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.

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.

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.

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.

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

Efficient nonlinear Bragg diffraction was observed as an intense infrared femtosecond pulse was focused on a plasma grating induced by interference between two ultraviolet femtosecond laser pulses in air. The preformed electrons inside the plasma grating were accelerated by subsequent intense infrared laser pulses, inducing further collisional ionization and significantly enhancing the local electron density.

[Third harmonic enhancement and spectral analysis within plasma channel based on pump-probe effect].

The experiment realized third harmonic enhancement by using non-collinear dual plasma channels based on pump-probe effect. The coupling action between pump and probe beams can improve saturation effect within filament and overcome the limitation of harmonic enhancement due to intensity clamping. By non-collinearly focusing two ultrafast pulses in air which have respectively a single pulse energy of 4.4 and 10.2 mJ, both with duration of 60 fs and central wavelength of 810 nm, dual filaments as well as weak third harmonics can be generated. When strong beam is ahead of weak one, the former induces a plasma channel in advance which can modulate later weak beam and make obvious harmonic increase produced by probe beam. It was found that oscillation variation of spectral bandwidth in the region of harmonic energy increased significantly. When two beams intersect in a small angle of 27.3 mrad about 15 mm before geometric focus and the probe beam lags behind about 55 fs, the energy increase rate nearly reaches 70, and the corresponding spectral bandwidth is approximately 5 nm.

Plasma waveguide array induced by filament interaction.

We demonstrate that interference-assisted coalescence of two noncollinearly overlapped filaments creates a wavelength-scale periodic plasma density modulation to guide the input pulses equivalently as a photonic crystal plasma waveguide. The periodic self-channeling is evidenced by the direct observation of the filament coalescence, which reveals wavelength-scale spatial widths and periodicity dependent on the crossing angles and intensity ratios between the incident filaments.

[Study on spectral characteristics of third-order harmonic emission of plasma channels in atmosphere].

The authors observed the spectrum of third-order harmonic (TH) emission of the plasma channel in atmosphere by focused ultra-short laser pulses under various conditions. The authors used pulsed Ti: sapphire chirped pulse amplification (CPA) femtosecond laser system, with the central wavelength at 795 nm, repetition rate of 10 Hz, pulse duration of 30 fs and the pulse energy of 12 mJ, focused by a concave mirror with the focal length of 0.5 m, which can generate about 10(13) W x cm(-2) of power intensity. Under this condition, the dynamic balance between nonlinear Kerr self-focusing and plasma defocusing can support a long plasma channel in the interaction of the high intense laser pulses and gaseous media, and the interaction length between the laser pulse and air is greatly elongated, which is helpful to generating third-order harmonic emission. The full width at half maximum (FWHM) of the generated third-order harmonic spectrum is 15 nm with the central wavelength at 265 nm in the forward direction. The spectra of third-order harmonic emission red shift when the laser pulse is positive chirped. On the contrary, the spectra of third-order harmonic emission blue shift when the laser pulse is negative chirped. Proper dispersion can increase the intensity of third-order harmonic spectrum peak and sharped the spectrum peak. With the group velocity dispersion (GVD) of the pulses equal to +1.3 x 10(5) fs2, the peak of third-order harmonic spectrum red shifts and the group velocity dispersion of laser pulses equals to zero fs2. The experiment shows that the rising wing of pulses can obtain higher third-order harmonic conversion efficiency than the falling edge of the pulses. In addition, acoustic-optic programmable dispersive filter (AOPDF) in the laser system can control the spectrum shape of the laser. Changing the hole position of acoustic-optic programmable dispersive filter can also shift the spectrum shape of third-order harmonic emission in a certain range. Studies on spectral characteristics of the third-order harmonic emission in plasma channels at atmosphere are very useful to developing the technology of tuning the radiation in generation of ultraviolet or even soft X-ray domain.

Diode-pumped continuous-wave and passively mode-locked Yb:GSO laser.

We report on both continuous-wave and passively mode-locked laser actions of a new ytterbium-doped laser crystal Yb:Gd(2)SiO(5) (Yb:GSO) under high-power diode-end-pumping. Due to the anisotropic and compact crystal structure, Yb:GSO exhibits a large manifold splitting of Yb(3+) ions, and a Yb:GSO laser can be operated efficiently with negligible reabsorption losses at emission wavelengths. In the continuous-wave operation, a Yb:GSO laser was operated with a slope efficiency of 49% near 1094 nm--the longest laser wavelength achieved for Yb(3+) lasers. Passive mode locking was obtained with a semiconductor saturable-absorber mirror. At a pump power of 6.23 W, a maximum average output power of 638 mW was obtained with the repetition rate of 145.5 MHz.

[Anti-stokes blue fluorescence and stark splitting of 3H4 in Pr3+:Y2SiO5 (YSO)].

In the Pr3+ -doped Y2SiO5, the population in 1D2 of Pr3+ can be transferred to 3P0 state via non-radiative energy transfer by the laser excitation in resonance with 3H4-->1D2 , and we can experimentally study the Stark splitting of 3H4 energy level via 3P0 -->3H4 anti-Stokes emission spectra. Because the anti-Stokes emission spectra can avoid the energy transfer between different crystallographic site 1D2 energy levels, the above splitting lines attribution is more accurate than the assignment via 1D2-->3H4 Stokes spectra by the laser excitation in resonance with 3H4-->1D2. In addition, the character of the anti-Stokes fluorescence decay time was observed.