Intense Attosecond Pulses Research Articles

Research progress of high-order harmonics and attosecond radiation driven by interaction between intense lasers and plasma

The realizing of the detection and control of ultrafast process conduces to understanding and remoulding the physical world at a microcosm level. The attosecond light source with attosecond temporal resolution and nanometer spatial resolution can realize real-time detection and manipulation of the atomic-scale electronic dynamics and relevant effects of the substances. Therefore, attosecond science is considered as one of the most important milestones in the history of laser science. and has been listed as an important scientific and technological development direction in the coming 10 years. High-order harmonic generation (HHG) from intense laser-matter interaction is one of the most important routes to breaking through the femtosecond limit and achieving brilliant attosecond pulse radiations, and thus having aroused great interest in recent years. After more than 20-year development, the research about attosecond pulse generation by laser-gas interaction has reached a mature stage. This method produces the shortest isolated pulse in the world to date, with a pulse width being only 43 as. However, this method based on ionization-acceleration-combination encounters inevitable difficulties in pursuing the relativistically intense attosecond pulses and the highest possible photon energy. Quite a lot of studies have proved that the HHG efficiency from laser-plasma interaction can be a few orders of magnitude higher than that in gaseous media, which makes it possible to produce pulses with shorter pulse width and higher photon energy. In this article, we introduce the main generation mechanisms, research progress and frontier applications of HHG through the laser-plasma interaction process. In Section 2, we introduce the HHG generation mechanisms, including coherent wake emission, which is used to describe the HHG process driven by a nonrelativistic laser; relativistic oscillating mirror, which can well explain most of HHG processes generated from plasma-vacuum interface in relativistic regime; coherent synchrotron emission, which is suited to explain the HHG synchronously emitted from isolated electron sheets. The research progress is summarized in Section 3 from the aspects of radiation efficiency, polarization characteristics, phase characteristics, generation and diagnosis of isolated attosecond pulses, etc. Frontier applications of these ultra-broadband intense attosecond pulses are presented in the last section, such as the study of electronic dynamics, process, coherent diffraction imaging, diagnosis of extreme states of matter, the generation of extremely intense fields, etc. Finally, an outlook on the future development trends and innovation breakthroughs is also presented.

A Custom-Tailored Multi-TW Optical Electric Field for Gigawatt Soft-X-Ray Isolated Attosecond Pulses

Since the first isolated attosecond pulse was demonstrated through high-order harmonics generation (HHG) in 2001, researchers’ interest in the ultrashort time region has expanded. However, one realizes a limitation for related research such as attosecond spectroscopy. The bottleneck is concluded to be the lack of a high-peak-power isolated attosecond pulse source. Therefore, currently, generating an intense attosecond pulse would be one of the highest priority goals. In this paper, we review our recent work of a TW-class parallel three-channel waveform synthesizer for generating a gigawatt-scale soft-X-ray isolated attosecond pulse (IAP) using HHG. By employing several stabilization methods, we have achieved a stable 50 mJ three-channel optical-waveform synthesizer with a peak power at the multi-TW level. This optical-waveform synthesizer is capable of creating a stable intense optical field for generating an intense continuum harmonic beam thanks to the successful stabilization of all the parameters. Furthermore, the precision control of shot-to-shot reproducible synthesized waveforms is achieved. Through the HHG process employing a loose-focusing geometry, an intense shot-to-shot stable supercontinuum (50–70 eV) is generated in an argon gas cell. This continuum spectrum supports an IAP with a transform-limited duration of 170 as and a submicrojoule pulse energy, which allows the generation of a GW-scale IAP. Another supercontinuum in the soft-X-ray region with higher photon energy of approximately 100–130 eV is also generated in neon gas from the synthesizer. The transform-limited pulse duration is 106 as. Thus, the enhancement of HHG output through optimized waveform synthesis is experimentally proved.

Phase-matching analysis in high-order harmonic generation with nonzero orbital angular momentum Laguerre-Gaussian beams

Through high-order harmonic generation driven by intense ultrashort vortex infrared or midinfrared lasers, a nonzero orbital angular momentum can be imprinted onto extreme ultraviolet (XUV) or soft-x-ray (SXR) light pulses. Here we simulate the generation of vortex XUV harmonics in the gas medium as well as their propagation in vacuum till reaching the far field. We find that the intensity and phase of generated high harmonics are very sensitive to the position of gas jet with respect to the laser focus. The topological charge of the $q\mathrm{th}$ harmonic is found to be $q$ times that of the driving Laguerre-Gaussian beam. Each harmonic in the far field appears as a single ring in the transverse plane with an invariant diameter which is scalable with the fundamental topological charge only when the gas jet is placed after the laser focus. The underlying phase-matching mechanism is analyzed by examining the spatial map of the coherence length and by calculating the evolution of harmonic emission in the medium. We anticipate this work to stimulate interest in generating intense vortex XUV or SXR attosecond pulses for probing dynamics of molecules where special molecular features are difficult to be detected with linear or circular XUV or SXR pulses.

The effects of plasma density-gradient on laser-driven transmitted emission

High-order harmonic generation provides an opportunity for generating intense extreme ultraviolet attosecond pulses, which is useful to explore the property of materials. Transmitted radiation emitted by an ultrathin target illuminated by intense lasers is a promising way to generate such pulses. We show that the preplasma density gradient has an important influence on the transmitted radiation. An optimal scale length exists to support ultrathin nanobunches to generate coherent synchrotron emission and thus enormously enhance the radiation. Particle in cell simulations suggest that the optimal scale length depends on the laser intensity.

Control of the half-cycle harmonic emission process for generating the intense and ultrashort single attosecond pulses (SAPs)

Abstract In this paper, the half-cycle harmonic generation process has been controlled by using the asymmetric inhomogeneous chirped pulse combined with the ultraviolet (UV) pulse. It is found that by properly optimizing the chirps and chirp delays of the fundamental two-color pulse, the optimal negative and positive half-cycle laser profiles for the harmonic cutoff extension can be obtained. Further, with the introduction of the negative and positive inhomogeneous effect, respectively, the harmonic cutoff from the negative and positive half-cycle laser profiles can be further improved. Next, with the assistance of the UV pulse, the harmonic intensity can be enhanced due to the UV resonance ionization. Moreover, the single and double UV photon resonance ionizations are much better for generating the higher harmonic intensity. As a result, the stronger and broader harmonic plateaus with the larger harmonic cutoff can be obtained, which can support the generation of the high-intensity ultrashort attosecond pulses with the pulse durations of sub-45 as.

Generation of an intense single harmonic emission event and attosecond pulse from hydrogen molecular ion and its isotopic molecular ions with the assistance of half-cycle unipolar pulse

Through controlling the charge resonance enhanced ionization process of the isotope molecules of hydrogen molecular ion, deuterium molecular ion and tritium molecular ion, the intense single harmonic emission event has been produced, which is beneficial to generate the single attosecond pulse. Moreover, the optimal laser parameter combinations for producing the intense harmonic spectra and attosecond pulses have been shown from 4 × 1014 W/cm2 to 7 × 1014 W/cm2. Further, by properly adding a half-cycle unipolar pulse, the selected single harmonic emission event can be further extended, which can support the shorter attosecond pulse with higher photon energy. Finally, by superposing these harmonics, a number of intense attosecond pulses shorter than 40 as can be obtained.

Ultrafast electron–nuclear wavepacket in generated and probed with attosecond pulse trains

We observed the dynamical evolution of the electron–nuclear wavepacket composed of the and 32Πu states of with an evolution period of 1.3 fs. We created a dissociative wavepacket of by photoionization of O2 with the irradiation of an intense attosecond pulse train (APT) in the extreme ultraviolet (XUV) wavelength region, and probed it with another XUV APT by scanning delay between the two APTs. We revealed the evolution of the electron–nuclear wavepacket by the analysis of the spectral phase in the Fourier transform of the recorded delay dependent momentum spectrum of O+. We also found the correlation between the photon energy and the released kinetic energy originating from the dissociation of Rydberg states of O2 followed by the autoionization of the fragment O atom in the Rydberg states.

Production of 100-TW single attosecond x-ray pulse

Attosecond light sources have provided insight into the fastest atomic-scale electronic dynamics. True attosecond-pump–attosecond-probe experiments require a single attosecond pulse at high intensity and large photon energy, a challenge that has yet to be conquered. Here we show 100-TW single attosecond x-ray pulses with unprecedented intensity of 10 21 W / c m 2 and duration 8.0 as can be produced by intense laser irradiation of a capacitor-nanofoil target composed of two separate nanofoils. In the interaction, a strong electrostatic potential develops between the two foils, which drags electrons out of the second foil and piles them up in vacuum, forming an ultradense relativistic electron nanobunch. 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, intense attosecond pulse. Such a pulse enables the capture and control of electron motion at the picometer–attosecond scale.

Selection of intense spectral region and attosecond pulse from X2+ by using chirp-delay effect

A potential scheme to obtain intense harmonic spectra from X[Formula: see text] ([Formula: see text] and T) has been studied by using chirped-delay effect and half-cycle pulse. First, for the shorter pulse duration case, the main contributions of harmonic spectra of H[Formula: see text] and T[Formula: see text] are both coming from the falling part of laser field. While, for the longer pulse duration case, the rising and falling parts of laser field play the main role in harmonic spectra of H[Formula: see text] and T[Formula: see text], respectively. Second, according to the changed law of harmonic yields of H[Formula: see text] and T[Formula: see text] and by properly introducing the chirp-delay effect at some specific timing, the intense harmonic emission peak (HEP) can be obtained from both H[Formula: see text] and T[Formula: see text]. Third, with the help of the half-cycle pulse, the selected HEP can be further extended, showing the intense and broad spectral continua. Finally, by Fourier transformation of some harmonics on the spectral continua, the intense attosecond pulses with durations of 35 as can be produced.

Control of high-order harmonics from H2+ and D2+ for producing intense single attosecond pulse

Generally, the intensities of molecular high-order harmonic spectra from [Formula: see text] and its isotope molecules are quite different due to the effect of nuclear signals. Thus, in this paper, we investigate the change law of harmonic spectra from [Formula: see text] and [Formula: see text] driven by different laser fields and try to find the optimal harmonic spectra for producing intense single attosecond pulses (SAPs). The results show that in lower laser intensity case, the harmonic yield follows as [Formula: see text]; while, in higher laser intensity case, the harmonic yield meets the condition of [Formula: see text]. Further, by using this change law of harmonic yield and choosing the optimal harmonic emission peak (HEP), we can obtain the intense spectral continuum with the assistance of the half-cycle pulse (HCP). Next, with the superposition of some harmonics on the spectral continuum, the intense SAPs shorter than 37 as can be obtained from [Formula: see text] and [Formula: see text] harmonic spectra. Finally, the results show that the stronger attosecond signals can be obtained when the light nucleus or heavy nucleus molecules are driven by lower or higher laser intensities, respectively.

Asymmetry of molecular frame photoelectron angular distributions in by intense two-color attosecond pulses: coherent excitation and charge migration effects

Molecular frame photoelectron angular distributions (MFPADs) are theoretically studied by intense two-color attosecond linearly polarized laser pulses. Simulations are performed on aligned from numerical solutions of corresponding time-dependent Schrödinger equations. Asymmetric MFPADs are produced by a time-delayed soft x-ray attosecond pulse in the presence of an extreme ultraviolet (XUV) pump pulse. We present the dependence of the asymmetry ratio Γ of the MFPADs on the laser parameters of the pump-probe pulses. The asymmetry ratio Γ varies with the time delay, illustrating the coherent superposition of electronic states. Altering the phase, intensity and duration of the pump pulse gives rise to a modulation of the asymmetry ratio Γ as well. Moreover, the asymmetry of the MFPADs disappears in the case of long duration probe pulses, illustrating the coherent charge migration dynamics on the attosecond time scale.

Attosecond pulse generation from $H_2^ + $H2+ ions using a multicolor beam superposition method.

The behavior of the high-order harmonics and output attosecond pulses from hydrogen molecule ions with various internuclear distances that are exposed to high intensity incoming pulses are investigated. The incoming pulses that are spectrally wide yield from a superposition of monochromatic beams with a constant frequency distance. Our simulations show that the most intense and shortest attosecond pulses can result from hydrogen molecular ions with large internuclear distances which are exposed to irradiation of intense pulses with a frequency width greater than 0.03 a.u.

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.

Excited state effect of He+ ion on intense harmonic spectrum and attosecond pulse generation

We investigate the excited state effect of [Formula: see text] ion on intense harmonic spectrum and attosecond pulse (AP) generation. First step, by using the superposition state model, we find that as the excited state ratio increases, the intensity of harmonic spectrum can be enhanced, and a near-stable intense harmonic yield can be obtained when the ground state (GS) and the excited state remains within a certain ratio range. However, as the excited state ratio further increases, the intensity of harmonic spectrum is decreased. Further, with the introduction of a down-chirp modulation, the harmonic cutoff can be extended, showing a broader spectral continuum with a higher efficiency. Second step, by using the single GS model and with the help of the pump-probe-UV-resonance-ionization mechanism, the high-intensity and broader spectral continuum obtained in the first step can be realized in the experiment. Moreover, this method is not very sensitive to the delay time between the fundamental field and the UV pulse, which is beneficial to experimental implementation. As a result, 3 APs as short as 40 as can be obtained.

Attosecond pulse generation by relativistic flying mirrors in laser-plasma interaction: Effect of plasma density and driver amplitude on the generated pulse

The generation of high-intensity attosecond pulses by the interaction of two counterpropagating short laser pulses with underdense plasma is investigated. By using parallel fully kinetic particles in cell simulation, which shows the formation of relativistic flying mirrors in the wake wave of the intense driver laser pulse and the focusing reflection of the weak source pulse, it is demonstrated that intense attosecond pulses can be produced under the optimized conditions of plasma density and driver laser amplitude according to the relativistic similarity theory. In addition, it is shown that the frequency of the source pulse is upshifted by a factor from 10 to 80 corresponding to a reflected radiation wavelength from 20 to 164 nm which lies in the extreme ultraviolet region, while most of the energy lies around a frequency upshift of 20, in agreement with the measured Lorentz factor. The intensity of the main attosecond pulse is two orders higher than the source pulse intensity.

Waveform control in generations of intense water window attosecond pulses via multi-color combined field

The waveform control in the improvements of high-order harmonic generation (HHG) spectra and attosecond pulse signals driven by the two-color and three-color combined fields has been theoretically investigated. (a) The results show that by properly controlling the [Formula: see text]–2[Formula: see text] two-color laser beam (including the modulations of chirps, carrier envelope phases and delay time), either the harmonic cutoff can be extended, showing a water window spectral continuum, or the selective enhancement of the single-order and two-order harmonics can be found. Further, with the introduction of a third controlling field, the efficiency of spectral continuum can be enhanced by two orders of magnitude compared with that from the two-color field. Moreover, the enhancement of HHG is not very sensitive to the frequency of the third field (i.e., the frequency of the third field is chosen to be 3[Formula: see text], 4[Formula: see text] and 6[Formula: see text]). Thus, some water window attosecond pulses with the durations of 60 as can be obtained. (b) Furthermore, the harmonic cutoff can be further extended when using a half-cycle controlling pulse or introducing the inhomogeneous effect of the laser field. Moreover, the efficiency of HHG can be further improved when the initial state is prepared as the superposition state of the ground state and some excited state of He atom. Consequently, a much broader spectral continuum with an intensity enhancement of another two orders of magnitude can be found. Finally, through the Fourier transformation of some spectral continuum, the intense water window attosecond pulses with the durations of 60 as can be produced.

Behavioral study of high-order harmonics and attosecond pulse generation in a spatially non-homogeneous laser field

Using the metallic nano-antennas we can enhance the intensity of a laser pulse to prepare it for high-order harmonic generation. Plasmonic resonances in the metal surfaces cause a non-homogeneous enhancement of the incoming laser field. In this article behavior of the generated high-order harmonics and attosecond pulses with respect to the location of a helium ion in the non-homogeneous field is investigated. It is cleared that the broader supercontinuum can be observed for the case when the ion target located around the center of non-homogeneity which leads to more intense and shorter attosecond pulses.

Attosecond pulse generation via multicolor beam superposition in the over-barrier ionization regime

Based on coherent superposition of multicolor continuous wave beams, high order harmonics and attosecond pulse generation from a hydrogen molecular ion (HMI), with different internuclear distances, R, are studied. Driving pulse intensity was taken to be 400 TW cm−2, which can ionize an ion with R > Rb = 2.4 a.u. over the barrier. It is seen that cutoff harmonic with respect to R decreases until the over barrier ionization threshold, Rb, then becomes almost uniform to the end of R = 10 a.u. whereas it is almost constant with respect to the spectral width of the beams, Δω. Besides attosecond pulse intensity increases versus R, and decreases with respect to Δω so that the most intense attosecond pulses can be generated from HMIs with R ≥ 5 a.u. via the pulses by Δω ≤ 0.03 a.u. Attosecond pulse durations were insensitive to both Δω and R.

Harmonic Generation from Neutral Manganese Atoms in the Vicinity of the Giant Autoionization Resonance.

High harmonics from laser-ablated plumes are mostly generated from ionic species. We demonstrate that with ultrashort infrared (∼1.82 μm) driving lasers, high harmonics from laser-ablated manganese are predominantly generated from neutral atoms, a transition metal atom with an ionization potential of 7.4eV. Our results open the possibility to advance laser-ablation technique to study the dynamics of neutral atoms of low ionization potential. Moreover, as manganese contains giant autoionizing resonance, intense and broadband high harmonics have been demonstrated from this resonance at energies from 49 to 53eV. This opens the possibility to generate intense attosecond pulses directly from the giant resonances, as well as to study these resonances using high-harmonic spectroscopy.

Coherent synchrotron emission in transmission with double foil target

Generation of intense single attosecond pulses from coherent synchrotron emission (CSE), in the transmitted direction of the laser-irradiated double foil targets, has been investigated theoretically and numerically. Unlike conventional CSE in the single foil target case, here the dense electron nanobunch is formed in the vacuum gap between two foils, which is composed of the electrons blown out from the first ultrathin foil. Owing to the existence of the vacuum gap, the electron nanobunch can be accelerated to more energy. In addition, more laser energy can penetrate through the nanobunch and get reflected from the second foil. These reflected lasers and electron nanobunches interact with each other and results in enhanced CSE and consequently, the generation of intense attosecond pulses. Particle-in-cell simulations show that a single attosecond pulse with duration of 18 , photon energy and peak intensity of can be obtained from the double-foil targets irradiated by a laser at intensity of .