Spectral modulation and supercontinuum generation assisted by infrared femtosecond plasma grating

Spectral modulation and supercontinuum generation of a probe pulse is investigated by using the plasma grating induced by the interference of two infrared femtosecond laser pulses. The dependences of the supercontinuum generation from the probe pulse on the time delay, the relative polarization angle between the probe pulse and the two-pump pulses, and the input probe pulse energy are investigated. The far-field spatial profiles of the three pulses are measured with different time delays and relative polarization angle, and the core energy of the probe pulse as functions of the time delay and relative polarization angle are also shown.

High-Order-Harmonic Generation from a Relativistic Circularly Polarized Laser Interacting with Over-Dense Plasma Grating

The simple surface current model is extended to study the generation of high-order harmonics for a relativistic circularly polarized laser pulse interacting with a plasma grating surface. Both exact relativistic electron dynamics and optical interference of surface periodic structure are considered. It is found that high order harmonics in the specular direction are obviously suppressed whereas the harmonics of the grating periodicity are strongly enhanced and folded into small solid angles with respect to the surface direction. The conversion efficiency of certain harmonics is five orders of magnitude higher than that of the planar target cases. It provides an effective approach to generate a coherent radiation within the so-called ＇water window＇ while maintaining high conversion efficiency and narrow angle spread.

Detection of trace metals in water by filamentand plasma-grating-induced breakdown spectroscopy

Filament-and plasma-grating-induced breakdown spectroscopy(F-GIBS)was demonstrated as an efficient technique for sensitive detection of metals in water,where plasma gratings were established through synchronized nonlinear interaction of two noncollinear filaments and an additional filament was generated with another fs laser beam propagating along their bisector.A water jet was constructed vertically to the three coplanar filaments,overcoming side effects from violent plasma explosion and bubble generation.Three distinct regimes of different mechanisms were validated for nonlinear couplings of the third filament with plasma gratings.As the third filament was temporally overlapped with the two noncollinear filaments in the interaction zone,all the three filaments participated in synchronous nonlinear interaction and plasma grating structures were altered by the addition of the third filament.As the third filament was positively or negatively delayed,the as-formed plasma gratings were elongated by the delayed third filament,or plasma gratings were formed in the presence of plasma expansion of the ahead third filament,respectively.Using F-GIBS for trace metal detection in water,significant spectral line enhancements were observed.

Model of self-generated magnetic field generation from relativistic laser interaction with solid targets

Generation of self-generated annular magnetic fields at the rear side of a solid target driven by relativistic laser pulse is investigated by using theoretical analysis and particle-in-cell simulations.The spatial strength distribution of magnetic fields can be accurately predicted by calculating the net flow caused by the superposition of source flow and return flow of hot electrons.The theoretical model established shows good agreement with the simulation results,indicating that the magnetic-field strength scales positively to the temperature of hot electrons.This provides us a way to improve the magnetic-field generation by using a micro-structured plasma grating in front of the solid target.Compared with that for a common flat target,hot electrons can be effectively heated with the well-designed grating size,leading to a stronger magnetic field.The spatial distribution of magnetic fields can be modulated by optimizing the grating period and height as well as the incident angle of the laser pulse.

Plasma-grating-induced breakdown spectroscopy

.Laser-induced breakdown spectroscopy(LIBS)is a useful tool for determination of elements in solids,liquids,and gases.For nanosecond LIBS(ns-LIBS),the plasma shielding effect limits its reproducibility,repeatability,and signal-to-noise ratios.Although femtosecond laser filament induced breakdown spectroscopy(FIBS)has no plasma shielding effects,the power density clamping inside the filaments limits the measurement sensitivity.We propose and demonstrate plasma-grating-induced breakdown spectroscopy(GIBS).The technique relies on a plasma excitation source-a plasma grating generated by the interference of two noncollinear femtosecond filaments.We demonstrate that GIBS can overcome the limitations of standard techniques such as ns-LIBS and FIBS.Signal intensity enhancement with GIBS is observed to be greater than 3 times that of FIBS.The matrix effect is also significantly reduced with GIBS,by virtue of the high power and electron density of the plasma grating,demonstrating great potential for analyzing samples with complex matrix.

Depolarization of intense laser beams by dynamic plasma density gratings

As a typical plasma-based optical element that can sustain ultra-high light intensity,plasma density gratings driven by intense laser pulses have been extensively studied for wide applications.Here,we show that the plasma density grating driven by two intersecting driver laser pulses is not only nonuniform in space but also varies over time.Consequently,the probe laser pulse that passes through such a dynamic plasma density grating will be depolarized,that is,its polarization becomes spatially and temporally variable.More importantly,the laser depolarization may spontaneously take place for crossed laser beams if their polarization angles are arranged properly.The laser depolarization by a dynamic plasma density grating may find application in mitigating parametric instabilities in laser-driven inertial confinement fusion.