Growth and stabilization of a laser ripple on laser beam in a collisionless unmagnetized plasma

Summary In this paper we have studied the growth and stabilization of a radially symmetric ripple superimposed on a laser beam in a collisionless unmagnetized plasma. The saturation of the growth rate is due to pump depletion effect. On acoount of the nonuniform intensity distribution of the main beam, a d.c. component of the ponderomotive force becomes finite and it redistributes the carriers in the plasma in a plane transverse to the beam propagation. We have set up and solved the wave equations for main beam and ripple by using the WKB and paraxial ray approximations. As the ripple grows in the plasma, it draws energy from the background laser field. It is seen that the relative intensity of the laser ripple becomes critical due to the counteracting behaviour of the self-focusing and depletion of the main beam. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Nonlinear oscillatory waveguide propagation in a plasma

The presence of an intense Gaussian laser beam gives rise to a ponderomotive force on electrons in a collisionless plasma, leading to a redistribution of electron density along the wave-front and consequently to an intensity dependent dielectric constant which saturates with increasing intensity. The intensity dependent dielectric constant is responsible for beam propagation in an oscillatory waveguide. It is seen that (i) a beam of radiusr ₀ less thanr _0min (?c/ω _p) cannot be focused in the plasma regardless of its power, (ii) minimum dimension of oscillatory waveguide increases with increasing power of the beam. Similar results are also obtained for collisional plasma where nonlinearity arises due to nonuniform heating and consequent redistribution of carriers.     

Growth of a laser ripple in a magnetoplasma and its effect on plasma wave excitation

Summary This paper presents an investigation of the growth of a radially symmetrical ripple, superimposed on a Gaussian laser beam in a collisionless magnetoplasma. The effect of the magnetic field and the intensity of the laser on the growth of the ripple is presented in some detail. The effect of the presence of the ripple on the excitation of an electron plasma wave is also investigated. Coupling of a weak plasma wave with the main laser beam is through the modified background density. The combined effect of increased intensity of the laser beam and magnetic field is observed to suppress the growth of the ripple as well as the excitation of the plasma wave. The authors of this paper have agreed to not receive the proofs for correction.     

Dynamics of Filament Formation in Laser Produced Collisional Magnetoplasma

The paper presents an investigation of growth dynamics of spike in a collisional magneto plasma in the presence of externally applied magnetic field. We have set up and solved the non‐linear differential equations for growth and width of the laser spikes. We have considered the mechanism of nonuniform heating of carriers along the wavefront which is important in collisional plasmas. The effect of intensity of main beam and magnetic field on growth dynamics of spikes is studied (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Absolute parametric instability in a nonuniform plane plasma waveguide

The paper reports an analysis of the effect of spatial plasma nonuniformity on absolute parametric instability (API) of electrostatic waves in magnetized plane waveguides subjected to an intense high-frequency (HF) electric field using the separation method. In this case the effect of strong static magnetic field is considered. The problem of strong magnetic field is solved in 1D nonuniform plane plasma waveguide. The equation describing the spatial part of the electric potential is obtained. Also, the growth rates and conditions of the parametric instability for periodic and aperiodic cases are obtained. It is found that the spatial nonuniformity of the plasma exerts a stabilizing effect on the API. It is shown that the growth rates of periodic and aperiodic API in nonuniform plasma are less compared to that of uniform plasma.     

Effect of rippled laser beam on excitation of ion acoustic wave

Growth of a radially symmetrical ripple, superimposed on a Gaussian laser beam in collisional unmagnetised plasma is investigated. From numerical computation, it is observed that self-focusing of main beam as well as ripple determine the growth dynamics of ripple with the distance of propagation. The effect of growing ripple on excitation of ion acoustic wave (IAW) has also been studied.     

Enhanced Raman Scattering of Elliptical Laser beam in a Collisional Plasma

This paper presents the Enhanced Raman scattering of a elliptical laser beam in a collisional plasma. We have considered the mechanism of non‐uniform heating of carriers along the wave‐front, which is important in collisional plasma. The nonlinearity arising through non‐uniform heating leads to redistribution of carriers, which modifies the background plasma density profile in a direction transverse to pump beam axis. This modification in density effects the incident laser beam, plasma wave and back‐scattered beam. Non‐linear differential equations for the beam width parameters of pump laser beam, plasma wave and back‐scattered beam are set up and solved numerically. Numerical results predict the effect of self‐focusing of waves on the back‐scattered beam (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electromagnetic wave generation in a cylindrical resonator by electrons orbiting in a radial electrostatic field

The mechanisms leading to the generation of microwaves by electrons orbiting in a radial electrostatic field produced by a positively charged filament on the axis of a cylindrical resonator are investigated theoretically. The dispersion relations describing the interaction of the waves with the electrons are obtained. It is shown that the generation of electromagnetic fields is possible on account of both Cherenkov and plasma resonances. The frequencies and growth rates of waves under Cherenkov resonance conditions and also plasma resonance conditions in uniform and nonuniform electron layers are found. The advantages and disadvantages of different generation mechanisms are determined. Zh. Tekh. Fiz. 68, 91–94 (August 1998)     

Enhancement of self focusing of laser beams in a degenerate magnetoactive solid state plasma

This paper presents an investigation of the enhancement of self focusing of high frequency EM waves in a compensated degenerate magnetoactive solid state plasma. The nonlinearity in the dielectric constant responsible for self focusing arises due to nonuniform heating and consequent redistribution Of carriers. A rigorous kinetic treatment valid beyond the perturbation limit has been presented assuming the carrier relaxation time to be energy dependent. The results show that the self-focusing of extraordinary waves is enhanced while that of ordinary waves is reduced with $\text{ω}{}_{\mathrm{c}}\text{ω}$ increasing. However the focusing of extraordinary waves is not possible when ω_c > ω. The effect of degeneracy is also found to enhance the self-focusing of the waves while that of the enhanced absorption is to suppress the self-focusing process.     

Magnetic field generated in a plasma by a short, circularly polarized laser pulse

We study the generation of a quasistatic magnetic field by a short, circularly polarized laser pulse in a tenuous cold uniform plasma. It is shown that two physical mechanisms are responsible for the generation of the various components of the magnetic field. One mechanism is due to the ponderomotive forces and governs the generation of the azimuthal component of the magnetic field. The other is similar to the inverse Faraday effect (IFE) in a nonuniform plasma and gives rise to axial and radial components of the magnetic field. At moderate radiative intensities, all magnetic field components are proportional to the squared intensity. The spatial structure of the magnetic field depends strongly on the pulse shape and the plasma density. Zh. éksp. Teor. Fiz. 114, 849–863 (September 1998)     

Growth of a Gaussian Laser Ripple on a Gaussian Beam in a Collisionless Magnetoplasma and its Effect on the Excitation of Ion-Acoustic Wave

This paper presents an investigation of the growth of a radially symmetrical ripple, superimposed on a Gaussian laser beam in a collisionless magnetoplasma. Here we have presented the effect of magnetic field and the intensity of the laser beam on the growth of ripple in some detail. The effect of presence of ripple on the excitation of an ion-acoustic wave is also investigated. Coupling of a weak ion-acoustic wave with main laser beam is through modified background density. Interesting feature of the analysis is that the combined effect of increased intensity of the laser beam and magnetic field is observed to suppress the growth of the ripple as well as the excitation of the ion-acoustic wave.     

Absolute parametric instability of low-frequency waves in a 2D nonuniform anisotropic warm plasma

Using the separation method, absolute parametric instability (API) of electrostatic waves in a magnetized pumped warm plasma is investigated. In this case the effect of static strong magnetic field is considered. The problem of strong magnetic field is solved in two-dimensional (2D) nonuniform plane plasma. Equations which describe the spatial part of the electric potential are obtained. Also, the growth rates and conditions of the parametric instability for periodic and aperiodic cases are obtained. It is found that the spatial nonuniformity of the plasma exerts a stabilizing effect on the API. It is shown that the growth rates of periodic and aperiodic API in warm plasma are less when compared to that in cold plasma.     

Redistribution of photogenerated carriers in neutral and charged InAs quantum dot systems

Time-integrated and time-resolved photoluminescence spectra of neutral and negatively charged self-assembled InAs quantum dots (QDs) were studied. Obtained spectra have indicated that the redistribution of carriers in QDs occurs in all samples, but the temperature dependence of spectra are quite different for neutral and charged QDs. To clarify the origin of these behaviors, a model calculation based on two possible redistribution mechanisms has been carried out, and compared with experiments to show that the carrier tunneling between neighboring QDs is suppressed in charged QDs.     

Mode-coupling assisted electron accelerations by a plasma wave

The acceleration of electrons by a plasma wave in the presence of density ripple in plasma has been investigated. Plasma density ripple can excite higher harmonics of different phase velocities of the fundamental plasma wave. The combined role of the different harmonics of the plasma wave contributes significantly in electrons energy gain during acceleration by the fundamental plasma wave. Our calculation shows that the plasma electrons gain considerable energy during the acceleration by the plasma waves in the presence of a density ripple in plasma. The initial electron energy and the ripple density play an important role for the acceleration of an electron.     

Terahertz radiation generation by beating of two chirped laser pulses in a warm collisional magnetized plasma

Analytical equations of terahertz(THz) radiation generation based on beating of two laser beams in a warm collisional magnetized plasma with a ripple density profile are developed. In this regard, the effects of frequency chirp on the field amplitude of the terahertz radiation as well as the temperature and collision parameters are investigated. The ponderomotive force is generated in the frequency chirp of beams. Resonant excitation depends on tuning of the plasma beat frequency,magnetic field frequency, thermal velocity, collisional frequency, and effect of the frequency chirp with the plasma density.For optimum parameters of frequency and temperature the maximum THz amplitude is obtained.     

Two building blocks of microwave photonics filters in the presence of group delay ripple: a comparative survey

We have developed an analytic approach to investigate the effect of group delay ripple of the dispersive devices on the performance of two major building blocks of microwave-photonic filters. Firstly, performance of PM-based block in the presence of an arbitrary group delay ripple (GDR) is analyzed and compared with the ripple-free case to reveal the destructive effects of added group delay ripple. In the next step, we repeat the proposed approach for the AM-based one; again, the performance is compared with the ripple-free case. Two distortion metrics are also introduced to quantify this distortion. Comparison of the performance of two building blocks in the presence of group delay ripple unveils some interesting characteristics of microwave-photonic filters which have not been mentioned so far. We also add a general survey of two analyzed building blocks to present their respective most significant advantages and shortcomings. The simulated Optisystem results conform to our proposed analytical approach and verify the theoretical model.     

Relativistic nonlinearity and wave-guide propagation of rippled laser beam in plasma

In the present paper we have investigated the self-focusing behaviour of radially symmetrical rippled Gaussian laser beam propagating in a plasma. Considering the nonlinearity to arise from relativistic phenomena and following the approach of Akhmanov et al, which is based on the WKB and paraxial-ray approximation, the self-focusing behaviour has been investigated in some detail. The effect of the position and width of the ripple on the self-focusing of laser beam has been studied for arbitrary large magnitude of nonlinearity. Results indicate that the medium behaves as an oscillatory wave-guide. The self-focusing is found to depend on the position parameter of ripple as well as on the beam width. Values of critical power has been calculated for different values of the position parameter of ripple. Effects of axially and radially inhomogeneous plasma on self-focusing behaviour have been investigated and presented here.     

Surface instability and pattern formation by ion-induced erosion and mass redistribution

The contribution of curvature dependent sputtering and mass redistribution to ion-induced self-organized formation of periodic surface nanopatterns is revisited. Ion incidence angle-dependent curvature coefficients and ripple wavelengths are calculated from 3-dimensional collision cascade data obtained from binary collision Monte Carlo simulations. Significant modifications concerning mass redistribution compared to the model of Carter and Vishnyakov and also models based on crater functions are introduced. Furthermore, I find that curvature-dependent erosion is the dominating contribution to pattern formation, except for very low-energy irradiation of a light matrix with heavy ions. The major modifications regarding mass redistribution and ion-induced viscous flow are related to the ion incidence angle-dependent thickness of the irradiated layer. A smaller modification concerns the relaxation of inward-directed mass redistribution. Ion-induced viscous flow in the surface layer also depends on the layer thickness and is thus strongly angle dependent. Simulation results are presented and compared to a variety of published experimental results. The simulations show that in most cases curvature-dependent erosion is the dominant contribution to surface instability and ripple pattern formation and also determines the pattern orientation transition. The simulations predict the occurrence of perpendicular ripple patterns at larger ion incidence angles, in agreement with experimental observations. Mass redistribution causes stabilization of the surface at near-normal ion incidence angles and dominates pattern formation only at very low ion energies.     

On second-harmonic generation in nonuniformly magnetized media

This paper presents a theoretical study of new effects of double-frequency optical signal generation in magnetic media with a nonuniform magnetization distribution. The study is based on the hydrodynamic approximation of the motion of conduction electrons in the field of an electromagnetic wave and equations for the mean electron spin. Within this approach, the mechanisms of the toroidal moment effect, which was experimentally discovered earlier in a system of magnetic particles with a vertical magnetization distribution as well as in a multilayered magnetic system (which is a collinear nonuniform magnetic system), are demonstrated. A new effect that appears due to the presence of equilibrium spin currents in a nonuniform magnetic system is studied in detail. This effect was predicted from the symmetry and can occur only in noncollinearly magnetized media. It is shown that this effect has a resonance nature with a resonance at a pump frequency equal to the plasma frequency of conduction electrons. Estimates of the susceptibility at the double frequency for the parameters of nickel and the typical scale of variation in the magnetization, which is equal to 10 nm, show that the susceptibility near the frequency of the plasma resonance at the chosen parameters is on the order of 10^–9 esu, which holds up a hope of the experimental detection of this effect.     

Theory of the vertical Hall effect in a dimensionally quantized system

The transverse redistribution of carriers that occurs in a 2D system under the effect of a tangential electric field and a magnetic field possessing a tangential component is studied. It is shown that the redistribution of carriers gives rise to a Hall voltage across isolated electrodes positioned above and under the quantum film. This voltage is determined by the 2D conductivity tensor and the transverse static electric polarizability of the 2D layer. The additional contribution that appears in the vertical Hall voltage because of the electron spin orientation induced by magnetic field and the spin-orbit interaction of electrons with the quantum well potential is determined.