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The misalignment of the gratings in pulse compressor can introduce residual angular chirp and finally result in a reduction of focused intensity, especially for high peak power femtosecond laser systems with large beam size and broadband spectrum. In this work, we study the angular chirp produced by grating misalignment in different dimensions and comprehensively investigate the various effects on the deterioration of the spatiotemporal characteristics of focused beam. Moreover, two representative ultra-high peak power femtosecond laser systems, featuring with 10 PW peak power/500 mm beam diameter/100 nm spectral width and 50 PW peak power/640 mm beam diameter/200 nm spectral width, are studied. For a drop of effective focused intensity smaller than 10%, the numerical results show that the misalignment of gratings must be kept below ~ 2 μrad and ~ 1 μrad for aforesaid 10 PW and 50 PW lasers, respectively. This work provides meaningful guidelines for the design and construction of the grating compressors in ultra-high peak power femtosecond laser systems. [ABSTRACT FROM AUTHOR]

Femtosecond laser filament-generated plasma can generate electromagnetic pulses (EMPs). These pulses may reduce the instrument's precision, and, hence, influence the accuracy of the experimental results. They may even cause widespread disruption by disabling of the electronic control systems or distribution networks of power plants. This study investigated the characteristics of EMPs generated from the interaction of filament-generated plasmas with a solid target in air. In this study, ultrafast laser filamentation was used to produce plasma, which was focused on a 3 mm-thick aluminum (Al) alloy target for interaction, and the spatial distribution and main contributors of the EMPs were systematically and extensively studied. The results showed that the EMPs generated from ultrafast laser filament interaction with the Al alloy target had the following characteristics: the EMP energy generated from laser filament interaction with solid targets is tens of times higher than that generated only from the femtosecond laser filament; the maximum EMP signals appeared at a 20°–80°detection angle. The relationship between the energy of EMPs and the width and energy of the laser pulses is presented and discussed. These findings are beneficial for gaining insight into the EMP generation mechanism, spatial distribution, and transmission, and for providing more information for the design of EMPs' shielding. [ABSTRACT FROM AUTHOR]

The generation and transport of fast electrons play an important role in intense laser interaction with dense plasma. Here, we experimentally study fast electron generation using a planar aluminum target obliquely irradiated at a large incident angle by an ultrashort high-temporal-contrast relativistic laser pulse. Simulations show that for near-relativistic laser intensities, s-polarized lasers above a certain energy threshold can generate energetic electrons () with a high yield, which is different from the case of low laser contrast. It is also shown that the most energetic electrons are emitted along the target's surface and accelerated to high energies. The key features of the acceleration regimes of lasers with different polarizations are also given. [ABSTRACT FROM AUTHOR]

Long distance propagation of an energetic laser pulse with intensity slightly below that for multi-photon ionization in air is considered analytically, by noting that in the process, it is mainly the peak region of the pulse that interacts with the air molecules. Similar to that of much shorter femtosecond laser pulses of similar intensity, the affected air becomes slightly ionized and self-consistently forms a co-propagating thin and low-density plasma filament along the axis. It is found that a hundred-Joule-level laser pulse with a relatively large spot radius and pulse duration can propagate (also in the form of a self-consistent filament) tens of kilometers through the atmosphere. Such laser propagation properties should have applications in many areas. [ABSTRACT FROM AUTHOR]

Airflow induced by femtosecond laser (800 nm/1 kHz/25 fs) filamentation with different lengths was investigated in a laboratory cloud chamber. Various filament lengths were generated by adjusting laser energy and lens focal length. It was found that airflow patterns are closely related to filament intensity and length. Intense and long filaments are beneficial in updraft generation with large vortices above the filament, while intense and short filaments tend to promote the formation of well-contacted vortices below the filament. Differently patterned airflows induced elliptical snow piles with different masses. We simulated airflow in a cloud chamber numerically taking laser filaments as heat sources. The mechanisms of differently patterned airflow and snow formation induced by filaments were discussed. [ABSTRACT FROM AUTHOR]

The mechanics of generating MeV target surface fast electrons (SFEs) is investigated using a 5 J, 50 fs laser pulse focused on a Cu planar and nanowire target. The energy spectrum and spatial angular distribution of fast electrons emitted from the planar target are determined and compared with those from the nanowire target. When the laser intensity reaches 1 × 1019 W cm−2, the coupling from the laser to 1–3 MeV electrons reaches 1.1% on the planar target at a 45° incidence angle, while the number of SFEs generated from the nanowire target is about 10% of those from the planar target. The two-dimensional particle-in-cell simulation results reproduce the electron emission characteristics and reveal a strong continuous surface magnetic field on the surface of the planar target as compared with the discrete magnetic field on the nanowire target which decreases SFEs. A high, hot electron temperature in the forward direction at 556keV is achieved for the nanowire structure capable of guiding and confining fast electrons along the wire direction, compared with 178keV for the planar target. [ABSTRACT FROM AUTHOR]

• Laser ignition of a lean methane/air mixture by dual-color femtosecond pulses is investigated. • A strong wavelength effect in fs laser ignition (fs-LI) is found. • The role of the ionization enhancement in the wavelength effect is clarified. • The mechanism of fs-LI is ascribed to the non-resonant photochemical ignition. Laser ignition (LI) is promising for green combustion of lean-fuel mixtures with controllable ignition timing and location. It was recently discovered that despite the inferior energy deposition and low thermal temperature in femtosecond (fs) laser-induced plasma, fs laser pulses can achieve a robust ignition of lean-fuel mixture through forming a "line" kernel by filamentation. Here, to clarify fs-LI mechanism, we investigated a dual-color (DC: 800 nm at 1.5 mJ and 400 nm at 0.43 mJ, ∼50 fs) fs-LI of a lean methane/air mixture with an equivalence ratio of φ = 0.87. An optical emission spectroscopy study was conducted to probe the N 2 + and OH emissions and characterize the ignition success rate. It was demonstrated that fs-LI can be achieved at a lower minimum ignition energy (MIE) (<0.46 mJ) by the DC scheme than that (>0.7 mJ) by a single-color (SC: 800 nm at 2.0 and 2.4 mJ, ∼50 fs) scheme, indicating a strong wavelength effect on the successful ignition. A pump-probe measurement was carried out to reveal the effect of the ionization enhancement on the successful ignition. It was found that only when the two-color fs pulses are temporally overlapped, the OH yield is strongly enhanced and the MIE is decreased. By comparing the variation trend of the fluorescence intensity of OH with that of the direct ionization product N 2 +, we ascribed fs-LI to a non-resonant photochemical ignition mechanism, in which the enhancement in the multiphoton/tunnel ionization of the lean-fuel mixture by the high-energy 400-nm photon can increase the yields of the reactive radicals through various dissociation and chain reaction pathways, and thus result in the successful ignition at the micro-joule level. This work unravels the essential role of the non-resonant photochemical ignition mechanism in fs-LI, and provides a promising route for the ignition of lean-fuel engines by compact ultrashort-pulsed lasers in the filamentation regime. [ABSTRACT FROM AUTHOR]

Abstract: High-order harmonic generation (HHG) with 40fs laser pulse in a quasistatic gas cell filled with argon gas was studied. Experiment results in several aspects were given and analyzed. As a result of balancing diffraction and plasma defocusing, and a large harmonic-generation cross-section, the spatial quality of the 23rd harmonic is successfully optimized with a divergence angle of 0.38mrad. The modulation of spectral spatial divergence angle by the gas pressure was given, which was explained by the formation of plasma channel. The effect of initial laser pulse chirp on the harmonic generation and the harmonic spectral blueshift under different gas pressure were given and analyzed. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Spectacular spatial and spectral evolution of femtosecond pulse during laser ablation. • Laser polarization plays an important role on the pulse evolution. • Various micro- and nano- structures are formed the surface of fused silica sample. • New interpretation is provided for the ciliary white light phenomenon. Abstract We report on the optical transmission of near-infrared (1.8 μm) femtosecond laser pulses during multiple pulses laser ablation of fused silica surface. In addition to the previously reported ciliary white light phenomenon (Phys. Rev. Lett. 110, 097601 (2013)), two new optical transmission phenomena were observed for relatively low energy laser pulses. We systemically examined the damage morphologies of the ablation site as a function of laser pulse energy and laser shot number. Laser induced periodic surface structures (LIPSS) with different periods and orientations were observed for different pulse energy regimes. Comparison of the damage morphologies and the optical transmission reveals that the two new optical transmission phenomena origin from the refraction of the incident near-infrared pulses on the LIPSS covered damage surface followed by nonlinear propagation of the beamlets and even filamentation if the laser energy was high enough. We found that the interference of the neighboring white light emissions gives rise to radially orientated optical fringes. Based on this observation, we proposed another interpretation for the ciliary white light phenomenon. [ABSTRACT FROM AUTHOR]

Broadband optical parametric amplification (OPA) in the near-infrared region (1.3–1.8 μm) is demonstrated in YCOB crystal pumped by 1 kHz Ti:sapphire based femtosecond laser at near critical wavelength degeneracy phase matching condition at first time. The gain bandwidth is closed to BIBO or BBO crystal OPA gain bandwidth. The energy obtained with 3.5 mm-thick YCOB reached 20 μJ with RMS 1.9%. After second harmonic generation, 17.6 fs pulse is obtained, and the pulse compressibility is demonstrated. [ABSTRACT FROM AUTHOR]

In femtosecond high-peak-power laser system, the radial group delay (RGD) of the pulse front introduced by conventional lens-based beam expanders can significantly decrease the achievable focal intensity, especially when it is larger than the pulse duration. In order to quantitatively analyze and compensate the RGD, a novel measurement scheme based on self-reference and second-order cross-correlation technology is proposed and applied to measure the RGD of the large-aperture ultra-short laser pulses directly. The measured result of the RGD in a 200 TW Ti:sapphire laser system is in good agreement with the theoretical calculation. To our knowledge, it is the first time to realize the direct RGD measurement of large-aperture ultra-short laser pulses. [ABSTRACT FROM AUTHOR]

Broadening of white-light continuum is observed by filamentation of near-infrared femtosecond laser pulses with peak power exceeding the megawatt level in BK7 glass with the presence of the zero-dispersion point. The simulated results show that, due to the low dispersion at the zero-dispersion point, the broadening of white-light continuum can be wider and the filament can persist in propagating stably longer distance. [ABSTRACT FROM AUTHOR]

• Laser ignition of a CH 4 /air mixture using fs laser filament ablation is proposed. • Ignition of lean fuel with ultralow MPE at 1 mJ and 100% success rate is achieved. • Successful ignition is no longer sensitive to the ablator position. • Reliable Ignition with little amount sample removal of 80 ng/pulse is showed. • This approach is straightforwardly extendable to a designed remote distance. Laser ablation ignition is an efficient approach in igniting lean-fuels with significantly reduced minimum pulse energy (MPE), but it requires a precise position control between the laser focal spot and the target surface, and is hardly for long-term use due to laser-induced surface damage. Here we adopt femtosecond filamentating laser to ablate a tantalum target for igniting a premixed methane-air mixture, and demonstrate the experimental realization of lean-fuel ignition over 1-m distance with 100 % success rate and ultralow MPE at ∼1 mJ. We further show that this technique is insensitive to the ablator position and can operate at a durable time with a little amount (∼80 ng/pulse) of sample removal from the target surface. By characterizing the evolution of flame kernel using time-resolved spectroscopy, we clarify the ultralow MPE ignition mechanism, in which the exothermal chemical reactions occurring inside the plasma filament effectively extends the kernel reaction zone, resulting in a plasma-channel-assisted ablation ignition. [ABSTRACT FROM AUTHOR]

Abstract: The rotational wave packet of linear molecules excited non-adiabatically by a femtosecond laser pulse is controlled actively using the second laser pulse. We propose a novel strategy to determine the optimal delay times of the two laser pulses to modify the coherence of the molecular rotational wave packet, by which the molecular alignment generated by the first pulse is suppressed or enhanced by selecting the respective delay times. This control scheme can be applied to molecular alignment using a multi-pulse laser field. [Copyright &y& Elsevier]