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Pulse self‐compression can offer a more compact and efficient solution for improving the peak power of ultraintense laser pulses. By solving a modified nonlinear Schrödinger equation considering the fifth‐order susceptibility, it is found that self‐compression occurred even in a normally dispersive medium owing to the negative fifth‐order susceptibility inducing a mass of negative frequency chirp. Furthermore, negatively prechirped pulses enabled self‐compression at lower intensities, thus preventing damage to the medium. The optimal choices for prechirp, input intensity, and medium length are numerically analyzed. A proof‐of‐principle experiment confirmed the aforementioned theoretical findings. It is expected that petawatt or even exawatt laser pulses with 25 fs/15 fs transform‐limited pulse duration can be self‐compressed to ≈ 9.9 fs/7.6 fs in normally dispersive media, such as fused silica glass plate. [ABSTRACT FROM AUTHOR]

Compressing high-energy laser pulses to a single-cycle and realizing the "λ3 laser concept", where λ is the wavelength of the laser, will break the current limitation of super-scale projects and contribute to the future 100-petawatt and even Exawatt lasers. Here, we have realized ultra-broadband gold gratings, core optics in the chirped pulse amplification, in the 750–1150 nm spectral range with a > 90% −1 order diffraction efficiency for near single-cycle pulse stretching and compression. The grating is also compatible with azimuthal angles from −15° to 15°, making it possible to design a three-dimensional compressor. In developing and manufacturing processes, a crucial grating profile with large base width and sharp ridge is carefully optimized and controlled to dramatically broaden the high diffraction efficiency bandwidth from the current 100–200 nm to over 400 nm. This work has removed a key obstacle to achieving the near single-cycle 100-PW lasers in the future. Gratings are used for pulse compression and stretching in chirped pulse amplification (CPA) or optical parametric CPA methods. Here the authors demonstrate ultra-broadband gold gratings that can compress single-cycle pulses with powers around 100 petawatts. [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]

High-order harmonics and attosecond pulse generation with coherent wake emission are theoretically investigated for the effect of pulse duration and carrier envelope phase (CEP) of few-cycle laser pulse. We find that short pulse duration will cause the negative chirp for the high harmonics. When the laser pulse is shortened to a few cycles, the influence of the laser CEP on the chirp of the harmonics will also become more prominent. [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]

A high-temporal contrast femtosecond Stokes pulse near 1,053 nm is obtained simply without polarizer extinction ratio limitation based on the stimulated Raman frequency shift process in ethanol with an 800-nm femtosecond Ti:sapphire laser as a pump source. By optimizing the incident pump pulse chirp and the ethanol Raman cell length, a clean Stokes pulse near 1,053 nm with a maximum energy of 0.24 mJ is obtained with ~7.5 % conversion efficiency and 0.8 % (rms) energy fluctuation. Compared with the incident pump pulse, the temporal contrast of the Stokes pulse is improved by at least approximately three orders of magnitude. [ABSTRACT FROM AUTHOR]

The generation of intense carrier-envelope-phase-stabilized sub-1.5 cycle circularly polarized pulses at 1.8 μm is reported. After changing the polarization of the pulses produced by an optical parameter amplifier to circular, selected nonlinear medium parameters are found to be able to expand the spectrum to supercontinuum (1,300-2,100 nm) with a extremely high transmittance (>65 %). Using such laser pulses, polarization control of terahertz emission is demonstrated. [ABSTRACT FROM AUTHOR]

We demonstrate the generation and control of elliptically polarized terahertz (THz) waves from air plasma produced by circularly polarized few-cycle laser pulses. Experimental and calculated results reveal that electric field asymmetry in rotating directions of the circularly polarized few-cycle laser pulses produces the enhanced broadband transient currents, and the phase difference of perpendicular laser field components is partially inherited in the generation process of THz emission. The ellipticity of the THz emission and its major axis direction are all-optically controlled by the duration and carrier-envelope phase of the laser pulses. [ABSTRACT FROM AUTHOR]

The evolution of THz waveform generated in air plasma provides a sensitive probe to the variation of the carrier envelope phase (CEP) of propagating intense few-cycle pulses. Our experimental observation and calculation reveal that the number and positions of the inversion of THz waveform are dependent on the initial CEP, which is near 0.5π constantly under varied input pulse energies when two inversions of THz waveform in air plasma become one. This provides a method of measuring the initial CEP in an accuracy that is only limited by the stability of the driving few-cycle pulses. [ABSTRACT FROM AUTHOR]

A laser wakefield accelerator (LWFA) with a weak focusing force is considered to seek improved beam quality in LWFA. We employ super-Gaussian laser pulses to generate the wakefield and study the behavior of the electron beam dynamics and synchrotron radiation arising from the transverse betatron oscillations through analysis and computation. We note that the super-Gaussian wakefields radically reduce the betatron oscillations and make the electron orbits mainly ballistic over a single stage. This feature permits to obtain small emittance and thus high luminosity, while still benefitting from the low-density operation of LWFA (Nakajima et al. 2011 Phys. Rev. ST Accel. Beams14, 091301), such as the reduced radiation loss, less number of stages, less beam instabilities, and less required wall plug power than in higher density regimes. [ABSTRACT FROM PUBLISHER]

By designing a cascaded laser wakefield accelerator, high-quality monoenergetic electron beams (e beams) with peak energies of 340–360 MeV and rms divergence of <0.3 mrad were produced. Based on this accelerator, the e-beam betatron radiation spectra were measured exactly via the single-photon counting technique to diagnose the e-beam transverse emittance in a single shot. The e-beam transverse size in the wakefield was estimated to be ∼0.35 μm by comparing the measured X-ray spectra with the analytical model of synchrotron-like radiation. By combining the measured e-beam energy and divergence, the normalized transverse emittance was estimated to be as low as 56 μm mrad and consistent with particle-in-cell simulations. These high-energy ultralow-emittance e beams hold great potential applications in developing free electron lasers and high-energy X-ray and gamma ray sources. [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: Based on cross-polarized wave generation, a robust front-end is designed for improving pulse temporal quality of a petawatt Ti:sapphire laser. High-contrast seed pulses with 105μJ energy and excellent beam profile can be generated by the front-end. The spectral width of the clean pulses is over 65nm (FWHM), which can support sub-15fs pulse duration. Injecting the clean pulses into the petawatt Ti:sapphire laser, the pulse temporal contrast is enhanced from original ~10−8 to 1.7×10−11 (−100ps before the main pulse), while the recompressed pulse duration is improved to be 27fs by controlling the spectral evolution in the amplifiers, and the maximum pulse peak power can support ~1.3PW. [Copyright &y& Elsevier]

Fig.1. (a) SEM images of the fabricated low-oscillation dispersion mirrors (LODM). The surface GLAD SiO 2 layer has an ultra-low refractive index of 1.09, which is used to reduce the partial reflection of the surface layer, thereby reducing the dispersion oscillation of the entire mirror. (b) electric field intensity(EFI) distribution of three mirrors. (c) multi-wavelength Dynamic EFI distribution of LODM. In Brief. The dispersion oscillation and damage threshold of a dispersion mirror (DM) are the key influencing factors for high-quality pulses and outputting high power density. The sculptured structure film prepared by the glance-angle deposition(GLAD) technique has a porous structure, and its shape and porosity can be adjusted. In this study, we fabricated a low-oscillation DM (LODM) with a high damage threshold through a sculptured structure layer. Although the static electric field intensity (EFI) inside the LODM is 8.5 times that of the Bragg mirror, their thresholds are still close. As laser damage is a dynamic process, EFI is redistributed in time. A new EFI analysis model, which combines the influence of action time and different frequency components is employed. As a result, the EFI transfer to deep due to sculptured layer is the root cause of high damage threshold. In the depth of the film, there is a time difference in the formation of EFI peaks of different frequencies, which causes the EFI in the depth of the film to cancel out. The dynamic effect of few-cycle laser pulses or attosecond pulses damage will be more common. It can elucidate the ultrafast laser damage behavior of film components. [Display omitted] • A low-oscillation dispersion mirror (LODM) with a high damage threshold is fabricated through a sculptured structure layer; • A new electric field analysis model including time component is established to analyze ultrafast laser damage; • Introducing the standing wave field peak of the multilayer into the depth of the film may be a good way to increase the damage threshold. The dispersion oscillation and damage threshold of a dispersive mirror (DM) are the key influencing factors for high-quality pulses and outputting high power density. Here, we studied the femtosecond damage behavior of low-oscillation DMs(LODM) with SiO 2 sculptured layer. The monochromatic standing wave field(SWF) distribution widely used in ultrafast laser damage analysis is sufficient in Bragg mirror. In more complex systems, the actual electric field intensity(typically smaller) will dynamically change due to the broad bandwidth and the time difference of pulse propagation. Time-domain dynamic electric field intensity displays that the spatio-temporal evolution of the pulse within the stacks averages the electric field intensity, especially the electric field in the deeper layers. The dynamic effect of few-cycle pulses or attosecond pulses damage will be more meaningful. The time-domain dynamic electric field model can elucidate the ultrafast laser damage behavior. Introducing the time-domain dynamic electric model into the merit function can help us design optical coatings with high damage thresholds. [ABSTRACT FROM AUTHOR]