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A concept of achieving trifocal terahertz (THz) radiation is discussed based on the application of a periodic electric field in addition to two dark hollow laser beams impinging into collisional plasma. Nonlinear plasma medium is engineered for altering the dynamics of electrons under such a periodic electric field and obtaining an additional transverse component of nonlinear current. This additional component modifies the optics associated with the emitted radiation, leading to trifocal THz radiation. The parameters associated with the doughnut intensity of dark hollow laser beams, such as beamwidth and beam order, control the position and peak of the emitted THz field. Interestingly, the efficiency of the emitted THz field can also be tuned by appropriate combinations of the laser beam orders. [ABSTRACT FROM AUTHOR]

The linear and nonlinear theory of a plasma-filled relativistic Cerenkov generator with coaxial slow-wave structure (SWS) is developed. Linear analysis includes the space-charge limiting current, dispersion relations, dispersion curves, and coupling impedance. In nonlinear theory, the electron beam space-charge effect, nonsynchronous interaction, and complex reflection coefficients of the electromagnetic wave at the boundaries of the SWS are taken into account. With the assumption of quasi steady state, the self-consistent equations are numerically calculated using the fourth-order Runga-Kutta method. The effects of the relative phase between inner and outer corrugations, phase and amplitude of the complex reflection coefficient at the boundaries of the SWS, and plasma density on device efficiency are discussed. Efficiencies of up to 37% without plasma and about 59% with plasma density of np = 3-7 × 1011 cm-3 are obtained. [ABSTRACT FROM AUTHOR]

Transverse gaps are introduced in thin-wire conductors to extend the region of phase variations of currents that are induced on them by an incident electromagnetic wave. Periodically placed gaps lead to qualitative changes in the properties of reradiated waves, such as the appearance of resonances corresponding to total reflection and total transmission of the incident wave. It is shown that total reflection occurs in a narrow band and depends very weakly on the incident angle of the illuminating radiation. This idea leads to novel filters and switches, whose operation is practically independent of the structure of the incident wave. Total resonant reflection can occur for conductors with high optical transparency, a property that is useful for various microwave diagnostic devices. Because of the increased radiation resistance of such structures, the resonant effects under consideration depend weakly on conductivity. For gratings made using these conductors with gaps stacked above a metal ground plane, it is possible to controllably collimate the rays. Expansion of the scattered fields in terms of Rayleigh's series, together with Poisson's summation formula, are used for numerical simulations of such structures. [ABSTRACT FROM AUTHOR]

Arrays of ellipsoidal microcavities have been fabricated in glass by a micropowder blasting technique. Optical microscopy of the plasmas generated in arrays of these cavities provides evidence of coupling between each microplasma and its nearest neighbors. Scattering of incoherent atomic radiation from one microplasma into adjacent microcavities is clearly observed. After correcting for field-of-view distortion and saturation, the images also appear to reveal the perturbation of each microplasma by its neighbor in the form of an elongation of the emission distribution along the axis joining the two cavities. [ABSTRACT FROM AUTHOR]