of a traveling wave with frequency and the parameters of a semiconductor waveguide layer is obtained. Numerical analysis of the co-and contradirectional SCW-optical interaction for guided TE and TM modes in a waveguide structure based on n-GaAs is performed. It is demonstrated that the power exchange between the TE modes is more effective than the power exchange between the TM modes. The intervals of the waveguide layer thickness with a relatively high efficiency of the mode transformation are determined.     

Surface waves of printed antennas on planar artificial periodicdielectric 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     

Field theory on excitation of space-charge waves through a gridless modulating gap

A novel field-theoretic approach is presented to treat the system composed of a drift tube, a gridless modulating gap and an electron beam. It is mentioned that no space-charge wave can propagate just to the front of the plane of discontinuity of the electronic waveguide. The field component is expressed by a Fourier integral, and the amplitude of any of the excited space-charge waves is calculated from the residue at the corresponding pole, in a way similar to the one used for a passive waveguide. The analysis is applicable to the case of an arbitrary field distribution over the gap, not only the single gap but also the multiple gap. It is shown that the ratio of the amplitude of any excited space-charge wave to that of the lowest is independent of the field distribution over the gap. Finally, the beam loading is derived from the standpoint of the space-charge wave theory and is compared with this by the ballistic theory.     

Fabrication and study of p-n structures with crystalline inclusions in the space-charge region

A new model of connecting elements for monolithic multijunction solar cells based on III–V compounds is proposed, in which p-n junctions with crystalline inclusions of a foreign semiconductor material in the space-charge region are used instead of p ⁺⁺-n ⁺⁺ tunnel junctions. The study shows that the introduction of crystalline inclusions to the space-charge region of the p-n junction in a GaSb-based structure allows current densities of ～50 A/cm² at an ohmic loss of ～0.01 Ω cm². The obtained characteristics of the connecting elements with crystalline inclusions show their applicability to multijunction solar cells for concentrated light conversion.     

Equivalence between circular and rectangular posts in guiding structures

An accurate approximation is found to the impedance of a circular-post diode mount in a millimetre-wave guiding structure, through determination of the equivalence between a circular post and a rectangular post in such structures.     

On focusing of surface waves on impedance guiding surfaces

The process of focusing of surface electromagnetic waves on impedance guiding surfaces is considered in the ray approximation with the help of a 2D model of a plane surface. An estimate of the influence of the wave properties of the field on this process is given.     

Benzocyclobutene wafer bonding for III-V nanophotonic guiding structures

An adhesive wafer bonding technique for the fabrication of nanophotonic guiding structures, the design of which consists of a III-V semiconductor core buried in a polymer matrix, is reported. The bonding was realised owing to benzocyclobutene. Nanostructures are perfectly embedded in the void-free matrix to form high density photonic circuits.