Zuoye Liu

Effect of laser pulse energy on orthogonal double femtosecond pulse laser-induced breakdown spectroscopy

In this paper, the effect of laser pulse energy on orthogonal double femtosecond pulse laser induced breakdown spectroscopy (LIBS) in air is studied. In the experiment, the energy of the probe pulse is changeable, while the pump pulse energy is held constant. At the same time, a systematic study of the laser induced breakdown spectroscopy signal dependence on the inter-pulse delay between the two pulses is performed. It is noted that the double pulse orthogonal configuration yields 2-32 times signal enhancement for the ionic and atomic lines as compared to the single pulse LIBS spectra when an optimum temporal separation between the two pulses is used, while there is no significant signal enhancement for the molecular lines in the studied range of the delay. It is also noted that the dependence of the enhancement factor for ionic and atomic lines on the inter-pulse delay can be fitted by Gaussian function. Furthermore, the electron temperature obtained by the relative line-to-continuum intensity ratio method was used to explain the LIBS signal enhancement.

Measurement of nonlinear refractive index coefficient using emission spectrum of filament induced by gigawatt-femtosecond pulse in BK7 glass

A beam of 33 fs laser pulse with peak power of 15-40 GW was employed to explore a convenient method to determine the nonlinear refractive index coefficient of an optical glass. It is rare to investigate nonlinearities of optical glass with such an extreme ultrashort and powerful laser pulse. According to our method, only a single beam and a few experimental apparatuses are necessary to measure the nonlinear refractive index coefficient. The results from our method are in reasonable agreement with the others, which demonstrates that this new method works well, and the nonlinear refractive index coefficient is independent of measuring technology. Meanwhile, according to our results and those obtained by others in different laser power ranges, it seems that the nonlinear refractive index coefficient has a weak dependence on the laser peak power.

Optical Emission Spectroscopy Analysis of the Early Phase During Femtosecond Laser-Induced Air Breakdown

Single-pulse and double-pulse optical emission spectroscopy (OES) analyses were carried out in air by using ultrashort laser pulses at atmospheric pressure. The aim of this work is to use spectroscopic methods to analyze the early phase of laser-induced plasma after the femtosecond laser pulse. The temporal behavior of emission spectra of air plasma has been characterized. In comparison with the single-pulse scheme, the plasma emission obtained in the double-pulse scheme presents a more intense continuum along with several additional ionic lines. As only one line is available in the single-pulse scheme, the plasma temperature measurements were performed using only the relative line-to-continuum intensity ratio method, whereas the relative line-to-line intensity ratio method and the relative line-to-continuum intensity ratio method were used simultaneously to estimate the electron temperature in the double-pulse scheme. The results reveal that the temperature values obtained by the two methods in the double-pulse scheme agree. Moreover, this shows that the relative line-to-continuum intensity ratio method is suitable for early phase of laser-induced plasma diagnostics. The electron number density was estimated using the Stark broadening method. In the early phase of laser-induced plasma, the temporal evolution of the electron number density exhibits a power law decrease with delay time.

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.

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.

Effect of target composition on proton acceleration in ultraintense laser-thin foil interaction

The interactions of ultraintense circularly polarized laser pulses with a mixed solid target and a double-layer target are studied by two-dimensional particle-in-cell simulations. Different carbon and proton compositions in the targets are used in the simulations. It is shown that the proton acceleration mechanisms in both targets are very sensitive to the ion density ratios between protons and carbon ions. For a mixed solid target, a relatively low proton density gives rise to monoenergetic peaks in the proton energy spectrum while a high proton density leads to a large cut-off energy and wide energy spread. With the increase of the ratio, the so-called directed-Coulomb-explosion becomes dominated over the radiation pressure. Surprisingly, for a double-layer target with a front proton layer and an ultrathin rear carbon layer, a highly monoenergetic proton beam with a peak energy of 1.7 GeV/u, an energy spread of ∼4%, and a divergency angle of 2° can be obtained, which might have diverse applications in m...

QUASI-CLASSICAL STUDY OF STEREO-DYNAMICS FOR THE REACTION C + CH → C2 + H ON THE 12A′ POTENTIAL ENERGY SURFACE

The quasi-classical trajectory (QCT) method and the 12A′ potential energy surface (PES) [Boggio-Pasqua et al., Phys Chem Chem Phys2:1693, 2000] have been employed to study the stereo-dynamics of the reaction C + CH (v = 0, j) → C2 + H at different collision energies over the range of 0.01–0.6 eV and for different rotational quantum number j = 0 - 3. The reactive total cross section with initial revibrational state of v = 0 and j = 0 as a function of collision energy is presented and compared with the quantum mechanics results. The forward-backward asymmetry phenomenon has been found in the angular distribution of the products. The calculated distribution of P(θr) indicates a strong product alignment perpendicular to k, but this kind of product alignment is found to be rather insensitive to the collision energy. The calculated distribution of P(ϕr) revealed that at low collision energy the products tend to be oriented along the negative direction of the y-axis, while at high collision energy, this product orientation tends to be pointed to the positive direction of the y-axis. Such product orientation tends generally to become stronger with the increase of collision energy. Further, product polarization (i.e. orientation and alignment) becomes weak with high rotational excitation of the reagent CH molecule.

Spatiotemporal pulse-splitting of a filamentary femtosecond laser pulse via multi-filament interaction

We report the spatiotemporal pulse-splitting of a filamentary pulse with splitting gaps in 102 μm magnitude via interaction with a plasma grating. The spatiotemporal pulse-splitting is interpreted by the transient refractive index change of the interaction area based on the ionization model. The laser pulse is temporally split during the filamentation process. When the precedent part of the temporal splitting probe pulse interacts with the plasma grating, the refractive index of the interaction area is significantly changed, resulting in the spatial separation of the subsequent part from its original position. The splitting gaps between the original pulse and the splitting pulse depend on the crossing angle between the filamentary pulse and the plasma grating and the relative position of the pulse interacts on the plasma grating.

Fluorescence spectroscopy of the plasma induced by multifilament interaction in air

Plasma regions are created by a single probe filament, a plasma grating, and the probe filament interacting with the plasma grating. The continuum radiations and the linear radiations of the plasmas generated are used to investigate the dynamics of the three types of plasmas. As free electrons generated inside the plasma grating are accelerated by the probe laser field, the impact ionization rate of the molecules sharply increases, which results in particles inside the plasma dissociating through various reaction paths.