Relativistic flying forcibly oscillating reflective diffraction grating.

Relativistic flying forcibly oscillating reflective diffraction gratings are formed by an intense laser pulse (driver) in plasma. The mirror surface is an electron density singularity near the joining area of the wake wave cavity and the bow wave; it moves together with the driver laser pulse and undergoes forced oscillations induced by the field. A counterpropagating weak laser pulse (source) is incident at grazing angles, being efficiently reflected and enriched by harmonics. The reflected spectrum consists of the source pulse base frequency and its harmonics, multiplied by a large factor due to the double Doppler effect.

4π-spherically focused electromagnetic wave: diffraction optics approach and high-power limits.

The focused field and its intensity distribution achieved by the 4π-spherical focusing scheme are investigated within the framework of diffraction optics. Generalized mathematical formulas describing the spatial distributions of the focused electric and magnetic fields are derived for the transverse magnetic and transverse electric mode electromagnetic waves with and without the orbital angular momentum attribute. The mathematical formula obtained shows no singularity in the field in the focal region and satisfies the finite field strength and electromagnetic energy conditions. The 4π-spherical focusing of the transverse magnetic mode electromagnetic wave provides the highest field strength at the focus and the peak intensity reaches 1026 W/cm2 for the laser power of 100 PW at 800 nm wavelength. As an example of using the mathematical formula, the electron-positron pair production via the Schwinger mechanism is analyzed and compared with previous results.

High-current stream of energetic α particles from laser-driven proton-boron fusion.

The nuclear reaction known as proton-boron fusion has been triggered by a subnanosecond laser system focused onto a thick boron nitride target at modest laser intensity (∼10^{16}W/cm^{2}), resulting in a record yield of generated α particles. The estimated value of α particles emitted per laser pulse is around 10^{11}, thus orders of magnitude higher than any other experimental result previously reported. The accelerated α-particle stream shows unique features in terms of kinetic energy (up to 10 MeV), pulse duration (∼10 ns), and peak current (∼2 A) at 1 m from the source, promising potential applications of such neutronless nuclear fusion reactions. We have used a beam-driven fusion scheme to explain the total number of α particles generated in the nuclear reaction. In this model, protons accelerated inside the plasma, moving forward into the bulk of the target, can interact with ^{11}B atoms, thus efficiently triggering fusion reactions. An overview of literature results obtained with different laser parameters, experimental setups, and target compositions is reported and discussed.

Plasma channel formation in NIR laser-irradiated carrier gas from an aerosol nanoparticle injector.

Aerosol nanoparticle injectors are fundamentally important for experiments where container-free sample handling is needed to study isolated nanoparticles. The injector consists of a nebuliser, a differential pumping unit, and an aerodynamic lens to create and deliver a focused particle beam to the interaction point inside a vacuum chamber. The tightest focus of the particle beam is close to the injector tip. The density of the focusing carrier gas is high at this point. We show here how this gas interacts with a near infrared laser pulse (800 nm wavelength, 120 fs pulse duration) at intensities approaching 1016 Wcm-2. We observe acceleration of gas ions to kinetic energies of 100s eV and study their energies as a function of the carrier gas density. Our results indicate that field ionisation by the intense near-infrared laser pulse opens up a plasma channel behind the laser pulse. The observations can be understood in terms of a Coulomb explosion of the created underdense plasma channel. The results can be used to estimate gas background in experiments with the injector and they open up opportunities for a new class of studies on electron and ion dynamics in nanoparticles surrounded by a low-density gas.

Multi-GeV electron-positron beam generation from laser-electron scattering.

The new generation of laser facilities is expected to deliver short (10 fs-100 fs) laser pulses with 10-100 PW of peak power. This opens an opportunity to study matter at extreme intensities in the laboratory and provides access to new physics. Here we propose to scatter GeV-class electron beams from laser-plasma accelerators with a multi-PW laser at normal incidence. In this configuration, one can both create and accelerate electron-positron pairs. The new particles are generated in the laser focus and gain relativistic momentum in the direction of laser propagation. Short focal length is an advantage, as it allows the particles to be ejected from the focal region with a net energy gain in vacuum. Electron-positron beams obtained in this setup have a low divergence, are quasi-neutral and spatially separated from the initial electron beam. The pairs attain multi-GeV energies which are not limited by the maximum energy of the initial electron beam. We present an analytical model for the expected energy cutoff, supported by 2D and 3D particle-in-cell simulations. The experimental implications, such as the sensitivity to temporal synchronisation and laser duration is assessed to provide guidance for the future experiments.

Analysis on the longitudinal field strength formed by tightly-focused radially-polarized femtosecond petawatt laser pulse.

Tight focusing of radially- or azimuthally-polarized electromagnetic waves becomes attractive because of the strong field generation in the longitudinal direction. In this paper, we investigate the strength of longitudinal electric field when a radially-polarized femtosecond PW laser pulse is tightly focused by a parabolic surface. From the calculation using the vector diffraction approach, it has been shown that the highest strength of 2.2 × 1013 V/cm can be reached for the longitudinal field with a radially-polarized 11.2-fs, 11.2-J uniform-beam-profile laser pulse. The difference in the strength of longitudinal field with different beam profile and the spectrum of a laser pulse has been also carefully examined. The propagation of a laser spot has been simulated under an extremely-tight-focusing condition (0.25 in terms of f-number) and an achievable field strength for a standing longitudinal field has been examined by colliding two radially-polarized fs PW-level laser pulses.

QED cascade with 10 PW-class lasers.

The intensities of the order of 1023-24 W/cm2 are required to efficiently generate electron-positron pairs in laser-matter interaction when multiple laser beam collision is employed. To achieve such intense laser fields with the upcoming generation of 10 PW laser beams, focusing to sub-micron spot size is required. In this paper, the possibility of pair production cascade development is studied for the case of a standing wave created by two tightly focused colliding laser pulses. Even though the stronger ponderomotive force expels the seed particles from the interaction volume when a tightly focused laser beam is used, tight focusing allows to achieve cascade pair production due to the higher intensity in the focal spot. Optimizing the target density can compensate the expulsion by the ponderomotive force and lower the threshold power required for cascade pair production. This will in principle allow to produce pairs with 10 PW-class laser facilities which are now under construction and will become accessible soon.

Spatio-temporal modification of femtosecond focal spot under tight focusing condition.

The focusing property of a focal spot of a femtosecond laser pulse is presented under tight focusing conditions (below f-number of 1). The spatial and temporal intensity distributions of a focused electric field are calculated by vector diffraction integrals and coherent superposition method. The validity of the calculation method is examined by comparing the intensity distribution obtained under a high f-number condition to that obtained with the fast Fourier transform method that assumes the scalar paraxial approximation. The spatial and temporal modifications under tight focusing conditions are described for a focused femtosecond laser pulse. The calculation results show that a peak intensity of about 2.5x10(24) W/cm2 can be achievable by tightly focusing a 12-fs, 10 PW laser pulse with a f/0.5 parabolic optic. The precise information on intensity distributions of a femtosecond focal spot obtained under a tight focusing condition will be crucial in assessing a focused intensity and in describing the motion of charged particles under an extremely strong electric field in ultra-relativistic and/or relativistic laser matter-interaction studies.

High-power γ-ray flash generation in ultraintense laser-plasma interactions.

When high-intensity laser interaction with matter enters the regime of dominated radiation reaction, the radiation losses open the way for producing short pulse high-power γ-ray flashes. The γ-ray pulse duration and divergence are determined by the laser pulse amplitude and by the plasma target density scale length. On the basis of theoretical analysis and particle-in-cell simulations with the radiation friction force incorporated, optimal conditions for generating a γ-ray flash with a tailored overcritical density target are found.

Experimental and clinical investigation of efficiency and ablation profiles of new solid-state deep-ultraviolet laser for vision correction.

PURPOSE: To investigate the efficiency and ablation profiles of a newly developed, all-solid-state laser platform. SETTING: Experimental investigations performed at Katana Technologies GmbH, Kleinmachnow, Germany, and clinical study, at the Ophthalmology Clinic, University of Messina, Messina, Italy. METHODS: Experimental studies were performed on poly(methyl methacrylate) (PMMA) and in porcine eyes using an all-solid-state, Q-switched, frequency-shifted laser (LaserSoft, Katana Technologies GmbH) with a Gaussian spot with a diameter of 0.2 mm in the target plane, a peak fluence of 350 mJ/cm2, and a repetition rate of 1 kHz. The ablation profiles were determined using a profile meter (MicroProf, Fries Research and Technology GmbH), corneal topography was analyzed with a TMS 2N (Tomey Inc.), and corneal thickness was measured with an ultrasound pachymeter (DGH Technology). In the clinical study, 9 human eyes were treated with photorefractive keratectomy. The mean outcome measures were uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BSCVA), corneal topography, and corneal transparency. The follow-up was 1 month for all eyes and 3 months for 4 eyes. Safety, efficacy, and predictability were evaluated. RESULTS: Smooth profiles were found in the PMMA and the porcine eyes. The topographic maps showed central steepening after the hyperopic ablation and slight central flattening of the surface after the myopic treatment. No eye lost lines of BSCVA; the UCVA improved in all eyes. All eyes were within +/-1.00 diopter (D) of emmetropia, and 89% were within +/-0.50 D. CONCLUSION: The efficacy of the ablation was good, with the profile meter results confirmed by the topographic measurements.

Regenerative amplification of femtosecond laser pulses in Ti:sapphire at multikilohertz repetition rates.

We have generated and applied noncoherent x-ray radiation in an all-solid-state laser system operating at repetition rates up to 20 kHz. Based on a model that takes into account the strong thermal loading of the Ti:sapphire rod, a laser cavity with low sensitivity to thermal lensing was chosen. With a maximum pump power of 80 W, an output power as high as 27 W was obtained in gain-switched operation, and, with a seeding from a femtosecond oscillator, 60-fs, 0.8-mJ (8-W) pulses at 10 kHz and 0.32-mJ (6.5-W) pulses at 20 kHz were generated. High power femtosecond output was used to generate x-ray continuum radiation up to 5 keV from a liquid-gallium jet target.

Ultrashort 1-kHz laser plasma hard x-ray source.

We achieved a continuous, stable, ultrashort pulse hard x-ray point source by focusing 1.8-W, 1-kHz, 50-fs laser pulses onto a novel, 30-microm -diameter, high-velocity, liquid-metal gallium jet. This target geometry avoids most of the debris problems of solid targets and provides nearly 4pi illumination. Photon fluxes of 5x10(8) photons/s are generated in a two-component spectrum consisting of a broad continuum from 4 to 14 keV and strong K(alpha) and K(beta) emission lines at 9.25 and 10.26 keV. This source will find wide use in time-resolved x-ray diffraction studies and other applications.