Intense Isolated Attosecond Pulses Research Articles

Examination of optimized ultrashort three-color waveforms for generating short and intense isolated attosecond pulses in soft x rays

Isolated attosecond pulses (IAPs) can be readily generated via high-order harmonic generation driven by an ultrashort laser pulse. Here, it is shown that the best way to obtain the ultrashort waveform for producing a short and intense IAP in the soft x rays is to optimize the three-color (TC) laser pulse consisting of the fundamental field and its second and third harmonic fields. To calibrate it, another way of constructing the ultrashort waveform directly in time using a truncated basis set of B-spline functions is first proposed. The calibration waveform (CW) contains more frequency components up to the eighth harmonic order. It is found that the IAP by the TC waveform has a shorter duration after macroscopic propagation in a nonlinear gas medium compared to that by the CW field. It is uncovered that the CW field is additionally modified by the higher-order frequency components during propagation, dominated by the neutral atom dispersion. The effect of phase jitter in the TC waveform and the extension of the TC scheme into higher photon energies are also discussed. Currently, precise control of TC laser waveform synthesis is already achievable in the labs, thus paving an effective way for generating a useful attosecond light source in the soft x rays.

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, omega and second harmonic, 2omega) 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 omega -2omega 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.

Production of intense isolated attosecond pulses with circular polarization by using counter-propagating relativistic lasers

We demonstrate theoretically and numerically that intense isolated circularly polarized (CP) attosecond pulses can be generated from ultrathin foil targets irradiated by two relativistic lasers from opposite sides, where their polarizations are orthogonal to each other. With a proper matching condition, the compressed oscillating plasma mirrors on both sides of the foil are pushed inside by laser radiation pressures, eventually merging together to form a dense electron nanobunch under the effect of orthogonal laser fields. This nanobunch reaches both high density and high energy in only half a laser cycle and smears out in others, resulting in coherent synchrotron emission of a single attosecond pulse with circular polarization. Two-dimensional particle-in-cell simulations show that an intense isolated CP attosecond XUV pulse with an intensity of 1.2 × 1019 W cm−2 and a duration of ∼75 as can be obtained by two lasers with the same intensity of 2.1 × 1020 W cm−2.

Direct generation of relativistic isolated attosecond pulses in transmission from laser-driven plasmas.

Isolated attosecond pulses are useful to perform pump-probe experiments at a high temporal resolution, and provide a new tool for ultrafast metrology. However, it is still a challenging task to generate such pulses of high intensity, even for a few-cycle laser. Through particle-in-cell simulations, we show that it is possible to directly generate a giant isolated attosecond pulse in the transmission direction from relativistic laser-driven plasmas. Compared to attosecond pulse generation in the reflection direction, no further spectral filtering is needed. The underlying radiation mechanism is coherent synchrotron emission, and the transmitted isolated attosecond pulse can reach relativistic intensity. This provides a promising alternative to generate intense isolated attosecond pulses for ultrafast studies.

Techniques to generate intense isolated attosecond pulses from relativistic plasma mirrors

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 radiation pressure of the incident laser and thus focus the generated harmonic beams. This increases the harmonic beam divergence and makes it difficult to separate the attosecond pulses angularly. In this article, we propose two novel techniques readily applicable in experiments to significantly reduce the divergence of Doppler harmonics, and achieve the generation of isolated attosecond pulses from the lighthouse effect without requiring few-cycle laser pulses. Their validity is demonstrated using state-of-the-art simulations, which show that isolated attosecond pulses with $10$TW peak power in the X-UV range can be generated with PW-class lasers. These techniques can equally be applied to other generation mechanisms to alleviate the constraints on the duration on the laser pulses needed to generate isolated attosecond pulses.

Comparison of damage and ablation dynamics of multilayer dielectric films initiated by few-cycle pulses versus longer femtosecond pulses.

The importance of high intensity few- to single-cycle laser pulses for applications such as intense isolated attosecond pulse generation is constantly growing, and with the breakdown of the monochromatic approximation in field ionization models, the few-cycle pulse (FCP) interaction with solids near the damage threshold has ushered a new paradigm of nonperturbative light-matter interaction. In this Letter, we systematically study and contrast how femtosecond laser-induced damage and ablation behaviors of SiO2/HfO2-based reflective multilayer dielectric thin film systems vary between FCP and 110 fs pulses. With time-resolved surface microscopy and ex situ analysis, we show that there are distinct differences in the interaction depending on the pulse duration, specifically in the "blister" morphology formation at lower fluences (damage) as well as in the dynamics of debris formation at higher fluences (ablation).

Intense X-ray isolated attosecond pulse generation by using low-intensity chirped pulse combined with a UV seeding pulse

We theoretically investigate intense isolated attosecond pulse (IAP) generation from high-order harmonic generation (HHG) driven by a low-intensity chirped pulse combined with a UV seeding pulse. The results show that, driven by a two-color chirped pulse, the harmonic cutoff can be remarkably extended and a spectral continuum in X-ray region can be obtained. Moreover, as the pulse duration increases or the delay time of the two pulses changes, a larger harmonic cutoff can be found. Further, with the introduction of a UV seeding pulse, the efficiency of HHG can be enhanced by three orders of magnitudes due to the UV resonance ionization. Moreover, as the UV pulse intensity increases, the enhanced ratio of HHG yield can be further improved. The enhancement of HHG yield is related to the pulse duration and delay time of the UV pulse. For instance, when adding a shorter duration UV pulse, the enhancement of HHG is dependent on the delay time of the UV pulse. However, when adding a longer duration UV pulse, the HHG enhancement is not very sensitive to the delay time of the UV pulse. Finally, the obtained spectral continuum supports the generation of intense IAP with the duration of 36 as.

Near-circularly polarized isolated attosecond pulse generation from coherent superposition state by a circularly polarized laser field

We theoretically investigate high-order harmonic generation (HHG) from a coherent superposition state of hydrogen atom in a circularly polarized (CP) laser field. The results show that, driven by the CP field, HHG from the coherent superposition state is enhanced by 4 to 5 orders of magnitude compared to that from the ground state. The enhanced intensity is comparable to that of the ground state in a linearly polarized laser field. Based on the classical and quantum analyses, we demonstrate that HHG in the CP laser field mainly arises from the recombination of electrons ionized at non-zero initial positions. The enhancement of HHG from the coherent superposition state is due to the higher ionization rate and also the larger probability density of electrons located far from the nucleus of the involved excited state. The obtained harmonic spectra support the generation of intense isolated attosecond pulses (IAPs) with the ellipticity as high as 0.93. Moreover, we show that the generation of IAPs with large ellipticity is robust against the variation of the carrier envelope phase (CEP) of the laser field. Our results provide a new route toward generating near-CP IAPs, which will be beneficial for ultrafast detection of chiral-sensitive light-matter interactions.

Towards intense isolated attosecond pulses from relativistic surface high harmonics

Relativistic surface high harmonics have been considered a unique source for the generation of intense isolated attosecond pulses in the extreme ultra-violet and x-ray spectral ranges. Their practi ...

Isolated attosecond pulse in the water window from many-cycle laser-driven plasma mirrors without pulse engineering

High-order harmonic generation from relativistic laser-driven plasma mirrors is an attractive route to produce highly energetic attosecond pulses in the extreme ultraviolet to x-ray regime. To achieve an isolated attosecond pulse (IAP) driven by many-cycle intense laser pulses, pulse engineering techniques such as polarization modulation and wavefront rotation, are usually needed. Here we show that it is possible to generate an IAP without pulse engineering. Through particle-in-cell simulations, it is found that plasma mirrors can be rapidly heated and deformed in a relatively long preplasma regime. Intense IAP in the high-frequency spectral region is given rise once when the mirror parameters are suitable. The results may offer a new route to generate a bright IAP source for various applications such as bio-imaging and electronic dynamic studies.

Isolated elliptically polarized attosecond soft X-ray with high-brilliance using polarization gating of harmonics from relativistic plasmas at oblique incidence.

The production of intense isolated attosecond pulse is a major goal in ultrafast research. Recent advances in high harmonic generation from relativistic plasma mirrors under oblique incidence interactions gave rise to photon-rich attosecond pulses with circular or elliptical polarization. However, to achieve an isolated elliptical attosecond pulse via polarization gating using currently available long driving pulses remains a challenge, because polarization gating of high harmonics from relativistic plasmas is assumed only possible at normal or near-normal incidence. Here we numerically demonstrate a scheme around this problem. We show that via control of plasma dynamics by managing laser polarization, it is possible to gate an intense single attosecond pulse with high ellipticity extending to the soft X-ray regime at oblique incidence. This approach thus paves the way towards a powerful tool enabling high-time-resolution probe of dynamics of chiral systems and magnetic materials with current laser technology.

Intense keV IAP generation by orthogonally polarized multicycle midinfrared two-color laser fields

We theoretically investigate the generation of intense keV attosecond pulses in an orthogonally polarized multicycle midinfrared two-color laser field. It is demonstrated that multiple continuum-like humps, which have a spectral width of about twenty orders of harmonics and an intensity of about one order higher than adjacent normal harmonic peaks, are generated under proper two-color delays, owing to the reduction of the number of electron-ion recollisions and suppression of inter-half-cycle interference effect of multiple electron trajectories when the long wavelength midinfrared driving field is used. Using the semiclassical trajectory model, we have revealed the two-dimensional manipulation of the electron-ion recollision process, which agrees well with the time frequency analysis. By filtering these humps, intense isolated attosecond pulses are directly generated without any phase compensation. Our proposal provides a simple technique to generate intense isolated attosecond pulses with various central photon energies covering the multi-keV spectral regime by using multicycle driving pulses with high pump energy in experiment.

Enhanced high-order harmonic generation from periodic potentials in inhomogeneous laser fields

We theoretically study high-order harmonic generation (HHG) from solid-phase systems in spatially inhomogeneous strong laser fields originated by resonant plasmons within a metallic nanostructure. The intensity of the second plateau in HHG may be enhanced by two-three orders and be comparable with the intensity of the first plateau. This is due to bigger transition probabilities to higher conduction bands. It provides us a practical way to increase the yields of HHG with laser intensity below the damage threshold. It presents a promising way to triple the range of HHG spectra in experimental measurements. It also allows to generate intense isolated attosecond pulse from solids driven by few-cycle laser fields.

Nonlinear Attosecond Metrology by Intense Isolated Attosecond Pulses

For the future progress of nonlinear attosecond science, one of the most important issues is the development of high-energy isolated attosecond pulse sources. In this paper, we review our research on the energy-scaling method for isolated attosecond pulse generation, which combines infrared two-color gating and an energy-scaling method of high-order harmonic generation. By establishing the robust energy-scaling method, the maximum pulse energy of the isolated attosecond pulse reaches 1.3 $\mu$ J/pulse in the XUV region, which is almost 100-fold higher than the values ever reported before. The pulse duration of this isolated attosecond pulse is directly measured to be 500 as by the autocorrelation method using the nonlinear interaction of N $_2$ . This nonlinear experiment clearly shows that our developed isolated attosecond pulse has enough pulse energy to make a breakthrough for nonlinear attosecond metrology as well as for attosecond-pump/attosecond-probe experiments investigating electronic processes. In addition, we further extend the high-energy harmonic continuum by using the infrared two-color gating method up to the soft-x-ray region. With the aim to temporally characterize an isolated attosecond pulse in the soft-x-ray region, we demonstrate the first observation of above-threshold ionization of He with two-photon absorption of multiple higher harmonics in the soft-x-ray region.

Intense isolated attosecond pulse generation from relativistic laser plasmas using few-cycle laser pulses

We have performed a systematic study through particle-in-cell simulations to investigate the generation of attosecond pulse from relativistic laser plasmas when laser pulse duration approaches the few-cycle regime. A significant enhancement of attosecond pulse energy has been found to depend on laser pulse duration, carrier envelope phase, and plasma scale length. Based on the results obtained in this work, the potential of attaining isolated attosecond pulses with ∼100 μJ energy for photons >16 eV using state-of-the-art laser technology appears to be within reach.

Generation of intense isolated attosecond pulses with high spatiotemporal quality by two-color polarization-gating Bessel–Gauss beams

We propose an efficient approach to generate intense isolated attosecond pulses, which is based on the double optical gating mechanism of using two-color polarization-gating driving pulses. A relatively intense second harmonic field is added to the fundamental pulse and microscopically enhances the high harmonic emission insider polarization-gating. In the high harmonic generation, Bessel–Gauss beams are employed to macroscopically improve the propagation dynamics in both temporal and spatial domains. The results shows that, compared with the traditional Gaussian case, Bessel–Gauss beams can produce well phase-matched supercontinuum in the plateau region, and efficient isolated attosecond pulses with high spatiotemporal quality can be obtained successfully.

Intense attosecond pulse generated from a molecular harmonic plateau of H2+ in mid-infrared laser fields

High-order harmonic generation from the molecular ion H2+ exposed to intense laser fields is investigated by the time-dependent quantum wave packet method. Molecular and atomic plateaus of harmonic spectra are effectively distinguished at large internuclear distances, where the harmonic efficiency of the molecular plateau is several orders of magnitude higher than that of the latter. We report on a physical model of the origin of the molecular supercontinua and reveal that the creation of this plateau directly results from the interference of the intramolecular electronic wave packet localized in two potential wells following the laser field. This is our first effort in utilizing the efficient molecular plateau to generate intense isolated attosecond pulses by controlling the dynamics of the nucleus and electrons with a mid-infrared laser. Further, we show that the harmonic plateau is enhanced at the macroscopic level by solving the Maxwell wave equation coupled with the Schrödinger equation.

Attosecond nonlinear optics using gigawatt-scale isolated attosecond pulses

High-energy isolated attosecond pulses required for the most intriguing nonlinear attosecond experiments as well as for attosecond-pump/attosecond-probe spectroscopy are still lacking at present. Here we propose and demonstrate a robust generation method of intense isolated attosecond pulses, which enable us to perform a nonlinear attosecond optics experiment. By combining a two-colour field synthesis and an energy-scaling method of high-order harmonic generation, the maximum pulse energy of the isolated attosecond pulse reaches as high as 1.3 μJ. The generated pulse with a duration of 500 as, as characterized by a nonlinear autocorrelation measurement, is the shortest and highest-energy pulse ever with the ability to induce nonlinear phenomena. The peak power of our tabletop light source reaches 2.6 GW, which even surpasses that of an extreme-ultraviolet free-electron laser.

Two states hydrogenlike model for high-order harmonic generation and an isolated attosecond pulse generation in a He+ ion

We present an analytic quantum theory of high harmonic generation by low frequency laser fields for a two-state hydrogenlike atom model. In the case of an initial coherent superposition of two states, the theory clearly explains why high conversion efficiency can be obtained. Furthermore, we apply this model to generate an intense isolated attosecond pulse by the proposed combined field. By mixing an infrared chirped pulse with a fundamental chirped pulse, a supercontinuum spectrum can be formed. The long quantum trajectory is suppressed, and the short one is enhanced and an intense isolated 95-as pulse is obtained by He+ ion. Our simulation shows that the produced pulses with good signal-to-noise ratio are obtained without any phase compensation. In particular, these results are analyzed by using both classical and time-frequency analyses.

For the generation of an intense isolated pulse in hard X-ray region using X-ray free electron laser

AbstractFor a real, meaningful pump-probe experiment with attosecond temporal resolution, an intense isolated attosecond pulse is in demand. For that purpose we report the generation of an intense isolated attosecond pulse, especially in X-ray region using a current-enhanced self-amplified spontaneous emission in a free electron laser (FEL). We use a few cycle laser pulse to manipulate the electron-bunch inside a two-period planar wiggler. In our study, we employ the electron beam parameters of Pohang Accelerator Laboratory (PAL)-XFEL. The RF phase effect of accelerator columns on the longitudinal energy distribution profile and current profile of electron-bunch is also studied, aiming that these results can be experimentally realized in PAL-XFEL. We show indeed that the manipulation of electron-energy bunch profile may lead to the generation of an isolated attosecond hard X-ray pulse: 150 attosecond radiation pulse at 0.1 nm wavelength can be generated.