Generation of terahertz radiation from ionizing two-color laser pulses in Ar filled metallic hollow waveguides

The generation of THz radiation from ionizing two-color femtosecond pulses propagating in metallic hollow waveguides filled with Ar is numerically studied. We observe a strong reshaping of the low-frequency part of the spectrum. Namely, after several millimeters of propagation the spectrum is extended from hundreds of GHz up to ~150 THz. For longer propagation distances, nearly single-cycle near-infrared pulses with wavelengths around 4.5 um are obtained by appropriate spectral filtering, with an efficiency of up to 0.25%.Comment: 6 pages, 3 figure

Some theorems on stability of discrete circulatory systems

Stability theorems established for static and dynamic losses in discrete, linear, elastic system

Hydrodynamic Model for Conductivity in Graphene

Based on the recently developed picture of an electronic ideal relativistic fluid at the Dirac point, we present an analytical model for the conductivity in graphene that is able to describe the linear dependence on the carrier density and the existence of a minimum conductivity. The model treats impurities as submerged rigid obstacles, forming a disordered medium through which graphene electrons flow, in close analogy with classical fluid dynamics. To describe the minimum conductivity, we take into account the additional carrier density induced by the impurities in the sample. The model, which predicts the conductivity as a function of the impurity fraction of the sample, is supported by extensive simulations for different values of ${\cal E}$, the dimensionless strength of the electric field, and provides excellent agreement with experimental data.Comment: 19 pages, 4 figure

Cluster-Growth in Freely Cooling Granular Media

When dissipative particles are left alone, their fluctuation energy decays due to collisional interactions, clusters build up and grow with time until the system size is reached. When the effective dissipation is strong enough, this may lead to the `inelastic collapse', i.e. the divergence of the collision frequency of some particles. The cluster growth is an interesting physical phenomenon, whereas the inelastic collapse is an intrinsic effect of the inelastic hard sphere (IHS) model used to study the cluster growth - involving only a negligible number of particles in the system. Here, we extend the IHS model by introducing an elastic contact energy and the related contact duration t_c. This avoids the inelastic collapse and allows to examine the long-time behavior of the system. For a quantitative description of the cluster growth, we propose a burning - like algorithm in continuous space, that readily identifies all particles that belong to the same cluster. The criterion for this is here chosen to be only the particle distance. With this method we identify three regimes of behavior. First, for short times a homogeneous cooling state (HCS) exists, where a mean-field theory works nicely, and the clusters are tiny and grow very slowly. Second, at a certain time which depends on the system's properties, cluster growth starts and the clusters increase in size and mass until, in the third regime, the system size is reached and most of the particles are collected in one huge cluster.Comment: 16 pages, 21 figures. Chaos 9(3) (in press, 1999

Torsional Instability of Cantilevered Bars Subjected to Nonconservative Loading

Cantilever bar torsional instability under nonconservative compression loadin

On the stability of equilibrium of continuous systems Technical report no. 65-1

Stability of equilibrium of linear elastic continuum - Galerkin metho