Generation of the third harmonic of near IR femtosecond laser radiation tightly focused into the bulk of a transparent dielectric in the regime of plasma formation

The generation of the third harmonic of femtosecond laser radiation with a wavelength of 1.24 μm tightly focused into the bulk of fused silica was simulated numerically for the regime with light intensity of 3 × 10¹³ W/cm², which is extreme for solid bodies. The efficiency of third harmonic generation (THG) was found to be restricted to 0.1% in the regime of plasma formation. This is determined by two competitive processes: a decrease of THG efficiency due to an increase of wave detuning and an increase of THG efficiency due to a growth of focusing asymmetry. In an isotropic medium, determination of the threshold of plasma formation by use of the third harmonic signal is shown to be a more sensitive method as compared with the standard scheme of nonlinear transmittance detection.     

Generation of a terahertz supercontinuum by the self-scattering of a femtosecond pulse in the optical-rectification regime

The possibility of generating a terahertz spectral supercontinuum in the regime of the optical rectification of a femtosecond optical pulse is predicted. The transient and asymptotic stages of generation are distinguished. The supercontinuum consists of the soliton and nonsoliton components of a terahertz signal at the asymptotic stage. It is shown that the formation of supercontinuums is accompanied by a redshift of the median of the opticalpulse spectrum.     

Excitation of coherent acoustic phonons by a femtosecond pulse

Physics of the Solid State - Excitation of coherent acoustic phonons in superlattices or layered materials by a femtosecond pulse is considered. In superlattices, coherent acoustic phonons with a...     

Investigation of coherent phonons in bismuth by femtosecond laser and X-ray pulse probing

Using femtosecond laser pulses, coordinated oscillations (coherent phonons) of Bi single-crystal atoms have been excited and recorded. The comparison with experimental results on the X-ray probing of coherent phonons at the same excitation energy density demonstrates that the observed lifetime and frequency shift of the oscillations are similar in both cases. Moreover, it has been revealed that the relaxation (incoherent) contributions are identical. This coincidence of the photoinduced response parameters indicates that probing in the visible spectrum correctly reflects the coherent dynamics of the Bi crystal lattice.     

Generation of terahertz radiation by the axicon focusing of ionizing laser pulses

Experimental results on the generation of terahertz radiation in the gas breakdown by high-power quasi-monochromatic laser pulses focused by an axicon lens are presented. A theoretical model developed for the terahertzradiation generation includes the nonlinear interaction of a femtosecond pulse with an ionized gas and the excitation of transverse oscillations of the plasma column. The results of the theoretical analysis are compared with the experimental data.     

Generation of terahertz radiation by the axicon focusing of ionizing laser pulses

JETP Letters - Experimental results on the generation of terahertz radiation in the gas breakdown by high-power quasi-monochromatic laser pulses focused by an axicon lens are presented. A...     

Features of femtosecond laser pulse reflection from a sharp boundary of relativistic laser plasma

The features of femtosecond laser pulse reflection from a plasma target of near-critical density were analytically and numerically studied in a wide range of laser pulse intensities and durations. Based on analytical calculations, it is shown that the pulse reflectance at a constant intensity decreases with decreasing pulse duration. Based of numerical simulation results, it is shown that, as a superintense femtosecond laser pulse is incident on a plasma layer with a concentration close to the critical one, a quasi-periodic electron-density structure is formed in the plasma bulk, which can both increase and decrease the the laser pulse reflectance.     

Generation of a directed nanolocalized photoelectron beam by means of femtosecond laser pulses

A directed photoelectron beam is obtained when photoelectrons from a sharp metal tip irradiated by femtosecond laser pulses are passed through a quartz nanocapillary. Such a nanolocalized photoelectron train makes it possible to conduct experiments with simultaneous femtosecond time and nanometer spatial resolutions.     

Generation of few-cycle laser pulses by coherent synthesis based on a femtosecond Yb-doped fiber laser amplification system

We demonstrate a coherent synthesis system based on femtosecond Yb-doped fiber laser technology. The output pulse of the amplification system is divided into two replicas and seeded into photonic crystal fibers of two parallel branches for nonlinear pulse compression. Because of the different nonlinear dynamics in the photonic crystal fibers, the compressed pulses show different spectra, which can be spliced to form a broad coherent spectrum. The integrated timing jitter between the pulses of two branches is less than one tenth of an optical cycle.By coherently synthesizing pulses from these two branches, 8 fs few-cycle pulses are produced.     

Observation of coherent optical phonons excited by femtosecond laser radiation in Sb films by ultrafast electron diffraction method

The generation of coherent optical phonons in a polycrystalline antimony film sample has been investigated using femtosecond electron diffraction method. Phonon vibrations have been induced in the Sb sample by the main harmonic of a femtosecond Ti:Sa laser (λ = 800 nm) and probed by a pulsed ultrashort photoelectron beam synchronized with the pump laser. The diffraction patterns recorded at different times relative to the pump laser pulse display oscillations of electron diffraction intensity corresponding to the frequencies of vibrations of optical phonons: totally symmetric (A _1g ) and twofold degenerate (E _g ) phonon modes. The frequencies that correspond to combinations of these phonon modes in the Sb sample have also been experimentally observed.     

Generation of GW radiation pulses from a VUV free-electron laser operating in the femtosecond regime

Experimental results are presented from vacuum-ultraviolet free-electron laser (FEL) operating in the self-amplified spontaneous emission (SASE) mode. The generation of ultrashort radiation pulses became possible due to specific tailoring of the bunch charge distribution. A complete characterization of the linear and nonlinear modes of the SASE FEL operation was performed. At saturation the FEL produces ultrashort pulses (30-100 fs FWHM) with a peak radiation power in the GW level and with full transverse coherence. The wavelength was tuned in the range of 95-105 nm.     

Generation of terahertz radiation during reflection of femtosecond laser pulses from a metal surface

The features of terahertz radiation generated upon oblique incidence of a femtosecond laser pulse on a metal surface are investigated theoretically. We propose a Cherenkov-type generation mechanism associated with excitation of low-frequency surface currents by a p-polarized optical field. The nonlinear surface currents and corresponding electrodynamic characteristics of low-frequency radiation are calculated analytically and by numerical simulation using the hydrodynamic model. The features of Cherenkov generation of terahertz radiation are analyzed, and techniques are proposed for increasing efficiency.     

Wide-angle diffraction of the laser beam by a sharp edge

Using a rigorous theory of diffraction, we explain the origin and analyze the structure of a wide-angle illuminated area observed when a limited beam is diffracted by the sharp edge of an opaque screen. It is shown that the formed plume has the structure of a cylindrical wave traveling from the screen edge and its amplitude is proportional to the beam amplitude at this edge. The observed structure is Young’s boundary wave produced by diffraction of the limited beam.     

Generation of bubbles in glass by a femtosecond laser

Permanent microscale bubbles with varied size and number density are induced in borosilicate glasses by adjusting the focusing depth (FD) of a tightly focused femtosecond laser. With continuously increasing of the focusing depth, the average size of generated bubbles experiences an increase-decrease process. However, the number density of generated bubbles experiences an opposite changing process compared to the change of the size. The possible mechanism for the bubble generation and changing with the focusing depth has been discussed.     

Generation of short electron bunches by a laser pulse crossing a sharp boundary of inhomogeneous plasma

The formation of short electron bunches during the passage of a laser pulse of relativistic intensity through a sharp boundary of semi-bounded plasma has been analytically studied. It is shown in one-dimensional geometry that one physical mechanism that is responsible for the generation of electron bunches is their self-injection into the wake field of a laser pulse, which occurs due to the mixing of electrons during the action of the laser pulse on plasma. Simple analytic relationships are obtained that can be used for estimating the length and charge of an electron bunch and the spread of electron energies in the bunch. The results of the analytical investigation are confirmed by data from numerical simulations.     

Spatiotemporal stability of a femtosecond hard-x-ray undulator source studied by control of coherent optical phonons

We report on the temporal and spatial stability of the first tunable femtosecond undulator hard-x-ray source for ultrafast diffraction and absorption experiments. The 2.5-1 Angstrom output radiation is driven by an initial 50 fs laser pulse employing the laser-electron slicing technique. By using x-ray diffraction to probe laser-induced coherent optical phonons in bulk bismuth, we estimate an x-ray pulse duration of 140+/-30 fs FWHM with timing drifts below 30 fs rms measured over 5 days. Optical control of coherent lattice motion is demonstrated.     

Self‐focusing of a cosh‐Gaussian laser beam in magnetized plasma under relativistic‐ponderomotive regime

The combined effect of relativistic and ponderomotive nonlinearities on the self‐focusing of an intense cosh‐Gaussian laser beam (CGLB) in magnetized plasma have been investigated. Higher‐order paraxial‐ray approximation has been used to set up the self‐focusing equations, where higher‐order terms in the expansion of the dielectric function and the eikonal are taken into account. The effects of various lasers and plasma parameters viz. laser intensity (a₀), decentred parameter (b), and magnetic field (ω_c) on the self‐focusing of CGLB have been explored. The results are compared with the Gaussian profile of laser beams and relativistic nonlinearity. Self‐focusing can be enhanced by optimizing and selecting the appropriate laser‐plasma parameters. It is observed that the focusing of CGLB is fast in a nonparaxial region in comparison with that of a Gaussian laser beam and in a paraxial region in magnetized plasma. In addition, strong self‐focusing of CGLB is observed at higher values of a₀, b, and ω_c. Numerical results show that CGLB can produce ultrahigh laser irradiance over distances much greater than the Rayleigh length, which can be used for various applications.     

Excitation of terahertz surface polaritons in a cylindrical waveguide by femtosecond laser pulses

Generation of terahertz surface polaritons in homogeneous round cross-section plasma waveguides upon nonlinear optical rectification of femtosecond laser pulses is analyzed theoretically. It is assumed that nonlinear polarization inducing a surface electromagnetic wave is formed at the waveguide boundary in a thin layer of the nonlinear dielectric that surrounds the waveguide. The efficiency of the femtosecond radiation conversion into surface polaritons is studied as a function of the waveguide radius and duration of the exciting laser pulse.     

Generation of a dense electron beam in a thin film using an ultraintense femtosecond laser pulse

Electron dynamics in a thin target irradiated with femtosecond laser pulses at an intensity of 10²⁰ W/cm² is studied in the framework of the kinetic theory of laser plasma based on the construction of propagators (in classical limit) for electron and ion distribution functions in plasma. The calculations are performed for real densities and charges of plasma ions. Electrons are partly ejected from the target. The laser pulse energy is predominantly absorbed by electrons, and the electrons are accelerated to relatively high energies.     

Observation of laser satellites in a plasma produced by a femtosecond laser pulse

Laser satellites are detected in the emission spectra of magnesium and aluminum plasmas produced by femtosecond laser pulses. This is made possible by the realization of picosecond time resolution in a high-luminosity x-ray spectrograph with a spherically curved mica crystal. The temporal characteristics of these newly recorded spectral lines show unequivocally that they are formed as a result of nonlinear processes. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 7, 454–459 (10 October 1997)