Relativistic electron mirrors from nanoscale foils for coherent frequency upshift to the extreme ultraviolet

Generation of attosecond gigawatt soft x-ray pulses through coherent Thomson backscattering

Collision between relativistic electron sheets and counterpropagating laser pulses is recognized as a promising way to produce intense attosecond x rays through coherent Thomson backscattering (TBS). In a double-layer scheme, the electrons in an ultrathin solid foil are first pushed out by an intense laser driver and then interact with the laser reflected off a second foil to form a high-density relativistic electron sheet with vanishing transverse momentum. However, the repulsion between these concentrated electrons can increase the thickness of the layer, reducing both its density and subsequently the coherent TBS. Here, we present a systematic study on the evolution of the flying electron layer and find that its resulting thickness is determined by the interplay between the intrinsic space-charge expansion and the velocity compression induced by the drive laser. How the laser driver, the target areal density, the reflector, and the collision laser intensity affect the properties of the produced x rays is explored. Multidimensional particle-in-cell simulations indicate that employing this scheme in the nonlinear regime has the potential to stably produce soft x rays with several gigawatt peak power in hundreds of terawatt ultrafast laser facilities. The pulse duration can be tuned to tens of attoseconds. This compact and intense attosecond x-ray source may have broad applications in attosecond science.

Generation of attosecond electron bunches of tunable duration and density by relativistic vortex lasers in near-critical density plasma

Attosecond electron bunches have wide application prospects in free-electron laser injection, attosecond X/γ-ray generation, ultrafast physics, etc. Nowadays, there is one notable challenge in the generation of high-quality attosecond electron bunch, i.e., how to enhance the electron bunch density. Using theoretical analysis and three-dimensional particle-in-cell simulations, we discovered that a relativistic vortex laser pulse interacting with near-critical density plasma can not only effectively concentrate the attosecond electron bunches to over critical density, but also control the duration and density of the electron bunches by tuning the intensity and carrier-envelope phase of the drive laser. It is demonstrated that this method can efficiently produce attosecond electron bunches with a density up to 300 times of the original plasma density, peak divergence angle of less than 0.5 ∘, and duration of less than 67 attoseconds. Furthermore, by using near-critical density plasma instead of solid targets, our scheme is potential for the generation of high-repetition-frequency attosecond electron bunches, thus reducing the requirements for experiments, such as the beam alignment or target supporter.

Giant isolated half-cycle attosecond pulses generated in coherent bremsstrahlung emission regime

Giant half-cycle attosecond pulse generation in the coherent bremsstrahlung emission regime is proposed for laser pulses with normal incidence on a double-foil target, where the first foil is transparent and the second foil is opaque. The presence of the second opaque target contributes to the formation of a relativistic flying electron sheet (RFES) from the first foil target. After the RFES has passed through the second opaque target, it is decelerated sharply, and bremsstrahlung emission occurs, which results in the generation of an isolated half-cycle attosecond pulse having an intensity of ∼1.4×10^{22}W/cm^{2} and a duration of 3.6 as. The generation mechanism does not require extra filters and may open a regime of nonlinear attosecond science.

Intense isolated attosecond pulses from two-color few-cycle laser driven relativistic surface plasma

Ultrafast plasma dynamics play a pivotal role in the relativistic high harmonic generation, a phenomenon that can give rise to intense light fields of attosecond duration. Controlling such plasma dynamics holds key to optimize the relevant sub-cycle processes in the high-intensity regime. Here, we demonstrate that the optimal coherent combination of two intense ultrashort pulses centered at two-colors (fundamental frequency, [Formula: see text] and second harmonic, [Formula: see text]) can lead to an optimal shape in relativistic intensity driver field that yields such an extraordinarily sensitive control. Conducting a series of two-dimensional (2D) relativistic particle-in-cell (PIC) simulations carried out for currently achievable laser parameters and realistic experimental conditions, we demonstrate that an appropriate combination of [Formula: see text] along with a precise delay control can lead to more than three times enhancement in the resulting high harmonic flux. Finally, the two-color multi-cycle field synthesized with appropriate delay and polarization can all-optically suppress several attosecond bursts while favourably allowing one burst to occur, leading to the generation of intense isolated attosecond pulses without the need of any sophisticated gating techniques.

Relativistic slingshot: A source for single circularly polarized attosecond x-ray pulses

We propose a mechanism to generate a single intense circularly polarized attosecond x-ray pulse from the interaction of a circularly polarized relativistic few-cycle laser pulse with an ultrathin foil at normal incidence. Analytical modeling and particle-in-cell simulation demonstrate that a huge charge-separation field can be produced when all the electrons are displaced from the target by the incident laser, resulting in a high-quality relativistic electron mirror that propagates against the tail of the laser pulse. The latter is efficiently reflected as well as compressed into an attosecond pulse that is also circularly polarized.

Long propagating velocity-controlled Einstein's mirror for terahertz light conversion

We show that Einstein's relativistic mirror with long (hundreds of µm) propagation distance and controllable propagation velocity can be implemented in the form of a dense free carrier front generated by multiphoton absorption of tilted-pulse-front femtosecond laser pulses in a dielectric or semiconductor medium. The velocity control is achieved by varying the pulse front tilt angle. Simulations demonstrate that such fronts can serve as efficient Doppler-type converters of terahertz pulses. In particular, the pulse reflected from a front, generated by three-photon absorption of a Ti:sapphire laser in ZnS, can exhibit strong (up to more than an order of magnitude) pulse compression and spectrum broadening without a noticeable amplitude change. The proposed technique may be used to convert strong low-frequency terahertz pulses, generated by optical rectification of tilted-pulse-front laser pulses, to desirable temporal and spectral characteristics for a variety of applications.

Electron-Nanobunch-Width-Dominated Spectral Power Law for Relativistic Harmonic Generation from Ultrathin Foils

Relativistic high-order harmonic generation from solid-density plasma offers a compact source of coherent ultraviolet and x-ray light. For solid targets much thinner than the laser wavelength, the plasma thickness can be tuned to increase conversion efficiency; a reduction in total charge allows for balancing the laser and plasma driving forces, producing the most effective interaction. Unlike for semi-infinite plasma surfaces, we find that for ultrathin foil targets the dominant factor in the emission spectral shape is the finite width of the electron nanobunches, leading to a power-law exponent of approximately 10/3. Ultrathin foils produce higher-efficiency frequency conversion than solid targets for moderately relativistic (1<a_{0}<40) interactions and also provide unique insight into how the trajectories of individual electrons combine and interfere to generate reflected attosecond pulses.

Absorption and opacity threshold for a thin foil in a strong circularly polarized laser field

We show that a commonly accepted transparency threshold for a thin foil in a strong circularly polarized normally incident laser pulse needs a refinement. We present an analytical model that correctly accounts for laser absorption. The refined threshold is determined not solely by the laser amplitude, but by other parameters that are equally or even more important. Our predictions are in perfect agreement with particle-in-cell simulations. The refined criterion is crucial for configuring laser plasma experiments in the high-field domain. In addition, an opaque foil steepens the pulse front, which can be important for numerous applications.

High-order harmonic generation from a solid-surface plasma by relativistic-intensity sub-100-fs mid-infrared pulses

High-order harmonic generation (HHG) in plasmas induced by ultrashort, relativistic-intensity laser pulses on solid surfaces can provide an efficient source of attosecond pulses and opens routes toward new regimes of laser-matter interactions, x-ray generation, laser particle acceleration, and relativistic nonlinear optics. However, field intensities in the range of I_rel∼10¹⁹  W/cm² are typically needed to achieve the relativistic regime of HHG in experiments with near-infrared laser pulses. Here, we show that, in the mid-infrared range, due to the λ^-2 scaling of I_rel with the driver wavelength λ, relativistic HHG can be observed at much lower levels of laser field intensities. High-peak-power 80-fs, 3.9-μm pulses are focused in our experiments on a solid surface to provide field intensities in the range of 10¹⁷  W/cm². Remarkably, this level of field intensities, considered as low by the standards of relativistic optics in the near infrared, is shown to be sufficient for generation of high-order harmonics with signature properties of relativistic HHG-beam directionality, spectra with extended plateaus, and a high HHG yield sustained for both p- and s-polarized driver fields.

Dense relativistic electron mirrors from a Laguerre-Gaussian laser-irradiated micro-droplet

We investigate dense relativistic electron mirror generation from a micro-droplet driven by circularly polarized Laguerre-Gaussian lasers. The surface electrons are expelled from the droplet by the laser's radial electric field and evolve into dense sheets after leaving the droplet. These electrons are trapped in the potential well of the laser's transverse ponderomotive force and are steadily accelerated to about 100 MeV by the longitudinal electric field. Particle-in-cell simulations indicate that the relativistic electron mirrors are characterized by high beam charge, narrow energy spread, and large angular momentum, which can be utilized for bright X/γ-ray emission and photon vortex formation.

Generation of Gigawatt Circularly Polarized Attosecond-Pulse Pairs

A novel scheme for generating a pair of gigawatt attosecond pulses by coherent Thomson scattering from relativistic electron sheets is proposed. With a circularly polarized relativistic laser pulse, the scattered x-ray signal can have a saddlelike temporal profile, where the lower electromagnetic frequencies are found mostly in the center region of this saddlelike profile. By filtering out the latter, we can obtain two few-attosecond pulses separated by a subfemtosecond interval, which is tunable by controlling the energy of the sheet electrons. Such a pulse pair can be useful for an attosecond pump probe at an unprecedented time resolution and for ultrafast chiral studies in molecules and materials.

Amplification of Relativistic Electron Bunches by Acceleration in Laser Fields

Direct acceleration of electrons in a coherent, intense light field is revealed by a remarkable increase of the electron number in the MeV energy range. Laser irradiation of thin polymer foils with a peak intensity of ∼1×10^{20}  W/cm^{2} releases electron bunches along the laser propagation direction that are postaccelerated in the partly transmitted laser field. They are decoupled from the laser field at high kinetic energies, when a second foil target at an appropriate distance prevents their subsequent deceleration in the declining laser field. The scheme is established with laser pulses of high temporal contrast (10^{10} peak to background ratio) and two ultrathin polymer foils at a distance of 500  μm. 2D particle in cell simulations and an analytical model confirm a significant change of the electron spectral distribution due to the double foil setup, which leads to an amplification of about 3 times of the electron number around a peak at 1 MeV electron energy. The result verifies a theoretical concept of direct electron bunch acceleration in a laser field that is scalable to extreme acceleration potential gradients. This method can be used to enhance the density and energy spread of electron bunches injected into postaccelerator stages of laser driven radiation sources.

Picosecond metrology of laser-driven proton bursts

Tracking primary radiation-induced processes in matter requires ultrafast sources and high precision timing. While compact laser-driven ion accelerators are seeding the development of novel high instantaneous flux applications, combining the ultrashort ion and laser pulse durations with their inherent synchronicity to trace the real-time evolution of initial damage events has yet to be realized. Here we report on the absolute measurement of proton bursts as short as 3.5±0.7 ps from laser solid target interactions for this purpose. Our results verify that laser-driven ion acceleration can deliver interaction times over a factor of hundred shorter than those of state-of-the-art accelerators optimized for high instantaneous flux. Furthermore, these observations draw ion interaction physics into the field of ultrafast science, opening the opportunity for quantitative comparison with both numerical modelling and the adjacent fields of ultrafast electron and photon interactions in matter.

Experimental investigations of the response of matter to ionization would require extremely fast ion pump pulses. Here, the authors explore a different approach observing ionisation dynamics in SiO₂ glass by generating synchronized proton pulses from the interaction of high-power lasers on a solid target.

Short X-ray pulses from third-generation light sources

High-brightness X-ray radiation produced by third-generation synchrotron light sources (TGLS) has been used for numerous time-resolved investigations in many different scientific fields. The typical time duration of X-ray pulses delivered by these large-scale machines is about 50-100 ps. A growing number of time-resolved studies would benefit from X-ray pulses with two or three orders of magnitude shorter duration. Here, techniques explored in the past for shorter X-ray pulse emission at TGLS are reviewed and the perspective towards the realisation of picosecond and sub-picosecond X-ray pulses are discussed.

Ion Acceleration Using Relativistic Pulse Shaping in Near-Critical-Density Plasmas

Ultraintense laser pulses with a few-cycle rising edge are ideally suited to accelerating ions from ultrathin foils, and achieving such pulses in practice represents a formidable challenge. We show that such pulses can be obtained using sufficiently strong and well-controlled relativistic nonlinearities in spatially well-defined near-critical-density plasmas. The resulting ultraintense pulses with an extremely steep rising edge give rise to significantly enhanced carbon ion energies consistent with a transition to radiation pressure acceleration.

Bright subcycle extreme ultraviolet bursts from a single dense relativistic electron sheet

Double-foil targets separated by a low density plasma and irradiated by a petawatt-class laser are shown to be a copious source of coherent broadband radiation. Simulations show that a dense sheet of relativistic electrons is formed during the interaction of the laser with the tenuous plasma between the two foils. The coherent motion of the electron sheet as it transits the second foil results in strong broadband emission in the extreme ultraviolet, consistent with our experimental observations.

Quantum-relativistic hydrodynamic model for a spin-polarized electron gas interacting with light

We develop a semirelativistic quantum fluid theory based on the expansion of the Dirac Hamiltonian to second order in 1/c. By making use of the Madelung representation of the wave function, we derive a set of hydrodynamic equations that comprises a continuity equation, an Euler equation for the mean velocity, and an evolution equation for the electron spin density. This hydrodynamic model is then applied to study the dynamics of a dense and weakly relativistic electron plasma. In particular, we investigate the impact of the quantum-relativistic spin effects on the Faraday rotation in a one-dimensional plasma slab irradiated by an x-ray laser source.

Intense isolated few-cycle attosecond XUV pulses from overdense plasmas driven by tailored laser pulses

A new scheme to generate an intense isolated few-cycle attosecond XUV pulse is demonstrated using particle-in-cell simulations. By use of unipolarlike or subcycle laser pulses irradiating a thin foil target, a strong transverse net current can be excited, which emits a few-cycle XUV pulse from the target rear side. The isolated pulse is ultrashort in the time domain with duration of several hundred attoseconds. The pulse also has a narrow bandwidth in the spectral domain compared to other XUV sources of high-order harmonics. It has most energy confined around the plasma frequency and no low-harmonic orders below the plasma frequency. It is also shown that XUV pulse of peak field strength up to 8 × 10(12) Vm(-1) can be produced. Without the need for pulse selecting and spectral filtering, such an intense few-cycle XUV pulse is better suited to a number of applications.

Pulse compression and broadening by reflection from a moving front of a photonic crystal

Previously, the effect of pulse bandwidth compression or broadening was observed in reflection from a moving front together with the Doppler shift. In this letter, an approach is presented, which alters pulse bandwidth without change in the central frequency. It occurs when light is reflected from a moving front of an otherwise stationary photonic crystal. This means that the photonic crystal lattice as such is stationary and only its boundary to the environment is moving, thus extruding (or shortening) the photonic crystal medium. The compression (broadening) factor depends on the front velocity and is the same as for the conventional Doppler shift. Complete reflection and transformation of the pulse can be achieved even with weak refractive index contrast, what makes the approach experimentally viable.