Vivek Sajal

Structural, optical, vibrational, and magnetic properties of sol-gel derived Ni doped ZnO nanoparticles

With a view to study structural, optical, vibrational, and magnetic properties of solgel derived Zn1−xNixO (x = 0.02, 0.04, and 0.06) nanoparticles, systematic investigations have been carried out. The Rietveld refinement of X-ray powder diffraction data revealed a single hexagonal phase with space group P63mc. The secondary phase of NiO appeared only in 6% Ni doped sample. Phonon modes in Ni doped ZnO nanoparticles were studied through Fourier transform infrared measurements. Furthermore, the enhancement in optical band gap with Ni doping from 3.29 to 3.32 eV has been observed through UV-visible spectroscopic analysis. Photoluminescence spectra of Zn1−xNixO show the UV-emission peak showing the blue shift with increase in doping concentration followed by broad visible (blue) emission corresponding to the defect emission whose intensity decreased with increasing Ni concentration. A clear room temperature ferromagnetism is observed in all samples but saturation magnetization decreased with increasing Ni co...

Phase-matched second- and third-harmonic generation in plasmas with density ripple

The generation of second and third harmonics by the interaction of an ultrashort laser pulse with underdense plasma having a density ripple is studied at intensities Iλ2=1016−1019W cm−2μm2 using fully relativistic two-dimensional particle-in-cell simulations with high spectral resolution. A theoretical model is developed for second- and third-harmonic conversion efficiencies. When the laser is plane polarized in the simulation plane even and odd harmonics are excited in the same polarization as the laser polarization. The highest efficiency of generation of a specific harmonic occurs when the ripple wave vector value kq satisfies phase-matching conditions. The efficiency of phase-matched harmonic generation is an order of magnitude higher than the one without phase matching. The efficiency increases rapidly in weak and moderate relativistic regime and tends to saturate in strong relativistic regime. At moderately relativistic intensities and low plasma densities, the simulation and recent experimental res...

Strong terahertz radiation generation by beating of two x-mode spatial triangular lasers in magnetized plasma

Resonant THz radiation generation is proposed by beating of two spatial-triangular laser pulses of different frequencies (ω1, ω2) and wave numbers ( k1, k2) in plasma having external static magnetic field. Laser pulses co-propagating perpendicular to a dc magnetic field exert a nonlinear ponderomotive force on plasma electrons, imparting them an oscillatory velocity with finite transverse and longitudinal components. Oscillatory plasma electrons couple with periodic density ripples n ′ = nq0e iqz to produce a nonlinear current, i.e., responsible for resonantly driving terahertz radiation at (ω = ω1 − ω2, k = k1 − k2 + q). Effects of THz wave frequency, laser beam width, density ripples, and applied magnetic field are studied for the efficient THz radiation generation. The frequency and amplitude of THz radiation were observed to be better tuned by varying dc magnetic field strength and parameters of density ripples (amplitude and periodicity). An efficiency about 0.02 is achieved for laser intensity of 2 × 10 W/cm in a plasma having density ripples about 30%, plasma frequency about 1 THz and magnetic field about 100 kG.

Highly focused and efficient terahertz radiation generation by photo-mixing of lasers in plasma in the presence of magnetic field

A mechanism of efficient and highly focused terahertz (THz) radiation generation by photo-mixing of top-hat like lasers with frequencies ω1, ω2 and wave numbers k1, k2 in pre-formed rippled density (corrugated) plasma is proposed. In this mechanism, intensity variation of lasers offers nonlinear ponderomotive force at frequency ω′=ω1−ω2 and wave number k′=k1−k2 which couples with density ripples in the plasma and leads to a strong nonlinear oscillatory current that resonantly excites highly focused and intense THz radiation at frequency ωUH=(ωp2+ωc2) (where ωc is electron cyclotron frequency). The efficiency of emitted THz radiation of the order of 15% is obtained under optimum conditions. It is observed that focus and intensity of emitted radiation can be controlled by selecting a proper profile index of the lasers, ripple parameters, and tuning of external magnetic field.

Stimulated Raman scattering of a laser beam in a plasma with azimuthal magnetic field

A strong azimuthal magnetic field localizes the lower hybrid waves radially in laser produced plasmas. The laser pump parametrically excites a lower hybrid wave and a backscattered electromagnetic sideband wave. The density perturbation due to the lower hybrid wave couples with the oscillatory velocity of electrons due to the pump wave, to produce a nonlinear current driving the sideband. The pump and sideband waves exert a ponderomotive force on electrons driving the lower hybrid wave. The local effects reduce the growth rate of stimulated Raman scattering. The fundamental radial eigenmode (p=0) of the lower hybrid wave is the maximally growing mode. The scattering process can be used as a diagnostic for the azimuthal magnetic field.

Stimulated forward Raman scattering of a laser in a magnetized plasma

A Gaussian laser beam propagating through a low-density plasma in the presence of an azimuthal magnetic field undergoes stimulated forward Raman scattering producing a lower hybrid wave and two radially localized electromagnetic sideband waves. The radial widths of the sidebands are of the order of the spot size of the pump, whereas the radial width of the lower hybrid wave depends on the growth rate of the Raman process. The nonlocal effect arising, due to the azimuthal magnetic field, reduces the region of nonlocal interaction and hence the growth rate. The growth rate of stimulated Raman forward scattering first increases on increasing magnetic field, maximizes at some optimum value of magnetic field, and then decreases.

Quasi-monoenergetic GeV electrons from the interaction of two laser pulses with a gas

A scheme is proposed for the acceleration of electrons generated during the ionization of a gas by two laser pulses. The electrons created from the ionization of neutral atoms near the rising edge of the pulse do not gain sufficient energy. If a prepulse is used before the main pulse then the prepulse removes electrons from the outer shells, and the main laser pulse interacts with the electrons in the inner shells of high atomic number gases, such as krypton and argon. The electrons are generated close to the peak of the main laser pulse and gain energy in GeV with a small spread in the energy and low emittance angle.

Tunable and efficient terahertz radiation generation by photomixing of two super Gaussian laser pulses in a corrugated magnetized plasma

A scheme of terahertz (THz) radiation generation is investigated by photo-mixing of two super Gaussian laser beams having different frequencies (ω1, ω2) and wave numbers (k→1, k→2) in a performed corrugated plasma embedded with transverse dc magnetic field. Lasers exert a nonlinear ponderomotive force, imparting an oscillatory velocity to plasma electrons that couples with the density corrugations ( n′=nα0eiαz) to generate a strong transient nonlinear current, that resonantly derives THz radiation of frequency ∼ ωh (upper hybrid frequency). The periodicity of density corrugations is suitably chosen to transfer maximum momentum from lasers to THz radiation at phase matching conditions ω=ω1−ω2 and k→=k→1−k→2+α→. The efficiency, power, beam quality, and tunability of the present scheme exhibit high dependency upon the applied transverse dc magnetic field along with q-indices and beam width parameters ( a0) of super Gaussian lasers. In the present scheme, efficiency ∼10−2 is achieved with the optimization of ...

Quasimonoenergic collimated electrons from the ionization of nitrogen by a chirped intense laser pulse

A scheme is proposed for quasimonoenergic collimated GeV electrons generated during ionization of nitrogen by a chirped intense laser pulse. The electrons accelerated by a laser pulse without a frequency chirp are known for poor-quality beams. If a suitable frequency chirp is introduced, then the energy of the electrons increases significantly. It is shown that quasimonoenergic collimated GeV electrons can be produced using a right choice of laser spot size, frequency chirp, and pulse duration.

Relativistic forward stimulated Raman scattering of a laser in a plasma channel

A circularly polarized Gaussian laser beam propagating through a low density plasma creates a partially electron depleted channel. The laser undergoes stimulated forward Raman scattering, producing a plasma wave and two radially localized electromagnetic sideband waves. The laser and the sideband waves exert an axial ponderomotive force on electrons driving the plasma wave. The latter couples with the pump to drive the sidebands. The radial width of the electromagnetic sideband is of the order of the spot size of the pump, r0, whereas the radial width of the plasma wave is determined by the growth rate of the Raman process. The localization effect reduces the region of interaction and the growth rate. The algebraic equation of growth rate for forward Raman scattering is solved numerically for a typical laser wavelength and a plasma density The growth rate of the forward Raman process increases on increasing the normalized pump amplitude at lower values of pump amplitude, while the growth rate decreases on increasing at higher values of the pump amplitude. On increasing the laser spot size, maximum growth rate is obtained at higher values of a00. This analysis will be applicable in the study of x-ray lasers, inertial confinement fusion, and laser plasma accelerators.