High-intensity Laser Pulses Research Articles

Structured targets for detection of Megatesla-level magnetic fields through Faraday rotation of XFEL beams

A solid density target irradiated by a high-intensity laser pulse can become relativistically transparent, which then allows it to sustain an extremely strong laser-driven longitudinal electron current. The current generates a filament with a slowly varying MT-level azimuthal magnetic field that has been shown to prompt efficient emission of multi-MeV photons in the form of a collimated beam required for multiple applications. This work examines the feasibility of using an x-ray beam from the European x-ray free electron laser for the detection of the magnetic field via the Faraday rotation. Post-processed three dimensional particle-in-cell simulations show that, even though the relativistic transparency dramatically reduces the rotation in a uniform target, the detrimental effect can be successfully reversed by employing a structured target containing a channel to achieve a rotation angle of 10−4 rad. The channel must be relativistically transparent with an electron density that is lower than the near-solid density in the bulk. The detection setup has been optimized by varying the channel radius and focusing the laser pulse driving the magnetic field. We predict that the Faraday rotation can produce 103 photons with polarization orthogonal to the polarization of the incoming 100 fs long probe beam with 5 × 1012 x-ray photons. Based on the calculated rotation angle, the polarization purity must be much better than 10−8 in order to detect the signal above the noise level.

Ionization behavior and dynamics of picosecond laser filamentation in sapphire

Currently, laser-induced structural modifications in optical materials have been an active field of research. In this paper, we reported structural modifications in the bulk of sapphire due to picosecond (ps) laser filamentation and analyzed the ionization dynamics of the filamentation. Numerical simulations uncovered that the high-intensity ps laser pulses generate plasma through multi-photon and avalanche ionizations that leads to the creation of two distinct types of structural changes in the material. The experimental bulk modifications consist of a void like structures surrounded by cracks which are followed by a submicrometer filamentary track. By increasing laser energy, the length of the damage and filamentary track appeared to increase. In addition, the transverse diameter of the damage zone increased due to the electron plasma produced by avalanche ionizations, but no increase in the filamentary zone diameter was observed with increasing laser energy.

Experimental study of fast electron generation from intense laser irradiated mylar foil with thin metal coating on front or rear surfaces

We report angular and spectral distribution of fast electrons in the interaction of a high intensity laser pulse (30 fs, I∼1019 W/cm2) with uncoated transparent mylar foil (thickness: 8 μm). The effect of thin Al coating (50 nm) on either the front or rear surface of the mylar foil on fast electron generation was also investigated. An electron flux enhancement of ∼1.8× (from ∼45 pC to ∼80 pC) and an increase in the maximum electron energy from ∼ 400 keV to ∼ 800 keV were observed in the case of front coated foil compared to the uncoated one. In the case of rear coated foil, an enhancement of ∼1.4× in the electron flux with no change in maximum electron energy was observed compared to the uncoated foil. The observations are understood in terms of possible different preplasma conditions for various target configurations used, which is also supported by 1D hydrodynamic simulation carried out for the present experimental conditions. The observed enhancement in electron flux and temperature is also supported by 2D Particle in Cell (PIC) simulation.

Highly efficient laser-driven Compton gamma-ray source

The recent advancement of high-intensity lasers has made all-optical Compton scattering become a promising way to produce ultrashort brilliant γ-rays in an ultra-compact system. However, so far achieved Compton γ-ray sources are limited by low conversion efficiency and spectral intensity. Here we present a highly efficient gamma photon emitter obtained by irradiating a high-intensity laser pulse on a miniature plasma device consisting of a plasma lens and a plasma mirror. This concept exploits strong spatiotemporal laser-shaping process and high-charge electron acceleration process in the plasma lens, as well as an efficient nonlinear Compton scattering process enabled by the plasma mirror. Our full three-dimensional particle-in-cell simulations demonstrate that in this novel scheme, brilliant γ-rays with very high conversion efficiency (higher than 10−2) and spectral intensity (∼109 ) can be achieved by employing currently available petawatt-class lasers with intensity of 1021 W cm−2. Such efficient and intense γ-ray sources would find applications in wide-ranging areas.

A mathematical model of optical bistability and the multiplicity of its solutions

Abstract The problem of high-intensity laser pulse interaction with a semiconductor under the condition of the light energy nonlinear absorption is considered. Mathematical model of an optical bistability based on re-normalization of forbidden energy zone of a semiconductor because of the induced electric field is presented. The multiplicity of the problem solutions at the Neumann boundary conditions statement is discussed and ways of its overcoming are proposed. The effective method for the numerical solution of the problem is developed and computer simulation results are presented.

Laser propagation in a subcritical foam: Ion and electron heating

We develop a model for laser propagation and heating in a subcritical foam (homogeneous electron density as a fraction of critical ne,0/nc<1). Our model describes the partition of energy between ions and electrons in an expanding foam element irradiated by a laser, and we solve for the plasma conditions generated by burning down the foam microstructure. We find that a substantial fraction of laser energy goes into breaking down and homogenizing the foam microstructure, which slows down the laser heat front. We also find that the ion temperature in a plasma generated by burning down a foam can exceed the electron temperature. This is because laser energy is deposited into the expansion kinetic energy of ions as the foam microstructure burns down explosively. The higher ion temperature increases ion-acoustic wave damping which reduces stimulated Brillouin scattering (SBS). We test our model against data from an experiment that shot a subcritical foam with a high intensity laser pulse on the Janus laser facility at LLNL. We find that by modeling the effect of the foam microstructure, we can explain both the experimentally measured laser propagation velocity and the backscattered SBS power.

Biphotonic Ionization of DNA: From Model Studies to Cell.

Oxidation reactions triggered by low-intensity UV photons represent a minor contribution with respect to the overwhelming pyrimidine base dimerization in both isolated and cellular DNA. The situation is totally different when DNA is exposed to high-intensity UVC radiation under conditions where biphotonic ionization of the four main purine and pyrimidine bases becomes predominant at the expense of singlet excitation processes. The present review article provides a critical survey of the main chemical reactions of the base radical cations thus generated by one-electron oxidation of nucleic acids in model systems and cells. These include oxidation of the bases with the predominant formation of 8-oxo-7,8-dihydroguanine as the result of preferential hole transfer to guanine bases that act as sinks in isolated and cellular DNA. In addition to hydration, other nucleophilic addition reactions involving the guanine radical cation give rise to intra- and interstrand cross-links together with DNA-protein cross-links. Information is provided on the utilization of high-intensity UV laser pulses as molecular biology tools for studying DNA conformational features, nucleic acid-protein interactions and nucleic acid reactivity through DNA-protein cross-links and DNA footprinting experiments.

Tracking ultrafast dynamics of intense shock generation and breakout at target rear

We report upon the picosecond plasma dynamics at the rear surface of a thin aluminium foil (of either 5.5 μm or 12 μm thickness) excited by high contrast (picosecond intensity contrast of 10−10), 800 nm, femtosecond pulses at an intensity of 3 × 1019 W/cm2. We employ ultrafast pump-probe reflectometry using a second harmonic probe (400 nm) interacting with the rear surface of the target. A rise in the probe reflectivity 30 ps after the pump pulse interaction reveals the breakout of a shock wave at the target rear surface which reflects the 400 nm probe pulse. Simulations using the ZEPHYROS hybrid particle-in-cell code were performed to understand the heating of the target under the influence of the high intensity laser pulse, and the temperature profile was then passed to the radiation-hydrodynamic simulation code HYADES in order to model the shock wave propagation in the target. A good agreement was found between the calculations and experimental results.

Analytical solutions for nonlinear Thomson scattering including radiation reaction

Analytical solutions for the emitted nonlinear Thomson scattering spectrum with radiation reaction (RR) included are provided for a single electron colliding with a high intensity laser pulse. Further expressions are derived for the peak intensity for a given harmonic order and the downshift of the frequency when RR is included. Controlling the spectrum with shaping of the laser pulse frequency (chirp) has been investigated. It is shown that chirping of the laser pulse gives a distinct fingerprint of the effect of RR in the spectrum.

Ultrafast excitation of conduction-band electrons by high-intensity ultrashort laser pulses in band-gap solids: Vinogradov equation versus Drude model

Heating of conduction-band electrons is one of the major processes of energy absorption and transfer in high-intensity ultrafast laser–solid interactions. It is frequently simulated by assuming a high rate of electron-particle collisions. We explore the approximation of low-rate electron–phonon collisions based on the Vinogradov equation for the intraband absorption rate by conduction-band electrons performing laser-driven oscillations. Band-structure modification by the ponderomotive energy of the ultrafast oscillations is taken into account. The Vinogradov equation combined with the Keldysh formula for the interband transition rate delivers a highly nonequilibrium energy distribution of the conduction electrons. Reported results suggest a substantial revision of the traditional models of ultrafast free-carrier heating by intense laser pulses.

Direct electron acceleration for diagnostics of a laser pulse focused by an off-axis parabolic mirror

We develop a theoretical basis for a new method of high-intensity ultrashort laser pulse diagnostics via vacuum electron acceleration from an ultrathin foil. A laser pulse is focused by an off-axis parabolic mirror, which has practical interest for most experiments with high-intensity pulses. The field description is based on Stratton–Chu integrals, which allow covering all focusing ranges up to the diffraction limit where the six-component laser field is correctly described. The theoretical approach uses a test particle method applicable for quite thin foils and rarefied gases, where the plasma fields do not substantially affect electron acceleration. The diagnostic method is to use measurements of angular-spectrum characteristics of laser accelerated or scattered electrons to compare them with the theory developed here. The proposed method can diagnose not only the intensity but also the quality of a laser pulse.

Treatment of post-traumatic joint contracture in a rabbit model using pulsed, high intensity laser and ultrasound

Post-traumatic joint contracture induced by scar tissues following a surgery or injury can leave patients in a permanent state of pain and disability, which is difficult to resolve by current treatments. This randomized controlled trial examines the therapeutic effect of pulsed high-intensity laser (PHIL) and pulsed high-intensity focused ultrasound (PHIFU) for post-traumatic joint contracture due to arthrofibrosis. The peak power levels of both PHIL and PHIFU are much higher than that of laser or ultrasound currently used in physical therapy, while short pulses are utilized to prevent damage. To test the effectiveness of these treatments, a rabbit knee model for joint contracture was established. Twenty-one rabbits were split into four groups: untreated control (n = 5), PHIL (n = 5), PHIFU (n = 5), and a PHIL + PHIFU group (n = 6). Maximum extension of the surgically modified rabbit knee was compared to that of the contralateral control knee over the course of 16 weeks. The rabbits in the untreated control group maintained a relatively consistent level of joint contracture, while every rabbit in each of the treatment groups had improved range of motion, eventually leading to a restoration of normal joint extension. Average recovery time was 7.6 ± 1.5 weeks for the PHIL treatment group, 9.8 ± 3.7 weeks for the PHIFU group, and 8.0 ± 2.2 weeks for the combined treatment group. Histopathology demonstrated reduced density and accelerated resorption of scar tissues in the treated knee joints. This study provides evidence that both PHIL and PHIFU are effective in treating post-traumatic joint contracture in rabbits and warrant further investigation into the underlying mechanisms to optimize PHIL and PHIFU based treatments in a larger number of animals.

Ultrafast modification of band structure of wide-band-gap solids by ultrashort pulses of laser-driven electron oscillations

The electric field of high-intensity ultrashort laser pulses substantially perturbs an electron subsystem of a crystal and affects its band structure. Laser-driven oscillations of electrons and holes are frequently referred to as a major mechanism of the perturbation in dielectrics and semiconductors. New physical effects arise when the band structure is modified by an ultrashort pulse of the oscillations driven by a few-cycle laser pulse. Assuming the laser-pulse envelope varies slowly compared to the carrier frequency, we derive analytical relations for the laser-modified band structure by utilizing the Keldysh cycle-averaged nonperturbative approach under the approximation of constant effective mass. Formation of indirect-gap transient bands, suppression of the nonlinear absorption on the leading edge of a laser pulse, and cycle-averaged photocurrent generation driven by the pulse envelope are predicted. Analytical scaling with six laser and material parameters is obtained. The reported results establish the limits of validity of the Keldysh photoionization model and advance understanding of the fundamental effects involved in high-intensity ultrafast laser-solid interactions.

Vacuum birefringence in the head-on collision of x-ray free-electron laser and optical high-intensity laser pulses

The focus of this article is on providing compact analytical expressions for the differential number of polarization flipped signal photons constituting the signal of vacuum birefringence in the head-on collision of x-ray free electron (XFEL) and optical high-intensity laser pulses. Our results allow for unprecedented insights into the scaling of the effect with the waists and pulse durations of both laser beams, the Rayleigh range of the high-intensity beam, as well as transverse and longitudinal offsets. They account for the decay of the differential number of signal photons in the far-field as a function of the azimuthal angle measured relative to the beam axis of the probe beam in forward direction, typically neglected by conventional approximations. Moreover, they even allow us to extract an analytical expression for the angular divergence of the perpendicularly polarized signal photons. We expect our formulas to be very useful for the planning and optimization of experimental scenarios aiming at the detection of vacuum birefringence in XFEL/high-intensity laser setups, such as the one put forward at the Helmholtz International Beamline for Extreme Fields (HIBEF) at the European XFEL.

Observation of Ultrafast Solid-Density Plasma Dynamics Using Femtosecond X-Ray Pulses from a Free-Electron Laser

The complex physics of the interaction between short pulse high intensity lasers and solids is so far hardly accessible by experiments. As a result of missing experimental capabilities to probe the complex electron dynamics and competing instabilities, this impedes the development of compact laser-based next generation secondary radiation sources, e.g. for tumor therapy [Bulanov2002,ledingham2007], laboratory-astrophysics [Remington1999,Bulanov2015], and fusion [Tabak2014]. At present, the fundamental plasma dynamics that occur at the nanometer and femtosecond scales during the laser-solid interaction can only be elucidated by simulations. Here we show experimentally that small angle X-ray scattering of femtosecond X-ray free-electron laser pulses facilitates new capabilities for direct in-situ characterization of intense short-pulse laser plasma interaction at solid density that allows simultaneous nanometer spatial and femtosecond temporal resolution, directly verifying numerical simulations of the electron density dynamics during the short pulse high intensity laser irradiation of a solid density target. For laser-driven grating targets, we measure the solid density plasma expansion and observe the generation of a transient grating structure in front of the pre-inscribed grating, due to plasma expansion, which is an hitherto unknown effect. We expect that our results will pave the way for novel time-resolved studies, guiding the development of future laser-driven particle and photon sources from solid targets.

Controlling the ellipticity of attosecond pulses produced by laser irradiation of overdense plasmas

The interaction of high-intensity laser pulses and solid targets provides a promising way to create compact, tunable, and bright XUV attosecond sources that can become a unique tool for a variety of applications. However, it is important to control the polarization state of this XUV radiation and to do so in the most efficient regime of generation. Using the relativistic electronic spring (RES) model and particle-in-cell simulations, we show that the polarization state of the generated attosecond pulses can be tuned in a wide range of parameters by adjusting the polarization and/or the angle of incidence of the laser radiation. In particular, we demonstrate the possibility of producing circularly polarized attosecond pulses in a wide variety of setups.

Low‐Power Continuous‐Wave Second Harmonic Generation in Semiconductor Nanowires

AbstractSemiconductor nanowires (NWs) are promising for realizing various on‐chip nonlinear optical devices, due to their nanoscale lateral confinement and strong light–matter interaction. However, high‐intensity pulsed pump lasers are typically needed to exploit their optical nonlinearity because light couples poorly with nanometric‐size wires. Here, microwatts continuous‐wave light pumped second harmonic generation (SHG) in AlGaAs NWs is demonstrated by integrating them with silicon planar photonic crystal cavities. Light–NW coupling is enhanced effectively by the extremely localized cavity mode at the subwavelength scale. Strong SHG is obtained even with a continuous‐wave laser excitation with a pump power down to W, and the cavity‐enhancement factor is estimated around 150. Additionally, in the integrated device, the NW's SHG is more than two orders of magnitude stronger than third harmonic generations in the silicon slab, though the NW only couples with less than 1% of the cavity mode. This significantly reduced power requirement of NW's nonlinear frequency conversion would promote NW‐based building blocks for nonlinear optics, especially in chip‐integrated coherent light sources, entangled photon pairs and signal processing devices.

On-chip polariton generation using an embedded nanograting microring circuit

Abstract We have proposed a model of polariton generation, which is normally generated by the dipole and the strong coupling field interaction. This system consists of a gold grating embedded on the plasmonic island, which is embedded at the center of the nonlinear microring resonator, which is known as a panda-ring resonator. The strong coupling between the plasmonic waves and the grating can be formed by the whispering gallery mode (WGM) of light within a Panda-ring resonator, in which the output is a dipole-like particle known as a polariton and seen at the system output. By varying the energy of high-intensity laser pulse in the system and gold granting a strong field is generated at the output. A dipole is formed by a pair of the grating signals, where one propagates in the opposite direction of the other. By using suitable parameters, dipole-like signals can be generated. Theoretical formulation is performed for a two-level system and polariton oscillation frequency i.e., the Rabi frequency is plotted. The obtained ground and excited state frequencies of this two-level system are 187.86 and 198.20 THz, respectively.

Nonlinear Polarization and Low-Dissipation Ultrafast Optical Switching in Phosphorene

Fundamental limits to the computation using silicon-based devices stimulate other emerging and viable alternatives. Manipulation of the electrical and optical properties with the ultrafast and intense electric fields offers one of such alternatives. Here, we study the interaction of high-intensity pulsed femtosecond laser of various intensities and carrier frequencies with a monolayer phosphorene using the real-time real-space time-dependent density functional theory. The nonlinear induced currents are entirely reversible but are phase-shifted compared to low-intensity lights within the period of incident laser. We observe optical Kerr effect associating the changes in the number of free charge carriers with the change in the refractive index of the material, which subsequently characterizes the material’s ability of dielectric switching. The transient changes in refractive index are reversible up to a certain threshold, suggesting that the properties can be switched on the timescale of an optical period,...

A setup for micro-structured silicon targets by femtosecond laser irradiation

We present a process to fabricate micro-structured thin silicon foils with highly light absorbing properties over a broad wavelength region based on short-pulse laser treatment. We employ this fascinating technique to the fabrication of targets for high-intensity laser-plasma experiments. A polished silicon wafer is processed with high-intensity femtosecond laser pulses resulting in conical spikes within the irradiated region. Height, distance, and shape of these spikes depend primarily on the number, energy, central wavelength and duration of the incident laser pulses, as well as the ambient medium. This method is of great value for the future development of target fabrication. The broad parameter base offers a huge selection regarding shape and dimensions of the resulting structure. It can be included in existing laser operations within the manufacturing chain while offering a high degree of customisability. Within the reach of higher repetition rates of high-power laser systems and an increased number of requested targets, this manufacturing method can be scaled while being cost efficient and easily adaptable.