Propagation regimes of intense circularly polarized laser beam in magnetoactive plasma

This paper presents an analytical and numerical investigation of an intense circularly polarized wave propagating along the static magnetic field parallel to oscillating magnetic field in magnetoactive plasma. In the relativistic regime such a magnetic field is created by pulse itself. The authors have studied different regimes of propagation with relativistic electron mass effect for magnetized plasma. An appropriate expression for dielectric tensor in relativistic magnetoactive plasma has been evaluated under paraxial theory. Two modes of propagation as extraordinary and ordinary exist; because of the relativistic effect, ultra-strong magnetic fields are generated which significantly influence the propagation of laser beam in plasma. The nature of propagation is characterized through the critical-divider curves in the normalized beam width with power plane For given values of normalized density (ω_p/ω) and magnetic field (ω_c/ω) the regions are namely steady divergence (SD), oscillatory divergence (OD) and self-focusing (SF). Numerical computations are performed for typical parameters of relativistic laser-plasma interaction: magnetic field B = 10-100 MG; intensity I = 10¹⁶ to 10²⁰ W/cm²; laser frequency ω = 1.1 × 10¹⁵ s⁻¹; cyclotron frequency ω_c = 1.7 × 10¹³ s⁻¹; electron density n_e = 2.18 × 10²⁰ cm⁻³. From the calculations, we confirm that a circularly polarized wave can propagate in different regimes for both the modes, and explicitly indicating enhancement in wave propagation, beam focusing/self-guiding and penetration of E-mode in presence of magnetic field.     

Absorption and second harmonic emission from interaction of femtosecond laser pulses with microspherical droplets

In this paper, the interaction of femtosecond laser pulses with droplets microplasma at the intensity of 10¹⁶ W/cm² is theoretically studied. Laser absorption, suprathermal electron generation, and second harmonic generation are discussed. Using an analytical model and a 2D particle-in-cell code, we find that the dominated mechanism is resonant absorption in the interaction of femtosecond laser pulses with droplets for the misrospherical geometry.     

Modulation instability of intense laser beam in an electron-positron plasma

Modulation instability of an intense right-hand elliptically polarized laser beam propagating through an electron-positron plasma is investigated by a new method. The nonlinear dispersion relation, in which the relativistic and ponderomotive nonlinearities are taken into account, is obtained for the laser radiation in electron-positron plasma by the Lorentz transformation. The Karpman equation is generalized to the case of three dimensions with three field components. When the nonlinear frequency shift of the electromagnetic field in plasma is involved, the nonlinear evolution equation for the slowly varying envelope of the laser field is obtained. Thus, modulation instability of the intense laser beam in electron-positron plasma is studied and the temporal growth rate of the instability is derived. The analysis shows that the growth rate of modulation instability is increased significantly near the critical surface in a laser-plasma.     

Modulation instability by intense laser beam in magnetized plasma

Modulation instability of an intense right-hand elliptically polarized laser beam propagating through magnetized plasma is investigated by a new method. The nonlinear dispersion relation, in which the relativistic and ponderomotive nonlinearities are taken into account, is obtained for the laser radiation in magnetized plasma by the Lorentz transformation. The Karpman equation is firstly generalized to the case of three dimensions with three field components. When the nonlinear frequency shift of the electromagnetic field in plasma is involved, the nonlinear evolution equation for the slowly varying envelope of the laser field is obtained. Thus, modulation instability of the intense laser beam in magnetized plasma is studied and the temporal growth rate of the instability is derived. The analysis shows that the peak growth rate of self-modulation instability is increased due to the axial magnetization of plasma. It is also shown that the growth rate of modulation instability is increased significantly near the critical surface in a laser-plasma.     

Theory of spin precession monitored by laser pulse

We first predict the splitting of a spin degenerate impurity level when this impurity is irradiated by a circularly polarized laser beam tuned in the transparency region of a semiconductor. This splitting, which comes from different exchange processes between the impurity electron and the virtual pairs coupled to the pump beam, induces a spin precession around the laser beam axis, which lasts as long as the pump pulse. It can thus be used for ultrafast spin manipulation. This effect, which has similarities with the exciton optical Stark effect we studied long ago, is here derived using the concepts we developed very recently to treat many-body interactions between composite excitons and which make the physics of this type of effects quite transparent. They, in particular, allow to easily extend this work to other experimental situations in which a spin rotates under laser irradiation.     

Chain propagator, mass, and universality in polymer solutions from Brownian relativity

A Lagrangian theory for single chains in polymer solutions is addressed via a recent Brownian relativity. By employing generalized diffusive coordinates, statements of covariance and diffusivity invariance result into free particle Lagrangians, where mass turns out to rise as a universal spacetime property. It descends from lowering diffusivity (or curving spacetime), so identifying a mechanism which conceptually resemble those ruling macromolecular scaling laws. An extended chain propagator recovers the Gaussian end-to-end distribution and, in the limits of time-like and space-like orbits, the dualism for diffusive paths and polymer random-walks.     

Synthesis of ZnO nanowires by pulsed laser deposition in furnace

ZnO nanowires were fabricated on Au coated (0 0 0 1) sapphire substrates by using a pulsed Nd:YAG laser with a ZnO target in furnace. ZnO nanowires have various sizes and shapes with a different substrate position inside a furnace. The length and the diameter of these ZnO nanowires were around 3-4 μm and 120-200 nm, respectively, confirmed by scanning electron microscopy (SEM). The diameter control of the nanowires was achieved by varying the position of substrates. The ultraviolet emission of nanowires from the near band-edge emission (NBE) was observed at room temperature. The formation mechanism and the effect of different position of substrates on the structural and optical properties of ZnO nanowires are discussed.     

Enhanced production of fast multi-charged ions from plasmas formed at cleaned surface by femtosecond laser pulse

We present atomic, energy, and charge spectra of ions accelerated at the front surface of a silicon target irradiated by a high-contrast femtosecond laser pulse with an intensity of 3×10¹⁶ W/cm², which is delayed with respect to a cleaning nanosecond laser pulse of 3-J/cm² energy density. A tremendous increase in the number of fast silicon ions and a significant growth of their maximum charge in the case of the cleaned target from 5+ to 12+ have been observed. The main specific features of the atomic, energy, and charge spectra have been analyzed by means of one-dimensional hydrodynamic transient-ionization modeling. It is shown that fast highly charged silicon ions emerge from the hot plasma layer with a density a few times less than the solid one, and their charge distribution is not deteriorated during plasma expansion.This revised version was published online in August 2005 with a corrected cover date.     

Spectroscopic analysis of femtosecond laser plasma filament in air

We report a spectroscopic analysis of a filament generated by a femtosecond laser pulse in air. In the filament spectra, the characteristic Stark broadened atomic oxygen triplet centered at 777.4 nm has been observed. The measured electron impact Stark broadening parameter of the triplet is larger than the theoretical value by Griem [H.R. Griem, Plasma Spectroscopy, McGraw Hill, New York, 1964] by a factor 6.7. Using the experimental value , the plasma densities derived from Stark broadening agree well with those most recently obtained from Théberge et al.’s measurement of the nitrogen fluorescence calibrated by longitudinal diffraction [F. Théberge, W. Liu, P.T. Simard, A. Becker, S. L. Chin, Phys. Rev. E 74 (2006) 036406]. However, the Stark broadening approach is much simpler and can be used to non-invasively measure the filament plasma density distribution in air under different propagation conditions.     

Comparison of two theories during self-focusing of Gaussian laser beam in thermal conduction-loss predominant plasmas

In the present paper, self-focusing phenomenon occurring as a result of non-linear interaction of intense laser beam with thermal conduction-loss predominant plasmas is studied by following both approaches viz. paraxial theory approach and moment theory approach. Non-linear differential equations for the beam width parameters of laser beam have been set up and solved numerically in both cases to study the variation of beam width parameters with normalized distance of propagation. Effects of laser intensity as well as plasma density on the beam width parameters have also been analyzed. It is observed from the analysis that in case of moment theory approach, strong self-focusing of laser beam is observed as compared to paraxial theory approach.     

Nonideality of Au/Si and Au/GaAs Schottky barriers due to process-induced defects

A mechanism of local lowering of the Schottky barrier height (SBH) is proposed, which causes nonideality in nearly ideal Au/n-Si and Au/n-GaAs Schottky barriers. Positively ionized defects generated by the process very close to the interface induce electrons in the metal-induced gap states (MIGS) and lower the SBH locally. The spatial density distribution of the ionized defects obtained from the SBH distribution is determined by the unique interaction with the MIGS. The defects are considered to have the negative-U property and are neutralized at very close positions to the MIGS. The potential distributions close to the interface have a considerable potential drop due to the large defect density. These inhomogeneous potentials are coincident with the energy level scheme of the defect identified as the defect causing the nonideality. This defect is Si self-interstitial in Au/Si SB, and As antisite in Au/n-GaAs SB. This MIGS with process-induced defect model supersedes the previously proposed two major Fermi level pinning models. The mystery of the T₀ effect is solved. The thermionic-field emission current taking place in the strong electric field has influence on the I-V characteristics at low temperatures. Regarding the C-V characteristics of Au/Si SB, the observed extra capacitance under the forward bias is an experimental evidence in accordance with the proposed model.     

Temperature dependence of excitonic emission in cubic CdSe thin film

A detailed photoluminescence investigation of the thermal redshift and broadening of the excitonic line of cubic CdSe film grown by molecular beam epitaxy is presented. Free excitonic emission from the cubic CdSe film was observed at low temperature. Temperature-dependent measurement was performed to obtain material parameters related to exciton-phonon interaction by fitting the experimental data to the phenomenological model. The relative contribution of both acoustic and optical phonon to the band gap shrinkage and exciton linewidth broadening are discussed. Exciton binding energy of 16±1.5 meV was determined from the Arrhenius analysis.     

Hard X-ray generation from microdroplets in intense laser fields

Microdroplets of 15-μm diameter are subjected to ultra-short laser pulses of intensities up to 10¹⁵Wcm⁻² to produce hot dense plasma. The hot electrons produced in the microdroplet plasma result in efficient generation of hard X-rays in the range 50–150keV at an irradiance as low as 8×10¹⁴Wcm⁻². The X-ray source efficiency is estimated to be about 2 ×10⁻⁷%. A prepulse that is about 11ns ahead of the main pulse strongly influences the droplet plasma and the resulting X-ray emission. For a similar laser prepulse and intensity, no measurable hard X-ray emission is observed when the laser is focused on a solid target of similar composition and this indicates that liquid droplet targets are best suited for hard X-ray generation in laser–plasma interactions.     

Nonlinear mixing between the emission of a cw laser and a femtosecond laser

We mix the emission of a femtosecond Ti:sapphire laser with the emission of a continuous wave infrared laser in a beta-barium borate crystal. Green light with a center wavelength of 527 nm and a spectral width of 2.5 nm resulting from sum frequency generation is detected. An intensity study verifies that a nonlinear χ⁽²⁾ process is at the origin of the green light generation. The experimentally obtained conversion efficiency of 7 × 10⁻¹⁰ is in good agreement to simple theoretical considerations.     

Incomplete normalization of probability on multifractals

This work is an extension of the incomplete probability theory from the simple case of monofractals previously studied to the more general case of multifractals that can occur in the phase space without equiprobable partition.     

Physical properties of double perovskite compounds ALaVMoO6 (A=Ca, Sr, Ba)—a possible half-metallic antiferromagnetic system

Double perovskite compounds ALaVMoO₆ (A=Ca, Sr, Ba) have been synthesized and their electrical and magnetic properties have been investigated. Magnetization measurements have indicated the possible antiferromagnetic transitions at 120 and 130 K for A=Ca and Sr samples, respectively. Electrical resistivity ρ for this system shows metallic temperature dependence from 300 to 20 K, though the sample with A=Ca shows weak semiconducting behavior in the low temperature region (<70 K). Considering the magnetic and electrical properties and assuming the V³⁺S=1 and Mo⁴⁺S=1 valence and spin states, the samples with A=Ca and Sr can be promising candidates for half-metallic antiferromagnets.     

Study of electronic states for V thin films deposited on 6H-SiC substrates by soft X-ray emission spectroscopy

Silicon carbide (SiC) is a candidate material for electronic devices to operate upon crucial environment. Electronic states of silicides and/or carbide/graphite formed in metal/SiC contact system is fundamentally important from the view point of device performance.We study interface electronic structure of vanadium (V) thin-film deposited on 6H-SiC(0 0 0 1) Si-face by using a soft X-ray emission spectroscopy (SXES). For specimens of V(38 nm)/6H-SiC (substrate) contact systems annealed at 850 °C, the Si L_2,3 emission spectra indicate different shapes and peak energies from the substrate. The product of materials such as silicides and/or ternary materials is suggested. Similarly, the C Kα emission spectra show the shape and peak energy characteristic of vanadium carbide including substrate 6H-SiC signal.     

Schottky barrier properties of various metal (Zr, Ti, Cr, Pt) contact on p-GaN revealed from I-V-T measurement

Schottky barrier contact using three different metal (Zr, Ti, Cr and Pt) and Ohmic contact using Ni were made on same epitaxial growth layer of p-GaN. Measurements were carried out using current-voltage-temperature (I-V-T) in the range of 27-100°C. Under forward bias and room-temperature (RT), the ideality factors (η) were determined to be 2.38, 1.82, 1.51 and 2.63, respectively, for Zr, Ti, Cr and Pt. The Schottky barrier height (SBH) and effective Richardson coefficient A^** were measured through modified Norde plot as one of the analysis tools. Barrier heights of 0.84, 0.82, 0.77 and 0.41 eV for Zr, Ti, Cr and Pt, respectively, were obtained from the modified Norde plot. Schottky barrier heights of Zr, Ti, or Cr/p-GaN were also measured through activation energy plot, and determined to be in the same range (∼0.87 eV) and Pt at 0.49 eV. These results indicate that the Fermi level seems to be pinned due to the value of slope parameter (S) was very low (S = −0.25).     

Production of nanoparticles of different materials by means of ultrashort laser pulses

Ultrashort pulsed laser ablation in vacuum of different targets was performed in order to investigate the possibility of producing nanoparticles with controlled size and shape. A systematic morphology characterization of deposited products was performed for nickel and silicon as a function of laser pulse intensity and wavelength, at a fixed pulse repetition rate. The nanoparticles were investigated by atomic force microscopy, and clear trends for their size and shape anisotropy were evidenced. The best conditions to obtain nanosized particles of oblate ellipsoidal shape, with the minor axis below 10 nm, were determined in the case of nickel targets. Our results show that ultrashort pulse laser deposition can be considered as an interesting technique for the tailoring of nanogranular films with the desired particles dimension and shape, according to the peculiar properties required in specific applications. Moreover, the preliminary features are very promising from the point of view of the production of magnetoresistive films with specific anisotropy.     

Kinetic theory of radiation in non-equilibrium relativistic plasmas

Many-particle QED is applied to kinetic theory of radiative processes in many-component plasmas with relativistic electrons and non-relativistic heavy particles. Within the framework of non-equilibrium Green’s function technique, transport and mass-shell equations for fluctuations of the electromagnetic field are obtained. We show that the transverse field correlation functions can be decomposed into sharply peaked (non-Lorentzian) parts that describe resonant (propagating) photons and off-shell parts corresponding to virtual photons in plasmas. Analogous decompositions are found for the longitudinal field correlation functions and the correlation functions of relativistic electrons. As a novel result a kinetic equation for the resonant photons with a finite spectral width is derived. The off-shell parts of the particle and field correlation functions are shown to be essential to calculate the local radiating power in relativistic plasmas and recover the results of vacuum QED. The influence of plasma effects and collisional broadening of the relativistic quasiparticle spectral function on radiative processes is discussed.