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.

Gas-phase pressure measurement using femtosecond laser-induced grating scattering technique.

Gas-phase pressure measurements remain challenging in situations where local pressure rapidly changes or in hostile environments such as turbulent combustion. In this work, we demonstrate the implementation of the recently developed femtosecond laser-induced grating scattering (fs-LIGS) technique for pressure measurement in ambient air. With an overall femtosecond laser pulse energy of 185 µJ, fs-LIGS signals were generated for various gas pressure ranging from 0.2 to 3.0 bar. By theoretically fitting the signal and extracting the time constant of the stationary density modulation damping, the pressure is successfully derived. The derived values were compared to the gauge pressure, which shows a quasi-linear dependence with a slope of 0.96, suggesting the feasibility of the fs-LIGS technique for gas-phase pressure measurements.

Resonant Perfect Absorption Yielded by Zero-Area Pulses.

We propose and study the manipulation of the macroscopic transient absorption of an ensemble of open two-level systems via temporal engineering. The key idea is to impose an ultrashort temporal gate on the polarization decay of the system by transient absorption spectroscopy, thus confining its free evolution and the natural reshaping of the excitation pulse. The numerical and analytical results demonstrate that even at moderate optical depths, the resonant absorption of light can be reduced or significantly enhanced by more than 5 orders of magnitude relative to that without laser manipulation. The achievement of the quasicomplete extinction of light at the resonant frequency, here referred to as resonant perfect absorption, arises from the full destructive interference between the excitation pulse and its subpulses developed and tailored during propagation, and is revealed to be connected with the formation of zero-area pulses in the time domain.

A redshift mechanism of high-order harmonics: Change of ionization energy.

We theoretically study the high-order harmonic generation of H2+ and its isotopes beyond the Born-Oppenheimer dynamics. It is surprising that the spectral redshift can still be observed in high harmonic spectra of H2+ driven by a sinusoidal laser pulse in which the trailing (leading) edge of the laser pulse is nonexistent. The results confirm that this spectral redshift originates from the reduction in ionization energy between recombination time and ionization time, which is obviously different from the nonadiabatic spectral redshift induced by the falling edge of the laser pulse. Additionally, the improved instantaneous frequency of harmonics by considering the changeable ionization energy can deeply verify our results. Therefore, this new mechanism must be taken into account when one uses the nonadiabatic spectral redshift to retrieve the nuclear motion.

Enhancement of the second plateau in solid high-order harmonic spectra by the two-color fields.

We theoretically investigate high-order harmonic generation (HHG) from solids in two-color fields. It is found that under the premise of maintaining the same amplitude, the intensity of the second plateau can be enhanced by two to three orders in a proper two-color field compared with the result in the monochromatic field with the same frequency as the driving pulse of the two-color field. This can be attributed to the fact that most excited electrons can be driven to the top of the first conduction band due to the larger vector potential of the two-color fields, which leads to the higher electron population of upper conduction bands. Moreover, we also find that isolated attosecond pulses can be generated from solids by choosing a proper two-color field that allows the electrons to reach the top of the first conduction band only once. This work provides a promising method for extending the range of solid HHG spectra in experiments.

Theoretical scheme for simultaneously observing forward-backward photoelectron holography.

Photoelectron angular momentum distribution of He+ driven by a few-cycle laser is investigated numerically. We simultaneously observe two dominant interference patterns with one shot of lasers by solving the 3D time-dependent Schrodinger equation. Analysis of a semiclassical model identifies these two interference patterns as two types of photoelectron holography. The interference pattern with Pz>0 is a type of forward rescattering holography, which comes from the interference between direct (reference) and rescattered (signal) forward electrons ionized in the same quarter-cycle. The interference pattern with Pz<0 is a type of backward rescattering holography, which comes from the interference between a direct electron ionized in the third quarter-cycle and rescattered backward electron ionized in the first quarter-cycle. Moreover, we propose a method to distinguish this backward rescattering holography and intracycle interference patterns of direct electrons.

Reexamining the high-order harmonic generation of HD molecule in non-Born-Oppenheimer approximation.

The high-order harmonic generation of the HD molecule is studied in non-Born-Oppenheimer approximation. It is found that there are only the odd harmonics in the harmonic spectrum of the HD molecule though the generation of even harmonics is possible in principle. Theoretical analysis [T. Kreibich et al., Phys. Rev. Lett. 87, 103901 (2001)] reveals that the nuclear dipole moment can contribute to the generation of the even harmonics, but the acceleration of the nucleus is about three orders of magnitude less than that of the electron. Hence, the even harmonics cannot be observed in the harmonic spectrum of the HD molecule.

Assessment of radioactive materials and heavy metals in the surface soil around uranium mining area of Tongliao, China.

Natural and artificial radionuclides and heavy metals in the surface soil of the uranium mining area of Tongliao, China, were measured using gamma spectrometry, flame atomic absorption spectrophotometry, graphite furnace atomic absorption spectrophotometry and microwave dissolution atomic fluorescence spectrometry respectively. The estimated average activity concentrations of (238)U, (232)Th, (226)Ra, (40)K and (137)Cs are 27.53±16.01, 15.89±5.20, 12.64±4.27, 746.84±38.24 and 4.23±4.76Bq/kg respectively. The estimated average absorbed dose rate in the air and annual effective dose rate are 46.58±5.26nGy/h and 57.13±6.45µSv, respectively. The radium equivalent activity, external and internal hazard indices were also calculated and their mean values are within the acceptable limits. The heavy metal concentrations of Pb, Cd, Cu, Zn, Hg and As from the surface soil were measured and their health risks were then determined. Although the content of Cd is much higher than the average background in China, its non-cancer and cancer risk indices are all within the acceptable ranges. These calculated hazard indices to estimate the potential radiological health risk in soil and the dose rate are well below their permissible limit. In addition the correlations between the radioactivity concentrations of the radionuclides and the heavy metals in soil were determined by the Pearson linear coefficient.

The signal quality improvement of laser-induced breakdown spectroscopy due to the microwave plasma torch modulation.

BACKGROUND: Laser-induced breakdown spectroscopy (LIBS) is a versatile analytical technique for element determination in solids, liquids, and gases. However, LIBS suffers from low detection sensitivity and high relative standard deviation (RSD), restricting its large-scale applications. the process of a physical sampling can, in some cases, compromise the mechanical strength of the component under examination. It should be considered that too large laser energy is bound to cause damage to samples which cannot be tolerated in the process of safe production in the nuclear industry. It is necessary to find a method to obtain high elemental signal intensity in low energy laser. RESULTS: Here, we present a novel approach by integrating microwave plasma torch (MPT) with LIBS, referred to as MPT-LIBS, which effectively addresses the limitations associated with traditional LIBS. The MPT-LIBS technique is evaluated using Cu samples with a low laser pulse energy of 0.55 mJ. A remarkable enhancement factor of over 70 for Cu I 521.82 nm line is demonstrated, while that of Cu I 324.75 nm and 327.40 nm lines exceeding two orders of magnitude. Furthermore, the RSDs of all Cu spectral lines are reduced, especially for Cu I 521.82 nm, which is decreased from 11.48 % to 1.36 %. This indicates a significant improvement in signal stability. Characterization of the tested samples using con-focal microscopy reveals that the ablation area of MPT-LIBS is only 1.36 times of that of LIBS. The limit of detection of Cu I 324.75 nm line is reduced from 52.8 ppk to 319 ppm. SIGNIFICANCE AND NOVELTY: This study not only offers valuable guidance for improving signal stability and the limit of detection in LIBS, but also demonstrates minimal sample damage due to its low ablation amount. Consequently, the proposed methodology has the potential to significantly advance LIBS technology, expanding its applicability in industrial applications.

Intense supercontinuum generation exceeding 300eV using a two-color field in combination with a 400-nm few-cycle control pulse.

We propose a method to control the harmonic process by using a two-color field in combination with a 400-nm few-cycle control pulse for the generation of an ultra-broadband supercontinuum with high efficiency. The ionization and acceleration steps in the harmonic process can be simultaneously controlled by using a three-color field synthesized by a 2000-nm driving pulse and two weak 800-nm and 400-nm control pulses. Then an intense supercontinuum covered by the spectral range from 140 eV to 445 eV is produced. The 3D macroscopic propagation is also employed to select the short quantum path of the supercontinuum, then intense isolated sub-100-as pulses with tunable central wavelengths are directly obtained within water window region. In addition, the generation of isolated attosecond pulses in the far field is also investigated. An isolated 52-as pulse can be generated by using a filter centered on axis to select the harmonics in the far field.

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.

Energy exchange between two noncollinear filament-forming laser pulses in air.

Energy exchange between two filament-forming pulses with initially free chirp in air was experimentally studied. It occurs because of the change of delayed nonlinear refractive index, which slightly chirps the incident filament-forming laser pulses. Accompanying energy exchange process, spectral characteristics of output laser pulses shows dramatic blueshift and supercontinuum generation. Nonlinear absorptive effect introduces an inbalance between energy exchange at the negative delays and that at the positive delays, and affects the energy exchange efficiency. These results may provide a more comprehensive understanding of energy exchange process between filament-forming laser pulses.

Control of ultra-intense single attosecond pulse generation in laser-driven overdense plasmas.

Ultra-intense single attosecond pulse (AP) can be obtained from circularly polarized (CP) laser interacting with overdense plasma. High harmonics are naturally generated in the reflected laser pulses due to the laser-induced one-time drastic oscillation of the plasma boundary. Using two-dimensional (2D) planar particle-in-cell (PIC) simulations and analytical model, we show that multi-dimensional effects have great influence on the generation of AP. Self-focusing and defocusing phenomena occur in front of the compressed plasma boundary, which lead to the dispersion of the generated AP in the far field. We propose to control the reflected high harmonics by employing a density-modulated foil target (DMFT). When the target density distribution fits the laser intensity profile, the intensity of the attosecond pulse generated from the center part of the plasma has a flatten profile within the center range in the transverse direction. It is shown that a single 300 attosecond (1 as = 10(-18)s) pulse with the intensity of 1.4 × 10(21) W cm(-2) can be naturally generated. Further simulations reveal that the reflected high harmonics properties are highly related to the modulated density distribution and the phase offset between laser field and the carrier envelope. The emission direction of the AP generated from the plasma boundary can be controlled in a very wide range in front of the plasma surface by combining the DMFT and a suitable driving laser.

Wavefront improvement in an injection-seeded soft x-ray laser based on a solid-target plasma amplifier.

The wavefront of an injection-seeded soft x-ray laser beam generated by amplification of high-harmonic pulses in a λ=18.9 nm molybdenum plasma amplifier was measured by a Hartmann wavefront sensor with an accuracy of λ/32 root mean square (rms). A significant improvement in wavefront aberrations of 0.51±0.03λ rms to 0.23±0.01λ rms was observed as a function of plasma column length. The variation of wavefront characteristic as a function time delay between the injection of the seed and peak of soft x-ray amplifier pump was studied. The measurements were used to reconstruct the soft x-ray source and confirm its high peak brightness.

Isolated attosecond pulse generation from pre-excited medium with a chirped and chirped-free two-color field.

We theoretically investigate the isolated attosecond pulse generation from pre-excited medium with a chirped and chirped-free two-color field. It is found that the large initial population of the excited state can lead to the high density of the free electrons in the medium and the large distortion of the driving laser field after propagation, though it benefits large enhancement of harmonic intensity in single atom response. These effects can weaken the phase-match of the macroscopic supercontinuum. On the contrary, the small initial population of 4% can generate well phase-match intense supercontinuum. We also investigate an isolated attosecond pulse generation by using a filter centered on axis to select the harmonics in the far field. Our results reveal that the radius of the spatial filter should be chosen to be small enough to reduce the duration of the isolated attosecond pulse due to the curvature effect of spatiotemporal profiles of the generated attosecond pulses in the far field.

Attosecond ionization control for broadband supercontinuum generation using a weak 400-nm few-cycle controlling pulse.

We theoretically demonstrate a method for generating the broadband supercontinuum. It is found that a weak 400-nm few-cycle pulse can be used to replace the ultraviolet attosecond pulse for controlling the ionization dynamics of the electron wave packets when a long-wavelength driving pulse is adopted. By adding a 400-nm few-cycle laser pulse to a 2000-nm driving pulse at proper time, only a quantum path can be selected to effectively contribute to the harmonics, leading to the efficient generation of a broadband supercontinuum. Moreover, our scheme is stable against nearly all the small parameter shift of the driving pulse and the controlling pulse. The macroscopic investigation reveals that the macroscopic supercontinuum with the bandwidth of about 165eV can be obtained. Then isolated sub-110-as pulses can be directly generated. Moreover, the generated attosecond pulse has a divergence angle of about 0.1mrad in the far field, which indicates its beam quality is good. Besides, it is also found that a near-field spatial filter can be used to select the different quantum paths (short or long) in the far field.

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.

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.

Chromosomal aberrations in the bone marrow cells of mice induced by accelerated (12)C(6+) ions.

The whole bodies of 6-week-old male Kun-Ming mice were exposed to different doses of (12)C(6+) ions or X-rays. Chromosomal aberrations of the bone marrow (gaps, terminal deletions and breaks, fragments, inter-chromosomal fusions and sister-chromatid union) were scored in metaphase 9h after exposure, corresponding to cells exposed in the G(2)-phase of the first mitosis cycle. Dose-response relationships for the frequency of chromosomal aberrations were plotted both by linear and linear-quadratic equations. The data showed that there was a dose-related increase in the frequency of chromosomal aberrations in all treated groups compared to controls. Linear-quadratic equations were a good fit for both radiation types. The compound theory of dual radiation action was applied to decipher the bigger curvature (D(2)) of the dose-response curves of X-rays compared to those of (12)C(6+) ions. Different distributions of the five types of aberrations and different degrees of homogeneity were found between (12)C(6+) ion and X-ray irradiation and the possible underlying mechanism for these phenomena were analyzed according to the differences in the spatial energy deposition of both types of radiation.

Broadband supercontinuum generation method combining mid-infrared chirped-pulse modulation and generalized polarization gating.

We present a method to control the harmonic process by a mid-infrared modulated generalized polarization gating for the generation of the broadband supercontinuum. Using a mid-IR generalized polarization gating modulated by a weaker mid-IR linearly polarized chirped field, the ionization, acceleration and recombination steps in the HHG process are simultaneously controlled, leading to the efficient generation of an ultra-broadband supercontinuum covered by the spectral range from ultraviolet to water window x-ray. Using this method we expect that isolated sub-100 attosecond pulses with tunable wavelength could be obtained straightforwardly.