Xiaolong Yuan

Control of filamentation induced by femtosecond laser pulses propagating in air

Filamentation formed by self-focusing of intense laser pulses propagating in air is investigated. It is found that the position of filamentation can be controlled continuously by changing the laser power and divergence angle of the laser beam. An analytical model for the process is given.

200 J high efficiency Ti:sapphire chirped pulse amplifier pumped by temporal dual-pulse

We report on an experimental and theoretical study of a large-aperture Ti:Sapphire (Ti:S) amplifier pumped with a novel temporal dual-pulse scheme to suppress the parasitic lasing (PL) and transverse amplified spontaneous emission (TASE) for high-energy chirped-pulse amplification (CPA). The pump energy distribution was optimized and the time delay between each pump pulse was controlled precisely. Both the numerical and experimental results confirm that the temporal dual-pulse pump technique can effectively suppress PL and TASE. The maximum output energy of 202.8 J was obtained from the final 150-mm-diameter Ti:S booster amplifier with a pump energy of 320.0 J, corresponding to a conversion efficiency of 49.3%. The compressed pulse duration of 24.0 fs was measured with a throughput efficiency of 64%, leading to a peak power of 5.4 PW. This novel temporal dual-pulse pump technique has potential applications in a 10 PW CPA laser system.

Effect of nuclear motion on spectral broadening of high-order harmonic generation.

High-order harmonic generation (HHG) in molecular targets is experimentally investigated in order to reveal the role of the nuclear motion played in the harmonic generation process. An obvious broadening in the harmonic spectrum from the H2 molecule is observed in comparison with the harmonic spectrum generated from other molecules with relatively heavy nuclei. We also find that the harmonic yield from the H2 molecule is much weaker than the yield from those gas targets with the similar ionization potentials, such as Ar atom and N2 molecule. The yield suppression and the spectrum broadening of HHG can be attributed to the vibrational motion of nuclear induced by the driving laser pulse. Moreover, the one-dimensional (1D) time-dependent Schrödinger equation (TDSE) with the non-Born-Oppenheimer (NBO) treatment is numerically solved to provide a theoretical support to our explanation.

Enhanced high-order harmonic generation from spatially prepared filamentation in argon

We experimentally demonstrate enhanced high-order harmonic generation (HHG) from spatially prepared filamentation in Argon. Upon shifting the focus position of an elliptically polarized laser pulse over the filament induced by a linearly polarized laser pulse, an obvious enhancement of harmonic yield by nearly one order of magnitude is observed. The result could be interpreted in terms of the double contributions from both the excited states of target atom and the phase-matching effect of harmonic beam. In contrast to the enhancement phenomena, an obvious suppression of harmonic yield is also presented, which could be attributed to both the ground-state depletion and the plasma effect.

Parasitic lasing in large aperture Ti:sapphire chirped pulse amplifier

We research some properties of parasitic lasing (PL) in the Ti: sapphire chirped pulse amplifier with the crystal diameter of 100 mm. The evolutionary process from the spontaneous emission to the PL and its influence on amplified output energy, spectrum, and beam profile are experimentally measured. The threshold of PL in the crystal is 22 J, and the output signal can still keep rising with the pump when the pump energy is below 38 J. The PL has no obvious impact on the output spectrum and beam profile besides the energy.

Enhanced high-order harmonic generation from excited argon

We experimentally demonstrate an effective scheme to generate the enhanced high-order harmonics from a coherent superposition state of ground state and excited states in Argon. By controlling the time delay between the pump and probe laser pulses, a surprising enhancement plateau of high-order harmonic generation is observed, with a half lifetime of dozens of picoseconds. In the plateau, the harmonic intensity is enhanced by nearly one order of magnitude compared to the case without pre-excitation. Moreover, the gradual enhancement process is also presented when the pump intensity is increasing for improving the population of excited states, which can be attributed to the Rydberg states created by frustrated tunneling ionization.

Probing electron–atom collision dynamics in gas plasma by high-order harmonic spectroscopy

Plasma is a complex system involving diverse collisional processes and interactions, such as electron-impact excitation, ionization, recombination, etc. One of the most important methods for studying the properties and dynamics of plasma is to analyze the radiations from plasma. Here, we demonstrate the high-order harmonic (HH) spectroscopy for probing the complex electron-atom collision (EAC) dynamics in a laser-induced gas plasma. These measurements were carried out by using an elliptically polarized pump and a time-delayed linearly polarized probe. The HH spectra from argon and krypton plasmas were recorded by scanning the time delay up to hundreds of picoseconds. We found that the delay-dependent HH yield contains three distinct regions, i.e., the first enhancement, the subsequent suppression, and the final restoration regions. A qualitative analysis shows that these features are clear signatures of the EAC processes and interactions involved in the delay-dependent HH spectroscopy.

Fast electron transport in high-intensity laser-plasma interactions diagnosed by optical and ion emission

Light and proton emission from rear surface of sandwich-like targets, which are irradiated by intense femtosecond laser pulses, have been measured. It is found that both of them depend on the transport material and density inside the targets. For metal transport layers the patterns of the optical images and the proton beams are uniform. While for the transport layers of dielectric, low-density foam and vacuum gap, the patterns are broken up. This indicates that the fast electron beams are also broken up probably due to instabilities.

Study of Cu Kα emission from frequency doubled intense femtosecond laser-plasma interactions

A laser-driven hard x-ray source with a small spatial size, high contrast characteristic Kα emission has been developed by a frequency doubled (~400nm) high contrast intense laser pulse focused on 50μm thick Cu foil. Simulations demonstrate the vacuum heating may be the main mechanism to generate hot electrons.