Optical-to-THz wave conversion via excitation of plasma oscillations in the tunneling-ionization process

A new ("linear-parametric") mechanism of a direct conversion of an ultrashort laser pulse into terahertz radiation is suggested. The conversion is due to the ionization-induced excitation and the subsequent electromagnetic emission of the superluminous polarization wave created by the axicon-focused laser pulse. For a few-cycle pulse with an optimum carrier-envelope phase, the considered mechanism is found to be much more effective than the alternative one based on the excitation of plasma oscillations in the laser wakefield by the ponderomotive force and able to provide THz radiation of the gigawatt power level with the use of moderate optical intensity (approximately 10(14)-10(15) W/cm2).     

Generation of strong Langmuir fields during optical breakdown of a dense gas

Results are presented from theoretical studies and computer simulations of the resonant excitation of Langmuir waves during the ionization of a homogeneous gas by high-intensity laser radiation. Two mechanisms for the formation of nonuniform resonant structures in the discharge are examined: plasma-resonance ionization instability, resulting in the density modulation along the electric field vector, and gas breakdown in the field of a transversely inhomogeneous laser beam (a Bessel beam produced by an axicon lens). In both cases, the transition of the plasma density through the critical value is accompanied by the generation of intense Langmuir waves, the formation of fast ionization fronts, and the appearance of long-lived quasi-turbulent states.     

Differential inhibition of postnatal brain,spinal cord and body growth by a growth hormone antagonist

Background  

Growth hormone (GH) plays an incompletely understood role in the development of the central nervous system (CNS). In this study, we use transgenic mice expressing a growth hormone antagonist (GHA) to explore the role of GH in regulating postnatal brain, spinal cord and body growth into adulthood. The GHA transgene encodes a protein that inhibits the binding of GH to its receptor, specifically antagonizing the trophic effects of endogenous GH.     

Dipole resonances of an ionized cluster

We study the resonance properties of an ionized spherical cluster that interacts with an optical radiation field in terms of linear hot-plasma polarizability models. Based on a generalization that includes spatial dispersion in the well-known Mie problem of the diffraction of a plane wave by a small-size plasma sphere, we calculate the eigenfrequencies, the radiative and collisional damping constants, and the resonance amplitudes of the cluster surface and bulk plasmon fields. The role of collisionless dissipation processes is analyzed in terms of a one-dimensional kinetic model. The latter allows the corresponding damping constants for both types of plasmons to be determined as functions of the characteristic electron-plasma boundary collision frequency. We show that both types of plasmons in certain domains of cluster and external-radiation parameters can undergo a strong resonance that causes both the amplitude of the scattered wave and the absorbed power and the field inside the cluster to increase significantly.     

Effect of three-body forces on the lattice dynamics of noble metals

A simple method to generate an effective electron-ion interaction pseudopotential from the energy wave number characteristic obtained by first principles calculations has been suggested. This effective potential has been used, in third order perturbation, to study the effect of three-body forces on the lattice dynamics of noble metals. It is found that three-body forces, in these metals, do play an important role. The inclusion of such three-body forces appreciably improves the agreement between the experimental and theoretical phonon dispersion curves.     

Adiabatic frequency up-conversion of a powerful electromagneticpulse producing gas ionization

The theory of strong frequency upconversion of the powerful ionizing electromagnetic radiation in gases is presented based on the modified nonlinear geometrical optics approximation. The permanent spectrum upshift versus propagation path, exceeding considerably the initial frequency, is demonstrated without strong wave dissipation for the cases of impact and field-induced ionization in the high-intensity field range. Reflectionless propagation into the supercritical plasma and broad-band tuning of the laser radiation are emphasized as highly promising physical applications of the phenomenon described     

Generation of terahertz radiation upon the optical breakdown of a gas

Terahertz radiation of plasma oscillations excited upon the optical (axicon) breakdown of a gas in the presence of external fields of other frequency ranges has been analyzed. It has been shown that the spectra and intensity of oscillations and radiation generated by them depend strongly on the character of the spatiotemporal evolution of the formed plasma. The intensity is maximal upon the rapid formation of the plasma with a sharp boundary and decreases to very low values for objects with a smooth density profile. New schemes and regimes of break-down have been proposed, which make possible, according to preliminary estimates, an emitted energy of about 10 mJ with moderate intensities of ionizing laser radiation.     

Nonlinear Dynamics of Wave Beams and Packets in an Ionizable Medium

We present a review of the studies on nonlinear dynamics of the plasma–field system formed in the processes of breakdown of a gas by high-intensity laser or microwave radiation. The ionization instability dominating these processes significantly modifies the known effects of self-action of waves in a medium and gives rise to a number of new effects which are absent for other nonlinearity mechanisms. We describe the most important among these effects, such as the ionization–field instability of a plane wave, the self-channeling of radiation in the form of surface or leaky waves, and the self-conversion of the spectrum of the ionizing radiation. The results of numerical simulations of the dynamics of nonequilibrium freely-localized discharges created by focused microwave and laser pulses are presented.