Modulation instability in filamentary self-compression

We numerically analyze filamentary propagation for various media and input pulse parameters and show that temporal self-compression can greatly benefit from refocusing events. Analyzing the dynamical behavior in the second focal spot, it turns out that a dispersive temporal break-up may appear due to the emission of a hyperbolic shock wave from the self-steepened trailing edge of the pulse. This break-up event in the refocus enhances the self-compression capabilities of laser filaments, enabling up to 12-fold temporal compression. Only slightly perturbing the input pulse parameters, we further identify a regime in which refocusing events give rise to extended subdiffractive propagation in a weakly ionized channel.     

Electron-beam instability in left-handed media

We predict that two electron beams can develop an instability when passing through a slab of left-handed media (LHM). This instability, which is inherent only for LHM, originates from the backward Cherenkov radiation and results in a self-modulation of the beams and radiation of electromagnetic waves. These waves leave the sample via the rear surface of the slab (the beam injection plane) and form two shifted bright circles centered at the beams. A simulated spectrum of radiation has well-separated lines on top of a broad continuous spectrum, which indicates dynamical chaos in the system. The radiation intensity and its spectrum can be controlled either by the beams' current or by the distance between the two beams.     

Nonlinear dynamics of breathing current filaments inn-GaAs andp-Ge

A novel model is presented for spatio-temporal pattern formation in semiconductors. It leads to self-generated nonlinear current oscillations due to “breathing” current filaments in the regime of impurity impact ionization. The four qualitatively different regimes which have been observed in Ge with increasing current are consistently explained as: a stationary nonconducting state; bulkdominated oscillations; breathing filaments; stable filaments. The physical origin of the breathing oscillations is impact ionization coupled with transverse diffusion and longitudinal dielectric relaxation. A method is developed to derive simple nonlinear dynamic equations for the filament radius and the position of the peak transverse electric field by a nonlinear mode expansion.     

Impact ionization induced negative far-infrared photoconductivity inn-GaAs

Far-infrared photoconductivity ofn-GaAs epitaxial layers showing impact ionization breakdown has been investigated by molecular lasers at photon energies below the 1s-2p shallow donor transition energy. Negative photoconductivity was observed if a magnetic field was applied to the crystals and if impact ionization of donors by the electric bias field was the dominant electron excitation mechanism. The experimental results are qualitatively explained on the basis of the generation-recombination kinetics of electrons bound to donors. Negative photoconductivity is attributed to optically induced free to bound transitions of electrons from theN=0 Landau band to donor levels shifted by the magnetic field above the low energy edge of the conduction band.     

Rotational instability of filamentary crystals undergoing plastic deformation

We investigated the kinetics of spontaneous torsion in filamentary crystals during plastic deformation by a tensile load. The nature of the observed effect is discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 76–81, August, 1988.     

Plasma induced pulse breaking in filamentary self-compression

A plasma induced temporal break-up in filamentary propagation has recently been identified as one of the key events in the temporal self-compression of femtosecond laser pulses. An analysis of the Nonlinear Schrödinger Equation coupled to a noninstantaneous plasma response yields a set of stationary states. This analysis clearly indicates that the emergence of double-hump, characteristically asymmetric temporal on-axis intensity profiles in regimes where plasma defocusing saturates the optical collapse caused by Kerr self-focusing is an inherent property of the underlying dynamical model.     

Causes of the filamentary instability of optical-beam self-channeling in photopolymerizable composites

The model of the formation of filamentary inhomogeneities in the waveguide structures formed upon opticalbeam self-channeling in photopolymerizable composites is constructed and numerically studied. The conditions for the formation of a homogeneous waveguide channel are determined and confirmed by the experimental results.     

Matter transport in pure gold induced by high direct current densities

In a new investigation the temperature dependence of electromigration in gold was measured by applying an improved tracer method. The results are in good agreement with those of Gilder and Lazarus. In the temperature range between 1137K and 1235K the effective charge varies from ?7.7 e to ?6.7 e. However, the linear relation betweenz _eff and 1/?, predicted from theory, could not be confirmed unequivocally. The activation enthalpy of electromigration,Q _E =42.3±1.1 kcal/mole, is practically identical to that for self-diffusion. A temperature independent value was found for the residual resistivity of the thermally activated ion-vacancy complexes, ? _d =1.76 µΩ cm/at-%, which is close to the residual resistivity of vacancies in gold. In addition, our earlier investigations are reviewed, in which electromigration in gold was measured using a marker method and a tracer method simultaneously. The results are critically examined in connexion with a theory proposed by Guy.     

Hysteresis losses of fine filamentary superconductors including field dependent critical current density

Using the force versus displacement curve for the flux lines, the hysteresis losses of thin superconducting slabs in transverse magnetic field were calculated. Especially, Kim's model for the field dependence of the critical current densityJ _c 1/(B _m+B _o) was used (B _m- the applied field,B _o = const.). The results are compared with the corresponding model usingJ _c = const., as well as with the critical state model without the force versus displacement curve. It is shown that the losses per volume are always decreasing with decreasing slab thickness. In the case of large background fieldB ₀ and small amplitude of the ripple fieldb ₀, the corrections are small with respect to the caseJ _c= const. Without background field, the hysteresis losses are somewhat larger than for field independentJ _c. Therefore, in accordance with our previous paper, the strong decrease of hysteresis losses with decreasing filament diameter in lower magnetic fields cannot be explained by the reversible motion of flux lines in the superconductors. It is suggested that this strong decrease is connected with the reversible motion of flux not connected with flux lines in superconductors, the diameter of which is comparable with the London penetration depth .