Li Ruxin

Low Resistance and Long Lifetime Plasma Channel Generated by Filamentation of Femtosecond Laser Pulses in Air

An experimental investigation of the length and resistance of the laser plasma channel generated by filamentation of fs laser pulses in air was presented. It was found that the length of the plasma channel was different from that of the laser filament. This phenomenon was commensurate with a special self-guide mechanism without ionization. Through increase of the laser energy the plasma channel could be prolonged and the resistance could be reduced. To get even lower resistance, more effective control would be needed over the multi-filament. The lifetime of the plasma channel was increased by a factor of 4.5 by pulse sequences, which were generated by detuning the regenerative amplifier in the chirped pulse amplification (CPA) laser system.

Efficient Spherical Wavefront Correction near the Focus for the 0.89 PW/29.0 fs Ti:Sapphire Laser Beam

We demonstrate a new loop system of the spherical wavefront (SW) correction near the beam focus to effectively improve the focusability of 0.89 PW/29.0 fs Ti:sapphire chirped pulse amplification laser. After wavefront correction, the Strehl ratio is improved to reach 0.91, and the focal spot size using the f/4 off-axis parabola is reduced to 6.34 ? 6.94 ?m2 (corresponding to 1.63 ? 1.78 times diffraction limitation). With full peak power of 0.89 PW, the peak intensity of 2.59 ? 1021 W/cm2 is obtained. The experimental results show that the SW correction scheme near the beam focus is comparatively simple, economic and high-efficient.

Pulse Temporal Cleaner Based on Nonlinear Ellipse Rotation by Using BK7 Glass Plate

We report a new pulse cleaning technique to enhance the contrast ratio of intense ultra-short laser pulses. A pulse temporal cleaner based on nonlinear ellipse rotation by using BK7 glass plate is developed, and a contrast ratio improvement of two orders of magnitude for the milli-joule level femtosecond input pulses is demonstrated, the total transmission efficiency of the pulse cleaner is 16.7%.

Further Acceleration of MeV Electrons by a Relativistic Laser Pulse

With the development of photocathode rf electron gun, electrons with high-brightness and mono-energy can be obtained easily. By numerically solving the relativistic equations of motion of an electron generated from this facility in laser fields modelled by a circular polarized Gaussian laser pulse, we find the electron can obtain high energy gain from the laser pulse. The corresponding acceleration distance for this electron driven by the ascending part of the laser pulse is much longer than the Rayleigh length, and the light amplitude experienced on the electron is very weak when the laser pulse overtakes the electron. The electron is accelerated effectively and the deceleration can be neglected. For intensities around 10(19) W(.)mu m(2)/cm(2), an electrons energy gain near 0.1 GeV can be realized when its initial energy is 4.5 MeV, and the final velocity of the energetic electron is parallel with the propagation axis. The energy gain can be up to 1 GeV if the intensity is about 10(21) W(.)mu m(2)/cm(2). The final energy gain of the electron as a function of its initial conditions and the parameters of the laser beam has also been discussed.

Temporal Behaviour of Harmonics from One-Dimensional H2+ in an Ultrashort Laser Pulse

With the method of wavelet transform, we consider the temporal behaviour of high-order harmonic generation from one-dimensional H2+ exposed to an ultrashort laser pulse with a duration of tens of femtoseconds. The results, which are calculated by numerically solving the corresponding time-dependent Schrodinger equation with the split-operator method in the non-Born-Oppenheimer approach, show that: (1) the high-order harmonics in the cut-off range emitted as a train of pulses have better coherence than those in the plateau; (2) the harmonics are emitted early in time when the intensity of the laser pulse increases.

Frontiers in ultrafast optics and ultra-intense laser technology

Ultrafast optics and ultra-intense laser technology, which are important frontiers in the fields of optics and lasers, are expected to promote fundamental science discoveries and high-technology developments. In this paper, the current status and future trends of ultrafast optics and ultra-intense laser technology are introduced briefly, including the applications of ultrafast and ultra-intense lasers in particle acceleration, attosecond science, ultrafast nonlinear optics, micro/nano-fabrication, broadband optical frequency combs, etc. The future prospects and key techniques for generating ultra-intense and ultra-short laser pulses with higher performance are also described.

Generation of an isolated sub-100 attosecond pulse in the water-window spectral region

We propose a scheme to generate isolated attosecond pulses in the water-window spectral region. Based on the numerical solutions of the single active electron model, we investigate high-order harmonic generation in helium atoms driven by a multi-cycle two-colour optical field synthesized by an intense 2000 nm, 20 fs pulse and its frequency-doubled pulse. When the latter is slightly detuned and properly phase shifted with respect to the fundamental laser pulse, an ultra-broad extreme ultraviolet supercontinuum with a spectral width of 130 eV can be generated in the 270–400 eV spectral regions. A supercontinuum from 280–340 eV in the water window can be selected to yield an isolated 67 attosecond pulse without employing any phase compensation. This water window coherent x-ray pulse with less than 100 attosecond duration is a potential tool for studying the ultrafast electronic dynamics of biological samples in water.

Wavefront Correction of Pettawat Laser System by a Deformable Mirror with 50 mm Active Aperture

We firstly propose and demonstrate a new economical approach that can correct the wavefront of the petwatt Ti:sapphire laser system with a beam size of 150mm. By using a deformable mirror with 50mm active aperture in the end of the laser, the focal spot size is reduced effectively. The experimental results show that the new approach is simple, less-expensive and valid from a technical and economical point. This technique could be applied to correct the wavefront of a large optical beam with a smaller aperture deformable mirror.

Nuclear fusion from Coulomb explosions of deuterated methane clusters subjected to ultraintense femtosecond laser pulses

This paper reports that Coulomb explosions taken place in the experiment of heteronuclear deuterated methane clusters ((CD4)n) in a gas jet subjected to intense femtosecond laser pulses (170 mJ, 70 fs) have led to table-top laser driven DD nuclear fusion. The clusters produced in supersonic expansion had an average size of about 5 nm in radius and the laser intensity used was 3 × 1017 W/cm2.The measured maximum and average energies of deuterons produced in the laser–cluster interaction were 60 and 13.5 keV, respectively. From DD collisions of energetic deuterons, a yield of 2.5(±0.4) × 104 fusion neutrons of 2.45 MeV per shot was realized, giving rise to a neutron production efficiency of about 1.5 × 105 per joule of incident laser pulse energy. Theoretical calculations were performed and a fairly good agreement of the calculated neutron yield with that obtained from the present experiment was found.

High-Conversion-Efficiency and Broadband Tunable Femtosecond Noncollinear Optical Parametric Amplifier

We build a compact high-conversion-efficiency and broadband tunable noncollinear optical parametric amplifier (OPA) in the infra-red (IR) pumped by a femtosecond Ti:sapphire CPA laser. The OPA consists of an internal seed of white-light continuum generator (WLG) and two noncollinear optical parametric amplifiers. The tunable wavelength range is from 1.2 μm to 2.4 μm for both signal and idle pulses. The total OPA efficiency in the last OPA stage reaches about 40% in a wider tunable spectral range (from 1.3 μm to 1.7 μm for signal pulse, from 1.5 μm to 2.0 μm for idle pulse respectively).