Single Attosecond Pulse Generation Research Articles

Density-dependent carrier-envelope phase shift in attosecond pulse generation from relativistically oscillating mirrors

Density-dependent carrier-envelope phase shift in attosecond pulse generation from relativistically oscillating mirrors

Comparative study of different optimization methods for single attosecond pulse generation with a two- or three-color gating scheme

The shaping of the waveform of a two- or three-color driving laser pulse is essential for generating an ultrashort isolated attosecond pulse (IAP) from high-order harmonic generation, which can be implemented in two different ways, i.e., direct coherent synthesizing of electric fields with appropriate parameters (wavelength, intensity, carrier envelope phase, time delay), and superposition of fields undergoing a spectral phase shaping in frequency space view, which additionally results in a chirp and pulse-width broadening. In this work, we perform a comparative study on the IAP generation with these two schemes by virtue of a genetic algorithm, and two very meaningful conclusions are clarified. First, weighing the advantages and disadvantages of the two methods, the width and strength of IAPs optimized from two schemes are basically the same. More optimization parameters in the second method of laser shaping do not show an advantage in shortening width or enhancing the intensity of the IAP compared with the first one. Second, in each case, one can get the shortest IAP when the carrier envelope phases for each spectral component are not required to be precisely controlled. This study provides useful theoretical guidance for generating a shorter IAP with multicolor gating in the experiment.

Generation of intense single short attosecond pulse with a multicycle spatially inhomogeneous field

We theoretically study the high-order harmonics and single attosecond pulse generation from a pre-excited He+ ion in a multicycle two-color spatially inhomogeneous field. It is shown that the broadband supercontinuum spectrum and single quantum path selection can be realized, and then a single 15.2 as pulse is directly obtained. By appending a uv attosecond pulse to the two-color spatially inhomogeneous field at a proper time, the conversion efficiency of the high-energy part of the supercontinuum is improved by at least an order of magnitude, which enables the production of an intense single 17.3 as pulse.

Generation of the high-intensity single harmonic energy peak and attosecond pulse by using resonance ionization schemes from atoms and molecules

The lower harmonic intensity is still the reason that limits the application of attosecond pulse. Thus, in this paper, on the basis of resonance ionization schemes, we propose the effective methods to improve the harmonic intensity of atomic and molecular systems. For atomic system (i.e. He atom), with the combination of chirped-UV pulse, not only the harmonic cutoff can be extended, but also the harmonic intensity can be enhanced, caused by the UV resonance ionization between ground state and excited state. As a result, the single harmonic energy peak (HEP) with the higher intensity and higher emitted photon energy can be obtained, which can support the attosecond pulses shorter than 40 as. For the molecular system (i.e. [Formula: see text] and [Formula: see text]), by properly choosing the pulse duration of the fundamental pulse, the time region of charge resonance enhanced ionization (CREI) can be well controlled. Therefore, the maximum HEP with the higher intensity can be obtained. Second, by properly adding a half-cycle unipolar pulse, the selected maximum HEP in the CREI region can be further extended. Thus, an intense and broad spectral region can be produced, which can generate the attosecond pulses with durations of 36[Formula: see text]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.

Simulations on the single attosecond pulse generation by optimized multicolor laser field

The attosecond (as) pulses are of great significance to observe and control the ultrafast electron dynamics in atoms, molecules and condensed matter. By utilizing some sub-cycle gating methods, i.e., amplitude gating, polarization gating, and double optical gating, isolated attosecond pulses (IAP) have been generated experimentally from high-order harmonics generation (HHG) by the interaction of intense femtosecond laser fields and gas targets. At present, about 50-as IAPs are now available only in few laboratories in the world. Thus, searching for the ways to generate more short IAPs is still desirable and important for the strong field physics and attosecend science. The color gating is another potential technique generating ultrashort IAP, which could generate continuum high-order harmonics without using sub-cycle laser pulses. On the theoretical side, many schemes with two- and three-color combination field have been proposed to generate short IAP, and some works propose that in this way the IAP with width less than 10 as may be generated. However, these investigations are performed only on the single-atom level, and the real experimental conditions and strength of IAPs are not considered in their simulations. In this work, we perform a numerical experiment on the generation of strong IAP from HHG driven by multicolor driving fields. Unlike the previous studies, the macroscopic effects are included in the present simulations. Considering the detection range of spectrograph, the centering energy of IAP is confined as ~200 eV. To obtain single attosecond pulse with short width, a genetic algorithm is used to optimize waveforms of two- and three-color excitation laser pulses, which is determined by intensity, wavelength and carrier envelope phase for each component. We choose the fundamental pulse as Ti:sapphire laser field with the wavelength of 800 nm to ensure the intensity of the generated attosecond pulse, and the second or third component is not confined as the commensurate harmonics of fundamental pulse. In order to possess an acceptable computation time, the model of strong field approximation is adopted in the simulations of HHG. Our simulations show that by coherent synthesizing two or three multi-cycle pulses (the full width at half maximum of each laser field is 6 fs) we obtain sub-optical-cycle laser pulse, which can easily generate supercontinuum high-order harmonic spectra with wide bandwidth. Different from the single-color driving field, which produces the attosecond pulse train, the single attosecond pulses are generated from optimized laser fields. The width of IAP decreases with the increase of intensity of driving laser field. On the single atom level, our simulations show that about 50 as and 40 as IAP can be obtained by superimposing appropriate harmonics generated by optimized two- and three-color laser pulses, respectively. More importantly, after considering the propagation effects, about 50 as IAPs are obtained with spherical spatial filtering of harmonics in the far field, under reasonable conditions, i.e., the length and position of gas jet, gas pressure. With great advent of the ultrafast lasers technology, such as coherent waveform synthesizing, optical parametric amplification and optical parametric chirped-pulse amplification, this work gives helpful guidance for intense short IAP generation in the experiment.

Generation of Single Attosecond Pulse by Using Asymmetric Polarization Gating Technology

The generation of high-order harmonics and single attosecond pulses via the asymmetric polarization gating technology has been theoretically investigated. It is shown that when the two circularly polarized laser fields are asymmetric in amplitude and phase, not only can the modulations of the harmonic spectrum be decreased, but also the efficiencies of the harmonics can be enhanced. As a result, a super-continuum with the bandwidth of 85 eV, contributed by the single harmonic emission peak and the near-single short quantum path, can be obtained. Finally, through the Fourier transformation of the selected harmonics on this supercontinuum, a near-single attosecond pulse with a full width at half maximum of 52 as can be obtained.

Generation of single cycle attosecond pulse via Thomson scattering of Gaussian beam interacting with a single electron in the electrostatic field environment

The new way of generating the attosecond pulses via Thomson scattering of linear polarization Gaussian beam propagating on solid surface is investigated. It is found that a single attosecond laser pulse is generated which renders the appropriate strength electrostatic field to the environment. After studying the influence of the strength of the electrostatic field on it, the results show that a single attosecond pulse is generated when intensity of Gaussian beam is 5 and intensity of the electrostatic field is Es = 5 ×10−4, which is more intense and direction simplification.

Generation of water window single attosecond pulse from multi-cycle mid-infrared laser field with chirp gating modulation

A potential scheme to produce the water window single attosecond pulses (SAPs) from the multi-cycle mid-infrared (MIR) laser field has been investigated with the help of the chirp gating modulation. It is found that with the introduction of the laser chirp (i.e. up-chirp or down-chirp modulation), the extension of the harmonic cutoff can be achieved and the single harmonic radiation peak (HRP) can be selected during the harmonic emission process. Moreover, the chirp gating modulation on the high-order harmonic generation (HHG) is sensitive to the few-cycle pulse duration, but it is not very sensitive to the multi-cycle pulse duration. Moreover, a larger extension of the harmonic cutoff covering the whole water window region can be found with the help of the multi-cycle down-chirp modulation. Further, by properly choosing the coherent superposition of the ground state and the high Rydberg state as the initial state, the efficiency of HHG can be enhanced by 5 orders of magnitude. Finally, a series of high-intensity sub-40as pulses covering the water window region can be obtained.

Generation of High-Intensity Single Attosecond Pulse by Using Inhomogeneous Chirped Pulse

The generation of high-order harmonics and single attosecond pulses from $$ {H}_2^{+} $$ has been theoretically investigated beyond the Born–Oppenheimer approximations. It is found that with the introduction of the inhomogeneous effects of the laser field in frequency and space, not only the harmonic cutoff can be remarkably enhanced, but also the single short quantum path can be selected. Further, with the increase of the initial vibrational state and by properly adding a terahertz controlling field, the harmonic yield can be enhanced by 2.5 orders of magnitude, showing a highintensity supercontinuum with the bandwidth of 511 eV. As a result, four single attosecond pulses shorter than 28 as can be produced. Moreover, the present scheme can also be achieved in the multicycle pulse duration, which is much better for practical application.

Single attosecond pulse generation for a dielectric driven by mid-infrared laser field

We theoretically investigate the high-order-harmonic generation (HHG) from solids by simulating the dynamics of a single active electron in a dielectric. Interband tunnelling is investigated, by which the harmonic generation can be modeled based on the classical trajectory analysis of electron-hole pairs. The supercontinuum region of the fundamental HHG plateau, whose intensity is about seven-order magnitude enhancement as compared with that of the second plateau, is used in the generation of isolated attosecond pulses. Thus, it provides us a practical way to increase the conversion efficiency of HHG with laser intensity below the damage threshold.

Single attosecond pulse generation by using plasmon-driven double optical gating technology in crossed metal nanostructures

An effective method to obtain the single attosecond pulses (SAPs) by using the multi-cycle plasmon-driven double optical gating (DOG) technology in the specifically designed metal nanostructures has been proposed and investigated. It is found that with the introduction of the crossed metal nanostructures along the driven and the gating polarization directions, not only the harmonic cutoff can be extended, but also the efficient high-order harmonic generation (HHG) at the very highest orders occurs only at one side of the region inside the nanostructure. As a result, a 93 eV supercontinuum with the near stable phase can be found. Further, by properly introducing an ultraviolet (UV) pulse into the driven laser polarization direction (which is defined as the DOG), the harmonic yield can be enhanced by two orders of magnitude in comparison with the singe polarization gating (PG) technology. However, as the polarized angle or the ellipticity of the UV pulse increase, the enhancement of the harmonic yield is slightly reduced. Finally, by superposing the selected harmonics from the DOG scheme, a 30 as SAP with intensity enhancement of two orders of magnitude can be obtained.

Generation of single attosecond pulse within one atomic unit by using multi-cycle inhomogeneous polarization gating technology in bowtie-shaped nanostructure

The generations of high-order harmonic spectra and single attosecond pulses (SAPs) driven by the multi-cycle inhomogeneous polarization gating (PG) technology in the bowtie-shaped nanostructure have been theoretically investigated. It is found that by setting the bowtie-shaped nanostructure along the driven laser polarization direction, not only the extension of the harmonic cutoff can be achieved, caused by the surface plasmon polaritons, but also the modulations of the harmonics can be decreased, caused by the PG technology and the inhomogeneous effect. As a result, the contribution of the harmonic plateau is only from one harmonic emission peak with the dominant short quantum path. Further, by properly adding a half-cycle pulse into the driven laser field, the harmonic emission process can be precisely controlled in the half-cycle duration and a supercontinuum with the bandwidth of 263 eV can be obtained. Finally, by directly superposing the harmonics from this supercontinuum, a SAP with the full width at half maximum of 23 as can be obtained, which is shorter than one atomic unit.

Gas position effect on single attosecond pulse generation in crossed metal nanostructure

An effect method to obtain the single attosecond pulses (SAPs) by using the inhomogeneous polarization gating (PG) technology in the crossed metal nanostructure has been proposed and investigated. It is found that by properly injecting the gas into the negative position of the nanostructure, the emitted harmonics at the very highest orders used for the SAPs only occurs at one side of the region inside the nanostructure. As a result, not only the extension of the harmonic cutoff can be achieved, but also the modulations of the harmonics can be decreased. Further, with the introduction of a weak near-infrared field, the harmonic yield can be enhanced by 3.5 orders of magnitudes in comparison with that from the single PG technology. Finally, by superposing the selected harmonics from the inhomogeneous PG scheme, a 32 as SAP with the high signal intensity can be obtained.

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 .

Intense single attosecond pulse generation from near-critical-density plasmas irradiated by a few-cycle laser pulse

Coherent synchrotron emission (CSE) from relativistic near-critical-density (NCD) plasmas irradiated by a few-cycle laser pulse is investigated theoretically and numerically. Due to the unique and larger laser-plasma interaction region in relativistic NCD plasmas, compared to those in solid targets, not only the required stringent conditions for CSE on laser and target are relaxed but also the radiation intensities are enhanced by two orders of magnitude. Moreover, it is found that a single attosecond pulse can also be easily obtained in the transmitted direction through CSE in this regime. Its energy conversion efficiencies from laser to emission can reach 10−3–10−2, which is more than one order of magnitude larger than those of attosecond trains from solids. Two-dimensional particle-in-cell simulations show that an intense single pulse at a peak intensity of ∼1019 W/cm2 and duration of ∼98 as in the transmitted direction is produced by the drive laser at an intensity of I0 = 8.6 × 1020 W/cm2.

Generation of high-intensity sub-30 as pulses by inhomogeneous polarization gating technology in bowtie-shaped nanostructure

The generation of high-order harmonics and single attosecond pulses (SAPs) from He atom driven by the inhomogeneous polarization gating technology in a bowtie-shaped nanostructure is theoretically investigated. The results show that by the proper addition of bowtie-shaped nanostructure along the driven laser polarization direction, the harmonic emission becomes sensitive to the position of the laser field, and the harmonics emitted at the maximum orders that generate SAPs occur only at one side of the region inside the nanostructure. As a result, not only the harmonic cutoff can be extended, but also the modulations of the harmonics can be decreased, showing a carrier envelope phase independent harmonic cutoff with a bandwidth of 310 eV. Further, with the proper introduction of an ultraviolet pulse, the harmonic yield can be enhanced by 2 orders of magnitude. Finally, by the Fourier transformation of the selected harmonics, some SAPs with a full width at half maximum of sub-30 as can be obtained.

Generation of isolated attosecond pulses by polarization gating of high-order harmonic emission from in intense ultrashort laser fields

This study presents a quantum-mechanical investigation of single attosecond pulse generation by gating high-order harmonic emission from H molecules in intense laser pulses with time-dependent ellipticity. The high-order harmonic generation from H molecules under left- and right-hand circularly polarized Gaussian pulses in superposition, is studied, and the effect of time duration and carrier-envelope phase of laser field on single attosecond pulse generation is investigated. Using a laser field formed by combination of left- and right-hand elliptically polarized Gaussian pulses, the effect of ellipticity of field on the duration and intensity of the attosecond pulses generated is studied. Numerical calculations show that, with the conventional polarization gating, a single attosecond pulse can be isolated from the pulse train emitted by the H molecule in a laser field.

Single attosecond pulse generation via continuum wave packet interference

A single attosecond pulse generation via continuum–continuum interference is investigated theoretically by exposing a single-electron atom in a femtosecond laser field with the intensity in over-the-barrier ionization regime. We show that the ground state of the atom is depleted in such intense laser field and the high-order harmonics (HHG) via continuum to continuum coherence are generated. In a few-cycle monochromatic laser field (5 fs/800nm, 1.2×1016Wcm−2), a single attosecond pulse with duration of 49 as is obtained from the HHG. With a two-color laser field combined by 1200nm (8 fs/7.5×1015Wcm−2) and 800nm (5 fs/1.0×1016Wcm−2), a shorter single pulse with duration of 29 as can further be produced by changing the relative carrier envelope phase of two laser pulses as a result of controlling the electronic quantum path in the intense electric field. Our results also show that a short single attosecond pulse can be generated in a wide range of the relative carrier envelope phase of the two laser pulses.

Near-circularly polarized single attosecond pulse generation from nitrogen molecules in spatially inhomogeneous laser fields

The generation of an attosecond pulse in nitrogen molecules using spatially inhomogeneous laser fields is investigated by numerically solving the time-dependent Schrödinger equation. It is found that an isolated attosecond pulse with elliptical polarization can be generated using linearly polarized laser fields. By changing polarization direction with respect to the molecular axis, the ellipticity of the attosecond pulse can be easily controlled. At some specific angles, the intensities of the two mutually vertical harmonic components, parallel and perpendicular to the driving laser polarization direction, are comparable. Additionally, the relative phase between the two components is about π/2. As a result, it supports the generation of the isolated near-circularly polarized attosecond pulse with a duration of 155 as.