Chirping and spreading of ultrashort laser pulses in underdense plasmas

A linear theory on the propagation of ultrashort pulses including only a few cycles in underdense plasmas is presented. It is shown that the dispersion in plasmas causes severe distortions in the pulse shape, including pulse chirping and spreading. The analytical calculations coincide very well with those obtained by particle-in-cell (PIC) simulations. The upper limit of the peak amplitude of the pulses, above which the linear theory breaks down due to the setting in of nonlinear effects of both the relativistic electron-mass increase and ponderomotive force, is also examined by PIC simulations. At certain high amplitudes, it is found that the ultrashort laser pulses can propagate like solitons.     

Doppler shift of femtosecond laser pulses from solid density plasmas

Doppler-shift of short UV pulses (248 nm, 700 fs) reflected from aluminum, polystyrene and gold targets were measured at focused intensities up to 5×10¹⁵ W/cm². In spite of the fact that the Doppler-shifts are expected to be dependent on the atomic weight of the target material, experimentally similar shifts are found for all targets. A possible explanation is provided by the well known dependence of the Doppler shift on the plasma temperature, if different absorption coefficients are assumed for the different targets. This coefficient shows good agreement with that one deduced from a theory based on collisional absorption in the case of aluminum, but in the case of the other targets, however, the experimentally observed Doppler-shift suggests smaller absorption for lighter and larger absorption for heavier target materials. PACS 52.25.Mq; 52.25.Qt; 52.40.Nk; 52.50.Jm     

Ion acceleration by superintense laser pulses in plasmas

Ion acceleration by petawatt laser radiation in underdense and overdense plasmas is studied with 2D3V-PIC (Particle in Cell) numerical simulations. These simulations show that the laser pulse drills a channel through the plasma slab, and electrons and ions expand in vacuum. Fast electrons escape first from the electron-ion cloud. Later, ions gain a high energy on account of the Coulomb explosion of the cloud and the inductive electric field which appears due to fast change of the magnetic field generated by the laser pulse. Similarly, when a superintense laser pulse interacts with a thin slab of overdense plasma, its ponderomotive pressure blows all the electrons away from a finite-diameter spot on the slab. Then, due to the Coulomb explosion, ions gain an energy as high as 1 GeV. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 2, 80–86 (25 July 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

强激光与等离子体相互作用产生的超强太赫兹辐射

超短强激光脉冲在等离子体中传播时会激发大振幅的等离子体尾波场，它是一种电子等离子体波．由于这是一种静电波，它一般不能转换成电磁辐射．我们发现在不均匀等离子体中激发的尾波场在一定条件下可以通过线性模式转换产生电磁辐射．由于用超短强激光脉冲尾波场可以达到的电场振幅达100GV／m，其振动频率在太赫兹（10^12Hz）附近，用这种方法可以产生电场强度达到GV／m的太赫兹辐射．     

Ion acceleration regimes in underdense plasmas

Acceleration of large populations of ions up to high (relativistic) energies may represent one of the most important and interesting tools that can be provided by the interaction of petawatt laser pulses with matter. In this paper, the basic mechanisms of ion acceleration by short laser pulses are studied in underdense plasmas. The ion acceleration does not originate directly from the pulse fields, but it is mediated by the electrons in the form of electrostatic fields originating from channeling, double layer formation and Coulomb explosion     

Intense laser beam guiding in self-induced electron cavitation channel in underdense plasmas

In underdense plasmas, the transverse ponderomotive force of an intense laser beam with Gaussian transverse profile expels electrons radially, and it can lead to an electron cavitation. An improved cavitation model with charge conservation constraint is applied to the determination of the width of the electron cavity. The envelope equation for laser spot size derived by using source-dependent expansion method is extended to including the electron cavity. The condition for self-guiding is given and illuminated by an effective potential for the laser spot size. The effects of the laser power, plasma density and energy dissipation on the self-guiding condition are discussed.     

Stimulated Raman backscattering from an ultrashort laser interacting with underdense plasmas

We present experimental results of the stimulated Raman backscattering instability (BSRS) in an ultrashort intense (45fs, 5.7×10¹⁷Wcm^-2) laser pulse interacting with an optically ionized helium gas. We have studied the stimulated Raman backscattering reflectivity and the phenomenon of the transition from strongly coupled BSRS to weakly coupled BSRS. We have obtained a good agreement between our experimental results and the theory of BSRS.     

The formation of relativistic electromagnetic solitons in plasma Bragg gratings induced by two counter-propagating laser pulses

The generation of relativistic electromagnetic solitons in plasma with spatiotemporal density modulation is investigated. When two counter-propagating laser pulses overlap in underdense plasma, the interaction between the pulses and plasma modulates the electron and ion densities resulting in localized, stable, long-living relativistic electromagnetic solitons. They are caused by the Stimulated Raman Scattering instability. The dependence of the formation of relativistic electromagnetic solitons on the ion motion, plasma parameters, and laser parameters is studied by particle-in-cell simulations as well.     

Amplification of short laser pulses by Raman backscattering in capillary plasmas

Short laser pulses can be significantly amplified in the process of Raman backscattering in plasma inside an oversized dielectric capillary. A dielectric capillary allows obtaining high intensities of the output radiation by sustaining efficient amplification at large distances compared to the diffraction length. The efficiency of the interaction between the pump wave and the amplified pulse is shown not to be critically sensitive to the transverse structure of the wave fields. For a quasi-single-mode initial seed pulse and a low pump intensity, the amplified pulse tends to preserve its transverse structure due to nonlinear competition of the capillary eigen-modes. At a high power of the pump wave, multimode amplification always takes place but the growth of the front peak of the pulse still follows the one-dimensional model. The Raman backscattering instability of the pump wave resulting in the noise amplification can be suppressed in detuned interaction by chirping the pump wave or arranging an inhomogeneous plasma density profile along the trace of amplification. The efficiency of the desired pulse amplification does not significantly depend on detuning in the case of a smooth detuning profile. Density inhomogeneities are shown to exert less influence on the amplification within a capillary than in the one-dimensional problem. Parameters of a future experiment on the Raman amplification of a short laser pulse inside a capillary are proposed.     

Nonlinear interaction of an intense laser beam with millimeter sized underdense plasmas

Experimental results are reported on the nonlinear interaction of a 0.5 μm laser light with long scalelength preformed plasmas. Spectra from stimulated Brillouin and Raman scattering, as well as measurements of the transmitted laser light clearly indicate the occurence of strong nonlinearities driven by filamentation of the laser beam.     

Excitation of multiple wakefields by short laser pulses in quantum plasmas

We present a theoretical investigation of the excitation of multiple electrostatic wakefields by the ponderomotive force of a short electromagnetic pulse propagating through a dense plasma. It is found that the inclusion of the quantum statistical pressure and quantum electron tunneling effects can qualitatively change the classical behavior of the wakefield. In addition to the well-known plasma oscillation wakefield, with a wavelength of the order of the electron skin depth (λe=c/ωpe, which in a dense plasma is of the order of several nanometers, where c is the speed of light in vacuum and ωpe is the electron plasma frequency), wakefields in dense plasmas with a shorter wavelength (in comparison with λe) are also excited. The wakefields can trap electrons and accelerate them to extremely high energies over nanoscales.     

Characterization and dynamics of gratings induced in glasses by femtosecond pulses

We characterize the gratings induced in commercial SF57 glass by femtosecond pulses. A simple technique enables us to measure both the real and the imaginary parts of the refractive index and shows that the induced grating is mainly a phase grating. An index variation of as much as 6x10(-3) is reported. Then we study the diffraction efficiency of the grating with respect to the intensity of the femtosecond pulse. Permanent structures are induced above the damage threshold. Below this threshold, the grating relaxes. The long-time dynamics of the decay is studied and described by two models. The formation of transient and permanent gratings in various other glasses is observed.