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 results are fairly reproduced by an analytical theory.

Ion acceleration from submicron thick foil target irradiated by a circularly polarized laser is studied using multidimensional particle-in-cell simulations. Convex, flat, and concave target shapes are considered. Radius of curvature of curved target is of the order of laser width in transverse direction. Accelerated ion beam of highest peak energy and least energy spread is obtained from concave target, whereas total accelerated charge is highest in convex target. It is attributed to the change in the growth of transverse instabilities and geometrical effects due to target curvature in initial stages of acceleration process. The variation in the radius of curvature of the foil depends on the ratio of initial spot size to the radius of curvature. Faster reduction in curvature is achieved for tightly focused Gaussian pulses as conjectured by the model.

An analytical formalism is developed and particle-in-cell simulations are carried out to study plasma channel formation in air by a two pulse technique and subsequent relativistic self focusing of the third intense laser through it. The first prepulse causes tunnel ionization of air. The second pulse heats the plasma electrons and establishes a prolonged channel. The third pulse focuses under the combined effect of density nonuniformity of the channel and relativistic mass nonlinearity. A channel with 20% density variation over the spot size of the third pulse is seen to strongly influence relativistic self focusing at normalized laser amplitude ∼0.4–1. In deeper plasma channels, self focusing is less sensitive to laser amplitude variation. These results are reproduced in particle-in-cell simulations. The present treatment is valid for millimeter range plasma channels.

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.

An alternative free electron laser concept for producing coherent terahertz radiation is investigated. A weakly relativistic electron beam impinged on a rippled underdense plasma, at an angle to the ripple wave vector, interacts with a seed terahertz signal to produce a space charge beam mode. The oscillatory electron velocity due to the beam mode beats with the density ripple to generate a coherent electromagnetic wave in the terahertz frequency range in forward direction. For an electron beam with 5 MW power, one can obtain ∼40 kW power at 1 THz frequency.