Narrow Results

In this paper, we investigate the influence of electron-lattice interaction on the stability of uniform electronic wavepackets on chains as well as on several types of fullerenes. We will use an effective nonlinear Schrödinger equation to mimic the electron–phonon coupling in these topologies. By numerically solving the nonlinear dynamic equation for an initially uniform electronic wavepacket, we show that the critical nonlinear coupling above which it becomes unstable continuously decreases with the chain size. On the other hand, the critical nonlinear strength saturates on a finite value in large fullerene buckyballs. We also provide analytical arguments to support these findings based on a modulational instability analysis.

In this paper, self-focusing of intense laser pulse propagating along the obliquely external magnetic field on the collisional magnetoactive plasma by using the perturbation theory have been studied. The wave equation describing the interaction of intense laser pulse with collisional magnetoactive plasma is derived. In addition, employing source-dependent expansion (SDE) method, the analysis of the laser spot-size is discussed. It is shown that with increasing of the angle in obliquely external magnetic field, the spot-size of laser pulse decreases and as a result laser pulse becomes more focused. Furthermore, it is concluded that the self-focusing quality of the laser pulse has been enhanced due to the presence of obliquely external magnetic field in the collisional magnetoactive plasma. Besides, it is seen that with increasing of $\nu /{\mathrm{\Omega }}_0$, the laser spot-size reduces and subsequently the self-focusing of the laser pulse in plasma enhances. Moreover, it is found that changing the collision effect in the magnetoactive plasma leads to increases of self-focusing properties.

Those who measure success with culmination do not seem to be aware that life is a journey not a destination. This spirit is best reflected in the unceasing failures in efforts for solving the problem of controlled thermonuclear fusion for even the simplest pinches for over decades; and the nature keeps us challenging with examples. However, these efforts have permitted researchers the obtention of a dense plasma with a lifetime that, albeit short, is sufficient to study the physics of the pinch effect, to create methods of plasma diagnostics, and to develop a modern theory of plasma processes. Most importantly, they have impregnated the solid state plasmas, particularly the electron–hole plasmas in semiconductors, which do not suffer from the issues related with the confinement and which have demonstrated their potential not only for the fundamental physics but also for the device physics. Here, we report on a two-component, cylindrical, quasi-one-dimensional quantum plasma subjected to a radial confining harmonic potential and an applied magnetic field in the symmetric gauge. It is demonstrated that such a system, as can be realized in semiconducting quantum wires, offers an excellent medium for observing the quantum pinch effect at low temperatures. An exact analytical solution of the problem allows us to make significant observations: Surprisingly, in contrast to the classical pinch effect, the particle density as well as the current density display a determinable maximum before attaining a minimum at the surface of the quantum wire. The effect will persist as long as the equilibrium pair density is sustained. Therefore, the technological promise that emerges is the route to the precise electronic devices that will control the particle beams at the nanoscale.

The nonlinear propagation of femtosecond Airy laser filaments in water is numerically investigated in this paper. We mainly consider the influences of confinement parameter, pulse duration and beam waist on the deposited energy of filaments. The values of confinement parameter are found to have a significant impact on the temporal and spatial dynamics of the pulse. The characteristics of energy deposition also differ widely for Airy beams with different confinement parameters. The less the confinement parameter, the more energy deposited by filamentation. However, the relative deposited energy evolves relatively small changes under different confinement parameters. We also found the longer pulse duration and longer beam waist are beneficial to the total deposited energy increase.

In this paper, electron acceleration by relativistic pondermotive force has been studied in the interaction of intense laser pulse with an axially inhomogeneous underdense plasma. The results show that the relativistic ponderomotive nonlinearity causes obvious beam self-focusing and a highly energetic electron can be achieved in the plasma, moreover, the high-energy electron would be automatically ejected by the ponderomotive force without adding any additional extractor. In addition, it is found that the real part and imaginary part of Lorentz factor can directly affect the amplitude and change the trend of electron acceleration, and electrons acceleration can be strengthened when we can elect properly the laser intensity with higher momentum and smaller beam width, but nonuniformity of plasmas can slightly decrease the electron acceleration. The results can provide useful physical support for novel design of table accelerator and the optimized parameters of electron acceleration in the plasma.

Self-focusing of a laser beam is one of the major consequences of nonlinear interaction of laser with plasma. In this paper, self-focusing of skew cosh Gaussian (skew-ChG) laser beam is studied in anisotropic, collisional plasma where in externally exponentially decreasing magnetic field is applied. Skewness parameters in the field profile of skew-ChG laser beam eventually determine intensity profile of the beam. The symmetry in skewness parameter in two transverse directions simplifies the calculations considerably. Second order, nonlinear, ordinary, coupled differential equations for Beam Width Parameters (BWP) are obtained using Akhmanovs approach under Wentzel–Kramers–Brillouin (WKB) and paraxial approximations which are solved numerically. For given amplitude of applied magnetic field, attenuated oscillatory self-focusing is observed with respect to change in attenuation coefficient.

This paper presents a way to achieve optical limiting using the self-focusing of a laser beam in a photorefractive medium. In this view, the protection is not based on the absorption of the beam energy in the limiting system but on a global defocusing of the light in the optical system. We have studied experimentally and theoretically the self-focusing of a single laser beam in electrically biased Bi₁₂TiO₂₀ from the continuous to the pulsed regime. We show that photorefractive materials are, for given conditions, efficient against laser radiation on these two different time scales at a low energy level (nJ).

The propagation and stability of spatio-temporal solitons (STSs) under the combined influence of dispersion, diffraction, self-focusing and self-phase modulation in a medium possessing cubic quictic nonlinearity is investigated. Using variational formalism, it has been shown that a pair of soliton state exists, of which, one is stable, while the other quasistable. It has been established that while the stable one is possible only in cubic quintic nonlinear (CQNL) media, the quasistable one is well known in Kerr media. Existence of these solitons is also verified by means of numerical experiment. Moreover, it is also shown that unlike bistable temporal solitons, no spatio-temporal bistable soliton exists in CQNL media. Dynamic behavior of solitons spatial and temporal widths which are different from that of the stationary soliton state is also investigated. Bound oscillatory and unbound regions are identified.

Conical emission is investigated for Ti:sapphire femtosecond laser pulses propagating in water. The colored rings can be observed in the forward direction due to the constructive and destructive interference of transverse wavevector, which are induced by the spatio-temporal gradient of the free-electron density. With increasing input laser energy, due to filamentation and pulse splitting induced by the plasma created by multiphoton excitation of electrons from the valence band to the conduction band, the on-axis spectrum of the conical emission is widely broadened and strongly modulated with respect to input laser spectrum, and finally remains fairly constant at higher laser energy due to intensity clamping in the filaments.

Supercontinuum spectrum generation by femtosecond laser pulses propagating in transparent liquids with different band gap thresholds is investigated. As the laser energy increases, the laser spectrum is strongly broadened and modulated, and finally a supercontinuum spectrum can be observed. The spectral broadening can be mainly attributed to the optical Kerr effect at low laser energy and the plasma created by the multiphoton excitation of electrons from the valence band to the conduction band at high laser energy. It is found that the spectral broadening and modulation strongly depend on the band gap threshold E_gap of the medium.

In the present paper, the effect of self-focusing on stimulated Raman scattering (SRS) process by a Gaussian laser beam in collisional plasma is investigated. Due to non-uniform heating, collisional nonlinearity arises, which leads to redistribution of carriers. As a result of which, background plasma density profile in a direction transverse to pump beam axis is modified. This modification affects the incident laser beam, electron plasma wave and back-scattered beam. Nonlinear differential equations for the beam width parameters of the main beam, electron plasma wave, back-scattered wave have been set up by taking full nonlinear part of the dielectric constant of collisional plasma with the help of moment theory approach and are solved numerically by Runge–Kutta method. It is observed from the analysis that focusing of waves greatly enhances the SRS reflectivity.

In the present paper, enhanced Brillouin scattering of a gaussian laser beam in a collisional plasma has been investigated with the help of moment theory approach. The pump wave (ω₀, k₀) interacts with pre-excited ion-acoustic wave (ω, k), thereby generating a back-scattered wave (ω₀ - ω, k₀ - k). As a specific case, back scattering for which k ≃ 2k₀ has been discussed. Due to nonuniform heating, collisional nonlinearity arises, which leads to redistribution of carriers. As a result background plasma density profile in a direction transverse to pump beam axis is modified. Consequently, the pump beam becomes self-focused and in turn leads focusing of ion-acoustic wave and back-scattered wave under appropriate conditions. It is further observed from the analysis that focusing of waves greatly enhances the SBS back-reflectivity.

This paper presents an investigation of the laser pulse distortion/breakup and the effect of the plasma channel on the laser propagation through the collisionless plasma. Moment theory has been used to derive differential equations of the beam width parameter of the laser propagating through uniform homogenous plasma and preformed plasma channel having parabolic density profile. Differential equations have been set up and solved numerically by using Runge Kutta method. From analysis, it is observed that the low intensity front and rear parts of the laser pulse get defocused/diffracted while the high intensity central portion of the laser pulse gets self-guided during the propagation through uniform homogenous plasma. As a result of this, laser pulse gets distorted. This distortion of the laser pulse has not been observed when the laser beam is propagated through the plasma channel having parabolic density profile. The laser pulse is guided as a whole, even the low intensity front and rear parts of the laser pulse are also guided. Therefore, it is predicted that plasma channel plays a significant role to prevent the distortion/beam breakup of the laser.

In this work, the dynamics of a pulse train propagating through a bimodal optical fiber whose core refractive index varies periodically along its main axis are studied numerically. The mathematical model is based on a coupled system of nonlinear Schrödinger-like equations. First- and second-order dispersions are taken into account, as well as the number of solitons propagating through the fiber. The physical conditions for self-compression of coupled cnoidal waves are obtained and discussed.

We study the fundamental nonlinear modes in a dual-cylinder waveguide shell, which features a self-focusing Kerr effect, coupled by a double-well connection. This double-well connection is twisted by a pitch rate and creates a spatial dependent linear mixing, which plays the role of an effective rotating double-well potential. Symmetry transition between the waveform and the power distribution of the fundamental nonlinear modes in this system can be induced both by the total power and by the rotation speed of the connection. Four types of modes — wave symmetry and power symmetry (WSPS), wave symmetry and power asymmetry (WSPA), wave asymmetry and power symmetry (WAPS) and wave asymmetry and power asymmetry (WAPA) — are found from the system. The dependence of these modes on the total power of the light field, the rotation speed and the coupling strength of the connection are systematically studied through the paper. The finding of this paper may offer potential applications in fabrication of new types of nonlinear all-optical devices.

Simultaneous co-propagation of red nondiffracting Bessel beam and green Gaussian beam at milliwatt powers in a photosensitive azobenzene liquid crystal has been studied. The fine displacement of the narrow-width Gaussian beam relative to the Bessel beam central maximum showed the Bessel beam distortion and controlled bending of its core in the direction opposite to the location of green Gaussian beam. After switching off the green beam, the dynamics of restoration of Bessel beam demonstrated the pronounced optical-trapping feature of its central core and transient soliton-like propagation with reduced diameter and increased intensity and propagation depth thus forming a waveguiding channel.

We present the experimental demonstration of a self-trapped optical beam bending in a photorefractive Fe-doped lithium niobate (LN:Fe) crystal controlled by the pyroelectric effect. Formation of self-trapped beams with typical $\sim$50$\mu$m diameter and large bending of $\sim$140$\mu$m are depicted in a 1cm length LN:Fe crystal for a laser beam at 632.8nm wavelength and 0.5mW power with a 30^∘C crystal temperature change. The self-trapped beam bending is opposite to the crystal $C^{+}$-axis. The underlying physics is elaborated and numerical simulations are performed. The long-living waveguiding channels with controlled curvilinear trajectories are promising for optical information routing.

The model given by Patil et al., for the self-focusing of lowest-order Bessel–Gaussian laser beam, is revisited by introducing the exponential plasma density ramp in this paper. Plasma density changes due to combined effect of the relativistic and ponderomotive nonlinearities, which arise due to the nonuniform spatial profile of the laser beam leading to stronger self-focusing. Under paraxial ray approximation, the expression for the evolution of laser-beam width parameter is obtained and solved numerically in order to achieve self-focusing. We have observed that as Bessel beam propagates through the plasma, it does not diffract and spread out, which is in contrast to the usual behavior of light that spreads out after being focused down to the small spot. Laser spot size decreases considerably under the presence of exponential density ramp. This study will be useful for better understanding the self-focusing mechanism of lasers as it becomes effective for the optimized parameters under the proposed situation. Bessel laser beams play an important role in enhancing the energy gain in laser-driven accelerators. In addition, for all laser-driven plasma-based accelerators, it appears possible to guide a laser beam over large distances using a plasma density channel, thus enhancing the acceleration length and the energy gain.

In this work, the role of asymmetry in the beam profile of finite Airy-Gaussian (AiG) laser is presented to explore its propagation dynamics in collisionless magnetized plasma. Emphasis is on analyzing the self-trapping condition for both extraordinary (E) and ordinary (O) modes of propagation of AiG laser beam. Theoretical formulation is based on obtaining nonlinear coupled differential equations for beam-width parameters in both transverse dimensions of the beam with the help of parabolic equation approach under WKB and paraxial approximations. It is found that asymmetry in the modulation parameters significantly affects self-focusing of finite AiG beam. Moreover, the same initial intensity parameter for simultaneous self-trapping of both modes of propagation is noteworthy.

This work investigates the polarization modes i.e., Extraordinary (E-mode) and Ordinary (O-mode) on self-focusing of higher-order mode of elegant Hermite-cosh-Gaussian (EHChG) laser beams within collisionless magnetized plasma medium. In this study, the authors have taken transverse electromagnetic (TEM) mode with mode indices (0, 4) and explored. The ponderomotive force is primarily responsible for the dielectric constant’s nonlinearity in this context. Also, WKB and paraxial approximations via the parabolic wave equation approach are used to establish the general form of differential equations for beam-width parameters in both transverse dimensions of the beam. In this paper, the authors have theoretically investigated the dominance of polarization modes and absorption coefficient on self-focusing. The final results show good enhancement of laser propagation through collisionless plasma medium.