Intense XUV Research Articles

Parametric attosecond pulse amplification far from the ionization threshold from high order harmonic generation in He.

Parametric amplification of attosecond coherent pulses around 100 eV at the single-atom level is demonstrated for the first time by using the 3D time-dependent Schrödinger equation in high-harmonic generation processes from excited states of He+. We present the attosecond dynamics of the amplification process far from the ionization threshold and resolve the physics behind it. The amplification of a particular central photon energy requires the seed XUV pulses to be perfectly synchronized in time with the driving laser field for stimulated recombination to the He+ ground state and is only produced in a few specific laser cycles in agreement with the experimental measurements. Our simulations show that the amplified photon energy region can be controlled by varying the peak intensity of the laser field. Our results pave the way to the realization of compact attosecond pulse intense XUV lasers with broad applications.

Angular dispersion boost of high order laser harmonics with Carbon nano-rods.

Periodic surface gratings or photonic crystals are excellent tools for diffracting light and to collect information about the spectral intensity, if the target structure is known, or about the diffracting object, if the light source is well defined. However, this method is less effective in the case of extreme ultraviolet (XUV) light due to the high absorption coefficient of any material in this frequency range. Here we propose a nanorod array target in the plasma phase as an efficient dispersive medium for the intense XUV light which is originated from laser-plasma interactions where various high harmonic generation processes take place. The scattering process is studied with the help of particle-in-cell simulations and we show that the angular distribution of different harmonics after scattering can be perfectly described by a simple interference theory.

Intense attosecond pulses carrying orbital angular momentum using laser plasma interactions

Light beams with helical phase-fronts are known to carry orbital angular momentum (OAM) and provide an additional degree of freedom to beams of coherent light. While OAM beams can be readily derived from Gaussian laser beams with phase plates or gratings, this is far more challenging in the extreme ultra-violet (XUV), especially for the case of high XUV intensity. Here, we theoretically and numerically demonstrate that intense surface harmonics carrying OAM are naturally produced by the intrinsic dynamics of a relativistically intense circularly-polarized Gaussian beam (i.e. non-vortex) interacting with a target at normal incidence. Relativistic surface oscillations convert the laser pulses to intense XUV harmonic radiation via the well-known relativistic oscillating mirror mechanism. We show that the azimuthal and radial dependence of the harmonic generation process converts the spin angular momentum of the laser beam to orbital angular momentum resulting in an intense attosecond pulse (or pulse train) with OAM.

Time-resolved ionization measurements with intense ultrashort XUV and X-ray free-electron laser pulses

AbstractModern free-electron lasers (FEL) operating in XUV (extreme ultraviolet) or X-ray range allow an access to novel research areas. An example is the ultrafast ionization of a solid by an intense femtosecond FEL pulse in XUV which consequently leads to a change of the complex index of refraction on an ultrashort timescale. The photoionization and subsequent impact ionization resulting in electronic and atomic dynamics are modeled with our hybrid code XTANT(X-ray thermal and non-thermal transitions) and a Monte Carlo code XCASCADE(X-ray-induced electron cascades). The simulations predict the temporal kinetics of FEL-induced electron cascades and thus yield temporally and spatially resolved information on the induced changes of the optical properties. In a series of experiments at FERMI and LCLS, single shot measurements with spatio-temporal encoding of the ionization process have been performed by a correlation of the FEL pump pulse with an optical femtosecond probe pulse. An excellent agreement between the experiment and the simulation has been found. We also show that such kind of experiments forms the basis for pulse duration and arrival time jitter monitoring as currently under development for XUV-FELs.

Coherent xuv pulse induced resonant Auger process in the krypton atom

We theoretically investigate the resonant Auger (RA) process via multiple core-excited states in the Kr atom in the presence of a coherent intense xuv pulse. Our results show the possibility of controlling the total energy-integrated electron yield, in the Kr atom, by appropriate combination and careful adjustment of the laser parameters from a coherent superposition of core-excited states. The simultaneous excitation of closely spaced core-excited states sets off interference in their common continuum, commonly termed as lifetime interference. We estimate the laser parameters essential for increasing the strength of this interference in the common continuum of two core-excited states. Our calculated electron spectra shows that an appropriate combination of pulse parameters favourably increases RA decay from one of the constituting core-excited states in the coherent superposition of core-excited states. We also find that variation of laser parameters can enhance RA decay from a core-excited state of comparatively low partial RA decay rate.

Single-shot extreme-ultraviolet wavefront measurements of high-order harmonics

We perform wavefront measurements of high-order harmonics using an extreme-ultraviolet (XUV) Hartmann sensor and study how their spatial properties vary with different generation parameters, such as pressure in the nonlinear medium, fundamental pulse energy and duration as well as beam size. In some conditions, excellent wavefront quality (up to λ/11) was obtained. The high throughput of the intense XUV beamline at the Lund Laser Centre allows us to perform single-shot measurements of both the full harmonic beam generated in argon and individual harmonics selected by multilayer mirrors. We theoretically analyze the relationship between the spatial properties of the fundamental and those of the generated high-order harmonics, thus gaining insight into the fundamental mechanisms involved in high-order harmonic generation (HHG).

Semirelativistic Schrödinger Equation for Relativistic Laser-Matter Interactions.

A semirelativistic formulation of light-matter interaction is derived using the so called propagation gauge and the relativistic mass shift. We show that relativistic effects induced by a superintense laser field can, to a surprisingly large extent, be accounted for by the Schrödinger equation, provided that we replace the rest mass in the propagation gauge Hamiltonian by the corresponding time-dependent field-dressed mass. The validity of the semirelativistic approach is tested numerically on a hydrogen atom exposed to an intense extreme ultraviolet laser pulse strong enough to accelerate the electron towards relativistic velocities. It is found that while the results obtained from the ordinary (nonrelativistic) Schrödinger equation generally differ from those of the Dirac equation, demonstrating that relativistic effects are significant, the semirelativistic formulation provides results in quantitative agreement with a fully relativistic treatment.

Core-resonant ionization of helium by intense XUV pulses: Analytical and numerical studies on channel-resolved spectral features

Core-resonant ionization of helium by intense XUV pulses: Analytical and numerical studies on channel-resolved spectral features

Relativistically intense XUV radiation from laser-illuminated near-critical plasmas

Pulses of extreme ultraviolet (XUV) light, with wavelengths between 10 and 100$\,$nm, can be used to image and excite ultra-fast phenomena such as the motion of atomic electrons. Here we show that the illumination of plasma with near-critical electron density may be used as a source of relativistically intense XUV radiation, providing the means for novel XUV-pump--XUV-probe experiments in the non-linear regime. We describe how the optimal regime may be reached by tailoring the laser-target interaction parameters and by the presence of preplasma. Our results indicate that currently available laser facilities are capable of producing XUV pulses with duration $\sim 10~\text{fs}$, brilliance in excess of $10^{23}$ photons/s/mm$^2$/mrad$^2$ (0.1% bandwidth) and intensity $I\lambda^2 \gtrsim 10^{19}~\text{W}\text{cm}^{-2}\mu\text{m}^2$.

Enhanced high-order harmonic generation driven by a wavefront corrected high-energy laser

We developed and implemented an experimental setup for the generation of high-energy high-order harmonics in argon gas for nonlinear experiments in the extreme ultraviolet (XUV) spectral range. High-order harmonics were generated by loosely focusing a high-energy laser pulse centered around 800 nm into a gas cell filled with argon in a phase-matching condition. The wavefront distortions of the driving pulse were corrected by a deformable mirror, and the XUV conversion efficiency was significantly improved due to the excellent beam profile at focus. A high-damage-threshold beam splitter was used to eliminate high-energy driving pulses co-propagating with high-order harmonics. The setup has a potential to provide intense ultrashort XUV pulses reaching the intensity levels required for nonlinear experiments in the XUV spectral range.

Rabi oscillation in few-photon double ionization through doubly excited states

We theoretically investigate few-photon double ionization of helium in intense XUV laser fields by numerically solving the time-dependent Schr\"odinger equation. Our results show that the familiar single-ring structure in the joint electron momentum spectra is split into the double-ring and previously unobserved triple-ring structures at some specific photon energies. By tracing the electron population evolution of the corresponding states, we found that the triple-ring structure is induced by the coupled Rabi oscillations among the ground, a singly excited, and a doubly excited states. The intermediate detuning causes the asymmetry of the triple-ring structures, which can be controlled by changing the laser intensity and frequency.

Direct measurement of the pulse duration and frequency chirp of seeded XUV free electron laser pulses

We report on a direct time-domain measurement of the temporal properties of a seeded free-electron laser pulse in the extreme ultraviolet spectral range. Utilizing the oscillating electromagnetic field of terahertz radiation, a single-shot THz streak-camera was applied for measuring the duration as well as spectral phase of the generated intense XUV pulses. The experiment was conducted at FLASH, the free electron laser user facility at DESY in Hamburg, Germany. In contrast to indirect methods, this approach directly resolves and visualizes the frequency chirp of a seeded free-electron laser (FEL) pulse. The reported diagnostic capability is a prerequisite to tailor amplitude, phase and frequency distributions of FEL beams on demand. In particular, it opens up a new window of opportunities for advanced coherent spectroscopic studies making use of the high degree of temporal coherence expected from a seeded FEL pulse.

Plasma high-order-harmonic generation from ultraintense laser pulses.

Plasma high-order-harmonic generation from an extremely intense short-pulse laser is explored by including the effects of ion motion, electron-ion collisions, and radiation reaction force in the plasma dynamics. The laser radiation pressure induces plasma ion motion through the hole-boring effect, resulting in frequency shifting and widening of the harmonic spectra. The classical radiation reaction force slightly mitigates the frequency broadening caused by the ion motion. Based on the results and physical considerations, parameter maps highlighting the optimum regions for generating a single intense attosecond pulse and coherent XUV radiation are presented.

Correlated electronic decay in expanding clusters triggered by intense XUV pulses from a Free-Electron-Laser

Irradiation of nanoscale clusters and large molecules with intense laser pulses transforms them into highly-excited non- equilibrium states. The dynamics of intense laser-cluster interaction is encoded in electron kinetic energy spectra, which contain signatures of direct photoelectron emission as well as emission of thermalized nanoplasma electrons. In this work we report on a so far not observed spectrally narrow bound state signature in the electron kinetic energy spectra from mixed Xe core - Ar shell clusters ionized by intense extreme-ultraviolet (XUV) pulses from a free-electron-laser. This signature is attributed to the correlated electronic decay (CED) process, in which an excited atom relaxes and the excess energy is used to ionize the same or another excited atom or a nanoplasma electron. By applying the terahertz field streaking principle we demonstrate that CED-electrons are emitted at least a few picoseconds after the ionizing XUV pulse has ended. Following the recent finding of CED in clusters ionized by intense near-infrared laser pulses, our observation of CED in the XUV range suggests that this process is of general relevance for the relaxation dynamics in laser produced nanoplasmas.

Dynamic Interference Photoelectron Spectra in Double Ionization: Numerical Simulation of 1D Helium**Supported by the National Natural Science Foundation of China under Grant Nos 61178028, 11674243 and 11674242, and the National Basic Research Program of China under Grant No 2015CB755403.

We report our numerical simulation on the dynamic interference photoelectron spectra for a one-dimensional (1D) He model exposed to intense ultrashort extreme ultraviolet (XUV) laser pulses. The results demonstrate an unambiguous interference feature in the photoelectron spectra, and the interference is unveiled to originate from the dynamic Stark effect. The interference photoelectron spectra are prompted for intense sub-femtosecond XUV laser pulses in double ionization. The stationary phase picture is corroborated qualitatively in the two-electron system. The ability of probing the dynamic Stark effect by the photoelectron spectra in a pragmatic experiment of single-photon double ionization of He may shed light on further investigation on multi-electron atoms and molecules.

Sequential and nonsequential double ionization of helium by intense XUV laser pulses: Revealing ac Stark shifts from joint energy spectra

Double ionization of a model helium system by intense extreme ultraviolet laser pulses is investigated by solving the time-dependent Schr\odinger equation and extracting the photoelectron spectra. Considering a frequency regime where the nonsequential two-photon double ionization is expected to dominate the double ionization process according to the perturbation theory, we identify the sequential and nonsequential mechanisms from the joint energy spectra of two electrons in the strong-field regime. In the nonperturbative regime, we find that sequential multiphoton double ionization shows remarkable peaks in the joint energy spectra and these peaks spread their widths with increasing laser field strength, which is caused by the ac Stark effect. Thus in the strong-field regime, the ac Stark shifts of different ionic states can be observed from the sequential multiphoton features in the joint energy spectra and the signs of the ac Stark shifts can be controlled by the laser frequency. We also find that the nonsequential mechanism dominates double ionization only at low field strengths while the sequential multiphoton mechanism contributes substantially at high field strengths, in accordance with estimates from rate equations.

Role of heat accumulation in the multi-shot damage of silicon irradiated with femtosecond XUV pulses at a 1 MHz repetition rate.

The role played by heat accumulation in multi-shot damage of silicon was studied. Bulk silicon samples were exposed to intense XUV monochromatic radiation of a 13.5 nm wavelength in a series of 400 femtosecond pulses, repeated with a 1 MHz rate (pulse trains) at the FLASH facility in Hamburg. The observed surface morphological and structural modifications are formed as a result of sample surface melting. Modifications are threshold dependent on the mean fluence of the incident pulse train, with all threshold values in the range of approximately 36-40 mJ/cm2. Experimental data is supported by a theoretical model described by the heat diffusion equation. The threshold for reaching the melting temperature (45 mJ/cm2) and liquid state (54 mJ/cm2), estimated from this model, is in accordance with experimental values within measurement error. The model indicates a significant role of heat accumulation in surface modification processes.

Ionization of helium by intense XUV laser pulses: Numerical simulations on channel-resolved probabilities

Ionization of a helium atom by intense extreme ultraviolet laser pulses is investigated in a frequency regime where the high-frequency stabilization condition is only fulfilled for the lowest single ionization channel. Multiphoton double ionization substantially contributes to the total ionization probability for superintense fields. As a result, no obvious stabilization against total ionization occurs. A detailed view of probabilities into different single ionization channels as a function of the field strength is presented. We find that the probabilities into some ionic channels peak at field strengths corresponding to one-photon resonances between field-dressed ionic states in the high-frequency Floquet theory. Thus we propose a sequential ``ionization-excitation'' mechanism in the dressed energy picture: first, one-photon absorption causes single ionization, leaving the ion in its dressed ground state; second, the ion is excited to a new state via one-photon absorption at the field strength where the resonance condition in the dressed ionic system is fulfilled. To reveal the sequential mechanism in the time domain, we also take a time-dependent view on the channel-resolved probabilities, observing the decrease of the ground-state ionic channel probability during the laser pulse when the field strength is such that a resonance condition exists between the dressed states in the ion.

Generation of isolated sub-40-attosecond pulse with a multicycle chirped laser and a static electric field

We numerically investigate the high-order harmonic generation and isolated attosecond pulse generation in a waveform that linearly produced by chirped laser pulse, chirp-free laser pulse, and static electric field. When a chirp-free laser pulse is added to the produced field of the chirped driving pulse and the static electric field, the plateau harmonic yield is enhanced by two orders. The spectral modulation is also significantly decreased, and the bandwidth of XUV spectrum is further broadened. An intense and a clean isolated 38-as pulse can be produced from the intense broadband XUV supercontinuum. After proper phase compensation, an isolated sub-8-attosecond pulse can be obtained. Furthermore, quantum time–frequency analysis reveals that the selection of the short quantum path can be achieved in this scheme.

Molecular formation along the atmospheric mass loss of HD 209458b and similar Hot Jupiters

The chemistry along the mass loss of Hot Jupiters is generally considered to be simple, consisting mainly of atoms, prevented from forming more complex species by the intense radiation field from their host stars. In order to probe the region where the temperature is low (T<2000K), we developed a 1D chemical and photochemical reaction model of the atmospheric mass loss of HD 209458b, involving 56 species, including carbon chain and oxygen-bearing ones, interacting through 566 reactions. The simulation results indicate that simple molecules like OH+, H2O+ and H3O+ are formed inside the region, considering that residual H2 survives in the exosphere, a possibility indicated by recent observational work. The molecules are formed and destroyed within a radial distance of less than 107km, but the estimated integrated column density of OH+, a potential tracer of H2, is high enough to allow detection, which, once achieved, would indicate a revision of chemical models of the upper atmosphere of Hot Jupiters. For low density Hot Jupiters receiving less intense XUV radiation from their host stars than HD 209458b, molecular species could conceivably be formed with a higher total column density.