High harmonic generation by relativistically intense laser pulses from overdense plasma layers is surveyed. High harmonics are generated in form of (sub-)attosecond pulses when the plasma surface rebounds towards the observer with relativistic velocity. Different cases are considered. The "relativistically oscillating mirror" (ROM) model, describing the most typical case, is analyzed in detail. The resulting harmonic spectrum is usually a power law with the exponent -8/3 [baeva2006relativistic], but possible exceptions due to "higher order γ-spikes" are considered. It is shown that under certain conditions, ultra-dense electron nanobunches can be formed at plasma surface that emit coherent synchrotron radiation. The resulting spectrum is much flatter and leads to the formation of a giant attosecond pulse in the reflected radiation. The harmonics radiation is also considered in time domain, where they form a train of attosecond pulses. It is characterized and a possibility to select a single attosecond pulse via polarization gating is described. Further, the line structure in relativistic harmonic spectra is analyzed. It is shown that the harmonics have an intrinsic chirp and it can be responsible for experimentally observed spectral modulations. Finally, high harmonic generation is considered in realistic three-dimensional geometry. It is shown that free space diffraction can act as a high pass filter, altering the spectrum and the reflected field structure. The high harmonics tend to be self-focused by the reflecting surface. This leads to a natural angular divergence as well as to field boost at the focal position. Coherently focusing the harmonics using an optimized geometry may result in a significantly higher field than the field of the driving laser.
Sub-10-attosecond pulses with half-cycle electric fields provide exceptional options to detect and manipulate electrons in the atomic timescale. However, the availability of such pulses is still challenging. Here, we propose a method to generate isolated sub-10-attosecond half-cycle pulses based on a cascade process naturally happening in plasma. A 100s-attosecond pulse is first generated by shooting a moderate overdense plasma with a one-cycle femtosecond pulse. After that, the generated attosecond pulse cascadedly produce a sub-10-attosecond half-cycle pulse in the transmission direction by …
Doppler harmonic generation of a high-power laser on a relativistic plasma mirror is a promising path to produce bright attosecond light bursts. Yet, a major challenge has been to find a way to generate isolated attosecond pulses, better suited to timed-resolved experiments, rather than trains of pulses. A promising technique is the attosecond lighthouse effect, which consists in imprinting different propagation directions to successive attosecond pulses of the train, and then spatially filtering one pulse in the far field. However, in the relativistic regime, plasma mirrors get curved by the …
The creation of attosecond pulses via laser-plasma interaction has been a subject of great scientific interest for more than three decades. This process is investigated by using particle-in-cell simulation with varying the plasma and laser parameters. The steepness of the density gradient at the plasma-vacuum interface is examined to see how this parameter affects the high-order harmonic generations and isolated-attosecond pulse creation. The optimal density gradient lengths $L$ are explored within the relativistic oscillating mirror mechanism. Although the ideal gradient lengths for the full …
We have investigated the polarization of attosecond light bursts generated by nanobunches of electrons from relativistic few-cycle laser pulse interaction with the surface of overdense plasmas. Particle-in-cell simulation shows that the polarization state of the generated attosecond burst depends on the incident-pulse polarization, duration, carrier envelope phase, as well as the plasma scale length. Through laser and plasma parameter control, without compromise of generation efficiency, a linearly polarized laser pulse with azimuth $θ^i=10^\circ$ can generate an elliptically polarized attosec…
The microscopic dynamics of laser-driven coherent synchrotron emission transmitted through thin foils are investigated using particle-in-cell simulations. For normal incidence interactions, we identify the formation of two distinct electron nanobunches from which emission takes place each half-cycle of the driving laser pulse. These emissions are separated temporally by 130 attoseconds and are dominant in different frequency ranges, which is a direct consequence of the distinct characteristics of each electron nanobunch. This may be exploited through spectral filtering to isolate these emissio…
To describe the high harmonic generation at plasma surfaces in the relativistic regime, we introduce the concept of {\it apparent reflection point} (ARP). It appears to an external observer that the radiation electric field is zero at the ARP. The relativistic dynamics of the ARP completely defines the generation of high harmonics and attosecond pulses. The ARP velocity is a smooth function of time. The corresponding $γ$-factor, however, has sharp spikes at the times when the tangential vector potential vanishes and the surface velocity becomes close to the speed of light. We show that managin…
Relativistic surface high harmonics have been considered a unique source for the generation of intense isolated attosecond pulses in the extreme ultra-violet (XUV) and X-ray spectral range. However, its experimental realization is still a challenging task requiring identification of the optimum conditions for the generation of isolated attosecond pulses as well as their temporal characterization. Here, we demonstrate measurements in both directions. Particularly, we have made a first step towards the temporal characterization of the emitted XUV radiation by adapting the attosecond streak camer…
A new regime in the interaction of a two-colour ($ω$,$2ω$) laser with a nanometre-scale foil is identified, resulting in the emission of extremely intense, isolated attosecond pulses - even in the case of multi-cycle lasers. For foils irradiated by lasers exceeding the blow-out field strength (i.e. capable of fully separating electrons from the ion background), the addition of a second harmonic field results in the stabilization of the foil up to the blow-out intensity. This is then followed by a sharp transition to transparency that essentially occurs in a single optical cycle. During the tra…
Half-cycle picosecond pulses have been produced from thin photo-conductors, when applying an electric field across the surface and switching on conduction by a short laser pulse. Then the transverse current in the wafer plane emits half-cycle pulses in normal direction, and pulses of 500 fs duration and 1e6 V/m peak electric field have been observed. Here we show that single half-cycle pulses of 50 as duration and up to 1e13 V/m can be produced when irradiating a double foil target by intense few-cycle laser pulses. Focused onto an ultra-thin foil, all electrons are blown out, forming a unifor…
We develop an analytical model for ultraintense attosecond pulse emission in the highly relativistic laser-plasma interaction. In this model, the attosecond pulse is emitted by a strongly compressed electron layer around the instant when the layer transverse current changes the sign and its longitudinal velocity approaches the maximum. The emitted attosecond pulse has a broadband exponential spectrum and a stabilized constant spectral phase $ψ(ω)=\pmπ/2-ψ_{A_m}$. The waveform of the attosecond pulse is also given explicitly, to our knowledge, for the first time. We validate the analytical mode…
Harmonic generation from solid surfaces is a promising tool for producing high energy attosecond pulses. We report shaping of the harmonic spectrum to achieve the bandwidth necessary for attosecond pulse generation. The shaping is demonstrated for lower as well as for higher harmonics using single and two-pulse pumping. The measured harmonic field autocorrelation function exhibits attosecond spikes in good agreement with the harmonic spectrum. Double slit experiments reveal a high spatial coherence of the harmonic beam.
A robust plasma gating to generate a single ultra-intense attosecond pulse is developed. It is a manifestation of the hole-boring effect that limits the strongest attosecond pulse emission within one laser cycle. The generated pulse is characterized by a stabilized harmonic phase $ψ\approx \pmπ/2$ and a slowly decaying exponential spectrum bounded by $γ$-spike scaling and CSE scaling. The phase oscillations in low-frequency region and fluctuations in high-frequency region are discussed. We also show that the phase fluctuations in high-frequency region can be reduced by including radiation reac…
With the generation of high order harmonics (HHG) on the plasma surface it is possible to turn the laser pulse into a train of attosecond or even zeptosecond pulses in the back radiation. These attosecond pulses may have amplitude several orders of magnitude larger than that of the laser pulse under appropriate conditions. We study this process in detail, especially the nanobunching of the plasma electron density. We derive the analytical expression that describes the electron density profile and obtain a good agreement with particle-in-cell simulations. We investigate the most efficient case …
For several decades, the interest of the scientific community in aneutronic fusion reactions such as proton-Boron fusion has grown because of potential applications in different fields. Recently, many scientific teams in the world have worked experimentally on the possibility to trigger proton-Boron fusion using intense lasers demonstrating an important renewal of interest of this field. It is now possible to generate ultra-short high intensity laser pulses at high repetition rate. These pulses also have unique properties that can be leveraged to produce proton-Boron fusion reactions. In this …
Laser-plasma electron accelerators can be used to produce high-intensity X-rays, as electrons accelerated in wakefields emit radiation due to betatron oscillations.Such X-ray sources inherit the features of the electron beam; sub-femtosecond electron bunches produce betatron sources of the same duration, which in turn allow probing matter on ultrashort time scales. In this paper we show, via Particle-in-Cell simulations, that attosecond electron bunches can be obtained using low-energy, ultra-short laser beams both in the self-injection and the controlled injection regimes at low plasma densit…
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 (PIC) 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 angle of…
A method to generate an intense isolated few-cycle attosecond XUV pulse is demonstrated using particle-in-cell simulations. When a tailored laser pulse with a sharp edge irradiates a 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 a duration of several hundred attoseconds. It 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 bel…
The birth of attosecond light sources is expected to inspire a breakthrough in ultrafast optics, which may extend human real-time measurement and control techniques into atomic-scale electronic dynamics. For applications, it is essential to obtain a single attosecond pulse of high intensity, large photon energy and short duration. Here we show that single 100-terawatt attosecond X-ray light pulse with intensity ${1\times10^{21}}\textrm{W}/\textrm{cm}^{ 2}$ and duration ${7.9} \textrm{as}$ can be produced by intense laser irradiation on a capacitor-nanofoil target composed of two separate nanof…
It is shown that the the interference of above-threshold electron de Broglie waves, generated by an intense laser pulse at a metal surface yields attosecond electron pulses. This inerference is an analogon of the superposition of high harmonics generated from rare gas atoms, resulting in trains of attosecond light pulses.Owing to the inherent kinematic dispersion, the propagation of attosecond de Broglie waves in vacuum is very different from that of attosecond light pulses, which propagate without changing shape. Above the metal surface there are "collaps bands" and "revival layers" of the el…
We present a new mechanism of attosecond extreme-ultraviolet (XUV) pulses generation from a relativistic laser-driven overdense plasma surfaces in the wavebreaking regime. Through particle-in-cell simulations and analysis, we demonstrate that the observed ultrashort XUV emission for the parameters we considered is predominantly due to a strong plasma-density oscillation subsequent to wavebreaking. The coupling of the strong density variation and the transverse fields in the front surface layer gives rise to the transmitted emission with frequencies mainly around the local plasma frequency. Thi…
We present a scheme for amplifying an extreme-ultraviolet (XUV) seed isolated attosecond pulse via stimulated Raman scattering of a pulse-train pump. At sufficient seed and pump intensity, the amplification is nonlinear, and the amplitude of the seed pulse can reach that of the pump, one order of magnitude higher than the initial seed amplitude. In the linear amplification regime, we find that the spectral signature of the pump pulse train is imprinted on the spectrum of the amplified seed pulse. Since the spectral signature is imprinted with its frequency downshifted by the plasma frequency, …
Attosecond bursts of coherent synchrotron-like radiation are found when driving ultrathin relativistic electron disks in a quasi-one-dimensional regime of wakefield acceleration, in which the laser waist is larger than the wake wavelength. The disks of overcritical density shrink radially due to the focusing wake fields, thus providing the transverse currents for the emission of an intense, radially polarized, half-cycle pulse of about 100 attoseconds in duration. The electromagnetic pulse first focuses to a peak intensity 10 times larger ($7\times10^{20}\rm W/cm^2$) than the driving pulse and…
The generation of ultra-strong attosecond pulses through laser-plasma interactions offers the opportunity to surpass the intensity of any known laboratory radiation source, giving rise to new experimental possibilities, such as quantum electrodynamical tests and matter probing at extremely short scales. Here we demonstrate that a laser irradiated plasma surface can act as an efficient converter from the femto- to the attosecond range, giving a dramatic rise in pulse intensity. Although seemingly similar schemes have been presented in the literature, the present setup deviates significantly fro…
Batch Bayesian optimization of attosecond betatron pulses from laser wakefield acceleration
Chalmers University of Technology（查尔姆斯理工大学）；University of Oxford（牛津大学）；Horia Hulubei National Institute for Physics and Nuclear Engineering（霍里亚·胡利贝国家物理与核工程研究所）
Accelerator-based X-ray free-electron lasers (XFELs) are the latest addition to the revolutionary tools of discovery for the 21st century. The two major components of an XFEL are an accelerator-produced electron beam and a magnetic undulator which tend to be kilometer-scale long and expensive. Here, we present an ultra-compact scheme to produce 10s of attosecond X-ray pulses with several GW peak power utilizing a novel aspect of the FEL instability using a highly chirped, pre-bunched and ultra-bright electron beam from a plasma-based accelerator interacting with an optical undulator. The self-…
