Generation of high-density electrons based on plasma grating induced Bragg diffraction in air

Excitation gratings in cross-beam filament wake channels in a dense argon gas: Formation, control, and Rabi sideband manifestation

When two intense laser beams cross at a small angle, the interference in the crossing area results in a finite intensity grating. We consider femtosecond laser filamentation in such a grating, in a situation when the process is largely confined to the grating maxima and leads to formation of a structured filament wake channel. In a dense gas, electron impact processes during the laser pulse cause a copious excitation of neutral atoms, resulting in formation of a finite grating of the density of excited atoms. Numerically solving the equations of laser-driven kinetics, we obtain the properties of this grating, as depending on the characteristics of the interfering beams and especially on the interbeam phase delay. The excitation gratings thus formed give rise to a hallmark effect of Rabi sideband emission when probed by a picosecond 800 nm laser pulse, which couples with transitions in the excited states manifold. Spectral and spatial interference of the emitted radiation forms four-dimensional spatial-spectral fringe patterns accessible for observation on a remote screen. The patterns are indicative of the excitation grating structure; their sensitivity to the phase delay between the crossing pump pulses warrants experimental verification.

Volume plasma grating by noncollinear interaction of femtosecond filament arrays

Stable propagation of multifilament arrays in transparent bulk media with adjustable separation distances between adjacent child filaments has always been desired for advanced manufacturing. Here, we report on the generation of an ionization-induced volume plasma grating (VPG) by the interaction of two batches of noncollinearly propagating arrays of multiple filaments (AMF). The VPG can externally arrange the propagation of the pulses along regular plasma waveguides via spatial reconstruction of electrical fields, which is compared with the self-formation of randomly distributed multiple filamentation originated from noises. The separation distances of filaments in VPG are controllable by readily changing the crossing angle of the excitation beams. In addition, an innovative method to efficiently fabricate multidimensional grating structures in transparent bulk media through laser modification using VPG was demonstrated.

Characterization of femtosecond laser-induced grating scattering of a continuous-wave laser light in air

Nanosecond laser-induced grating scattering/spectroscopy (LIGS) technique has been widely applied for measuring thermodynamic parameters such as temperature and pressure in gaseous and liquid media. Recently, femtosecond (fs) laser was demonstrated to induce the grating and develop the fs-LIGS technique for gas thermometry. In this work, we systematically investigated the fs-LIGS signal generation using 35 fs, 800 nm laser pulses at 1 kHz repetition rate in ambient air by varying the pump laser energies, the probe laser powers and the temporal delays between two pump laser pulses. The stability of single-shot fs-LIGS signal was studied, from which we observed that the signal intensity exhibits a significant fluctuation while the oscillation frequency shows a much better stability. A 4.5% precision of the oscillation frequency was achieved over 100 single-shot signals. By using a previously-developed empirical model, the fs-LIGS signals were fitted using nonlinear least-squares fitting method, by which crucial time constants characterizing the signal decay process were extracted and their dependences on the pump laser energy were studied. From the measured results and theoretical analysis, we found that the appropriate range of the overall pump laser energy for reliable fs-LIGS measurements is approximately located within 80 ∼ 300 μJ. The limitations on the accuracy and precision of the fs-LIGS measurements, the origin of destructive influence of plasma generation on the signal generation as well as the electrostriction contribution were also discussed. Our investigations could contribute to a better understanding of the fs-LIGS process and further applications of the technique in single-shot gas thermometry and pressure measurements in various harsh conditions.

Holographic Plasma Lenses

A hologram fully encodes a three-dimensional light field by imprinting the interference between the field and a reference beam in a recording medium. Here we show that two collinear pump lasers with different foci overlapped in a gas jet produce a holographic plasma lens capable of focusing or collimating a probe laser at intensities several orders-of-magnitude higher than the limits of a nonionized optic. We outline the theory of these diffractive plasma lenses and present simulations for two plasma mechanisms that allow their construction: spatially varying ionization and ponderomotively driven ion-density fluctuations. Damage-resistant plasma optics are necessary for manipulating high-intensity light, and divergence control of high-intensity pulses-provided by holographic plasma lenses-will be a critical component of high-power plasma-based lasers.

Interference-induced filament array in fused silica

One- and two-dimensional filament arrays are obtained in fused silica by using two and three interfered femtosecond laser beams, respectively. By modulating the number, cross angle, and azimuth of the beams, the dimension, period, orientation, and geometry of the filament-array can be controlled. The multiple beams interference method provides a convenient and effective method to generate and control the filament array in optical media with multiple degrees of freedom but without any external pulse modulation or focal element.

Filamentary plasma grating induced by interference of two femtosecond laser pulses in water

We present direct observation of filamentary plasma grating induced by interference between two noncollinear infrared femtosecond pulses in water by doping with gold nanoparticles. The gold nanoparticles act as scattering media in water and visualize the fine structure of local optical fields of plasma grating. By measuring the variation of local conductivity as laser undergoes filamentation in water, the generated electron density in water is qualitatively studied. Significant enhancement of local electron density is observed at the intersecting region as two laser beams form plasma grating, indicating the breakthrough of clamped intensity of a conventional filament in water.

Terahertz modulation induced by filament interaction

We experimentally demonstrated that nonlinear filament interaction could spectrally modulate terahertz (THz) radiation generated from asymmetric two-color filaments. It was the spatial plasma density modulation in plasma channels that induced the THz spectral modulation. As a result of optical manipulation of electron density in the filamentary plasma gratings, the proportion of high-frequency THz spectra increased, while that of low-frequency THz spectra decreased, indicating that the increase of free electron density in the filamentary plasma grating brought about THz frequency upshifts.

Laser filamentation induced bubbles and their motion in water

We demonstrate femtosecond filamentation induced convection in water by using a microscope directly observing the dynamic processes of the generated bubbles on a macroscopic time scale. The bubbles are driven by the filament in water and do directional movements. The angles between the bubbles' moving directions and the laser propagation direction varied at different positions along the filament, exhibiting a fusiform distribution. It indicates a fluid dynamic phenomenon depending on the local filament intensity, and reveals the convection processes induced by filamentation in water indirectly.

Plasma luminescence from femtosecond filaments in air: evidence for impact excitation with circularly polarized light pulses

Filaments produced in air by intense femtosecond laser pulses emit UV luminescence from excited N(2) and N(2)(+) molecules. We report on a strong dependence at high intensities (I≥1.4×10(14)  W/cm(2)) of this luminescence with the polarization state of the incident laser pulses. We attribute this effect to the onset of new impact excitation channels from energetic electrons produced with circularly polarized laser pulses above a threshold laser intensity.

Dissociative recombination in ultraviolet filamentary plasma gratings

We investigated collisions of nitrogen and argon gas mixture with energetic electrons accelerated by Bragg incident intense infrared femtosecond laser pulses in ultraviolet filamentary plasma gratings. Significant decrease of fluorescence spectra of argon atoms were observed when a small amount of nitrogen gas was mixed with argon gas that facilitated observable argon-nitrogen collisions. We experimentally measured the fluorescence emission from the argon and nitrogen gas mixture under different driving pulse energies, the fluorescence decay dynamics after the impact excitation, as well as the fluorescence intensity dependence on the nitrogen and argon pressures. The experimental measurements were based on the electron acceleration and its subsequent impact with the gas mixture in the filamentary plasma gratings, which was essential for the observation of the dominant dissociative recombination in the gas mixture.

Strong-field Kapitza-Dirac scattering of neutral atoms

Laser induced strong-field phenomena in atoms and molecules on the femtosecond (fs) time scale have been almost exclusively investigated with traveling wave fields. In almost all cases, approximation of the strong electromagnetic field by an electric field purely oscillating in time suffices to describe experimental observations. Spatially dependent electromagnetic fields, as they occur in a standing light wave, allow for strong energy and momentum transfer and are expected to extend strong-field dynamics profoundly. Here we report a strong-field version of the Kapitza-Dirac effect for neutral atoms where we scatter neutral He atoms in an intense short pulse standing light wave with fs duration and intensities well in the strong-field tunneling regime. We observe substantial longitudinal momentum transfer concomitant with an unprecedented atomic photon scattering rate greater than 10(16)s(-1).

Computational modeling of laser-plasma interactions: pulse self-modulation and energy transfer between intersecting laser pulses

The nonlinear interaction of intense femtosecond laser pulses with a self-induced plasma channel in air and the energy transfer between two intersecting laser pulses were simulated using the finite-difference time-domain particle-in-cell method. Implementation of a simple numerical code enabled modeling of various phenomena, including pulse self-modulation in the spatiotemporal and spectral domains, conical emission, and energy transfer between two intersecting laser beams. The mechanism for energy transfer was found to be related to a plasma waveguide array induced by Moiré patterns of the interfering electric fields. The simulation results provide a persuasive replication and explanation of previous experimental results, when carried out under comparable physical conditions, and lead to prediction of others. This approach allows us to further examine the effect of the laser and plasma parameters on the simulation results and to investigate the underlying physics.

Impact excitation of neon atoms by heated seed electrons in filamentary plasma gratings

We demonstrate impact ionization and dissociative recombination of neon (Ne) atoms by means of seeded-electron heating and subsequent electron-atom collisions in an ultraviolet plasma grating, allowing for a substantial fraction of the neutral Ne atomic population to reside in high-lying excited states. A buffer gas with relatively low ionization potential (nitrogen or argon) was used to provide high-density seed electrons. A three-step excitation model is verified by the fluorescence emission from the impact excitation of Ne atoms.

Dynamics of plasma gratings in atomic and molecular gases

The decay of the plasma grating formed at the intersection of two femtosecond filaments is measured in several molecular and atomic gases. The grating evolution is ruled by ambipolar diffusion in atomic gases and by a combination of ambipolar diffusion and collision-assisted free electron recombination in molecular gases. Electron diffusion and recombination coefficients are extracted for Ne, Ar, Kr, Xe, N2, O2, CO2, and air at 1 bar.

Control of third harmonic generation by plasma grating generated by two noncollinear IR femtosecond filaments

A plasma grating is formed by two femtosecond filaments, and the influence of probe filament on the plasma grating is shown. By using the plasma grating, the enhancement of the third harmonic (TH) generated from the probe filament is studied, and more than three orders of magnitude enhancement of TH generation is demonstrated as compared with that obtained from a single filament. The dependences of TH generation on the time delay, the spatial period of plasma grating, the relative polarization and the crossing position between the probe beam and the two pump beams are investigated. The spectral broadening of TH generated from the probe filament induced by the interaction between the probe filament and the plasma grating is also studied.