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Based on the three-step model of high-order harmonic generation, the effect of time-space waveform control on the extension of the harmonic cutoff energy has been theoretically studied. In the case of time region waveform control, by properly changing the laser intensity ratio, the laser phase and the time delay, the optimal three-color laser waveform can be obtained. Driven by this laser waveform, the harmonic cutoff can be extended. In the case of space region waveform control, based on the above best waveform as well as by choosing the appropriate positive laser inhomogeneous effect, the harmonic cutoff energy can be further extended. Moreover, with the increase of the inhomogeneous parameter, the harmonic cutoff can be extended. By analyzing the laser waveform, the spatial inhomogeneous effect and the harmonic emission process, the reason for the extension of the harmonic cutoff energy can be found. Furthermore, by properly superposing some harmonics, a single attosecond pulse with a duration of 45 as can be obtained. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Xinyang Normal University Natural Science Edition is the property of Journal of Xinyang Normal University Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

It is well known that X-ray crystallography is based on X-ray diffraction (XRD) for atoms and molecules. The diffraction pattern arises as a result of scattering of incident radiation, which makes it possible to determine the structure of the scattering substance. With the advent of ultrashort radiation sources, the theory and interpretation of X-ray diffraction analysis have remained the same. This work shows that when an attosecond laser pulse is scattered on a DNA molecule, including during its nicking and bending, the pulse duration is an important characteristic of the scattering. In this case, the diffraction pattern changes significantly compared to the previously known scattering theory. The results obtained must be used in XRD theory to study DNA structures, their mutations and damage, since the previously known theory can produce large errors and, therefore, the DNA structure can be "decoding" incorrectly. [ABSTRACT FROM AUTHOR]

We theoretically demonstrate a scheme for efficiently generating near-circularly polarized isolated attosecond pulses from mixed gases driven by non-integer frequency ratio two-color multicycle laser fields. It is shown that isolated attosecond pulses with a duration of 310 as can be generated, exhibiting non-zero ellipticity with values reaching as high as 0.9. Further analysis reveals that the ellipticities of these isolated attosecond pulses are attributed to the interference of high-order harmonics generated from different components of the mixture. When the components in the mixture have opposite orbital parity, constructive and destructive interference will occur for left- and right-circularly polarized harmonic components, or vice versa. Additionally, the temporal profile of the isolated attosecond pulse depends solely on the driving fields, while the polarization is determined by the gas mixture. The obtained elliptically polarized isolated attosecond pulses may serve as a promising tool for manipulating ultrafast dynamics in magnetic materials and chiral media.

Circularly and elliptically polarized high-order harmonics have unique advantages when used in studying the chiral and magnetic features of matter. Here, we studied the polarization properties of high-order harmonics generated from alignment nitrogen molecules driven by cross-linearly polarized two-color laser fields. Through adjusting various laser parameters and targets, such as the relative phase, the crossing angle, the intensity ratio of the driving fields, and the molecular alignment angle, we obtained highly elliptically polarized high-order harmonics with the same helicity in a wide spectral range. This provides a possible effective way to generate elliptically polarized attosecond pulses. Finally, we showed the probability of controlling the spectral range of elliptically polarized harmonics. [ABSTRACT FROM AUTHOR]

In this paper, the influence of role of quantum effects such as spin and tunneling on attosecond pulse generation in turbulent dense plasma is investigated. Calculations show that increasing the density gradient by a factor of 10, in the case of short-wavelengths, leads to about 90%, and in the case of long-wavelengths, leads to a 66% decrease in the nanobunching gain. Increasing the quantum parameter by a factor of 10 in the short-wavelength range will result in about 80% and in the long-wavelength range will lead to a 25% reduction in the nanobunching gain saturation time generated by the Weibel instability. Increasing the quantum parameter will increase the saturation value of the density perturbation. [ABSTRACT FROM AUTHOR]

In order to obtain the optimal inhomogeneous laser waveform of harmonic emission, the optimal waveforms of three - color pulse under the positive and negative inhomogeneous effects have been found by solving the time - dependent Schrodinger equation. The results show that the optimal extended high order harmonic spectra can be obtained by choosing the best positive and negative laser waveforms under the positive and negative inhomogeneous effects, respectively. Finally, the pulse duration of 43 as pulse can be obtained by Fourier transform in the harmonic plateau region. [ABSTRACT FROM AUTHOR]

It is well known that X-ray crystallography is based on X-ray diffraction (XRD) for atoms and molecules. The diffraction pattern arises as a result of scattering of incident radiation, which makes it possible to determine the structure of the scattering substance. With the advent of ultrashort radiation sources, the theory and interpretation of X-ray diffraction analysis have remained the same. This work shows that when an attosecond laser pulse is scattered on a DNA molecule, including during its nicking and bending, the pulse duration is an important characteristic of the scattering. In this case, the diffraction pattern changes significantly compared to the previously known scattering theory. The results obtained must be used in XRD theory to study DNA structures, their mutations and damage, since the previously known theory can produce large errors and, therefore, the DNA structure can be “decoding” incorrectly.

In this study, the attosecond transient absorption (ATA) spectrum below the excited states of the helium atom was investigated by numerically solving the fully three-dimensional time-dependent Schrödinger equation. Under single-active electron approximation, the helium atom was illuminated by a combined field comprising of extreme ultraviolet (XUV) and delayed infrared (IR) fields. The response function demonstrates that the absorption near the central frequency ( ω X ) of the XUV field is periodically modulated during the overlapping between the XUV and IR pulses. Using the time-dependent perturbation, the absorption near ω X is attributed to the wavepacket excited by the XUV pulse. The wave function oscillating at the frequency of the XUV pulse was obtained. Furthermore, the chirp-dependent absorption spectrum near ω X potentially provides an all-optical method for characterizing the attosecond pulse duration. Finally, these results can extend to other systems, such as solids or liquids, indicating a potential for application in photonic devices, and they may be meaningful for quantum manipulation. [ABSTRACT FROM AUTHOR]

We present the theory and simulation of attosecond transient absorption in helium atoms under the single-active-electron approximation. This study investigates the attosecond dynamics of intrinsic atomic states that interact with a field comprising vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) fields. The absorption spectrum of the helium atom is obtained from the response function, which is constructed by numerically solving the three-dimensional time-dependent Schrödinger equation. We observe a fine structure near the intrinsic atomic level, which is modulated with a 0.2 fs period. Based on high-order time-dependent perturbation theory, the frequency-dependent phase of the dipole response induced by the VUV and XUV fields is analytically obtained, and the fine structure is well explained by the phase difference. In addition, the absorption fringes are dependent on the chirp of the VUV field. This study investigates the features of the attosecond transient absorption in the VUV region, which may have valuable applications in the study of ultrafast phenomena in atoms, molecules, and solids. [ABSTRACT FROM AUTHOR]

A semi-classical optimization model is introduced for controlling the high-order harmonic generation process and extending the cutoff frequency. This method is capable of defining the driving laser shape interact with the F2 molecule for maximizing the cutoff frequency properly. This optimization procedure is evaluated by examining the high harmonic spectrum from the F­2 molecule irradiated by a two-color laser field. High harmonic spectrum is done using time-dependent density functional theory in a three-dimensional space. The results showed that adding two driving laser pulses with optimization could enhance the cutoff frequency by 96% compared to two driving laser pulses without optimization. In addition, this model for the F2 molecule is capable of reducing the output attosecond pulse duration from 200 as not optimized two-color laser to 135 as the optimized two-color laser.

Photoionization of atoms or molecules is a fundamental process in strong-field physics, essential for generating high-order harmonics and producing attosecond pulses. Here, we investigate how a three-color laser field, consisting of wavelengths at 800 nm and its second and third harmonics, photoionizes N 2 molecules and evaluates its effect on the properties of attosecond pulses. It has been discovered that by altering the relative phase, these harmonics periodically influence molecular ionization, increasing the discreteness and strength of the attosecond pulse trains. Three-color field driving dramatically raises the pulse peak while decreasing the width of the strongest peak pulse, whereas two-color field driving just slightly enhances the pulse intensity and duration. Furthermore, the effectiveness of harmonic production is significantly increased by varying the laser intensity ratios. Even though the efficiency and pulse intensity of the three-color field are limited by conversion efficiency, attosecond pulses with higher intensities can be produced with proper parameter tuning at lower ionization rates. These findings offer a significant foundation for precisely controlling attosecond pulse generation. 320.7110; 320.7120. [ABSTRACT FROM AUTHOR]

High-order harmonics and attosecond pulse generation with coherent wake emission are theoretically investigated for the effect of pulse duration and carrier envelope phase (CEP) of few-cycle laser pulse. We find that short pulse duration will cause the negative chirp for the high harmonics. When the laser pulse is shortened to a few cycles, the influence of the laser CEP on the chirp of the harmonics will also become more prominent. [ABSTRACT FROM AUTHOR]

In this paper, the variable coefficient nonlinear Schrödinger equation with fifth order dispersion in the inhomogeneous optical fiber is investigated to study the impact of fifth order dispersion on attosecond soliton propagation. Based on the Darboux transformation method, soliton solution is constructed with modulated coefficients though Lax pair. We show that the coefficients of GVD and fifth order term are the main keys to control the amplitude, width and phase shift in the model. With the suitable choices of coefficients for the obtained soliton solution, some new types of soliton management through fifth order are presented for the first time. Especially, the influence of fifth order dispersion of the higher order NLS equations with and without modulated coefficients is discussed in detail. We will also highlight the impact of the studies on application in ocean engineering. Obtained results may be potentially useful in the soliton shaping and management in inhomogeneous fibers.

In this paper, high-order harmonic generation by a bicircular field, which consists of two coplanar counterrotating circularly polarized fields of frequency ω and 3ω, is investigated for bromine (Br2) molecule. This field possesses dynamical symmetry, which can be adapted to the symmetry of the highest occupied molecular orbital (HOMO) and used to investigate the molecular symmetry. An accurate three dimensional(3D) calculation of molecular dynamics through time dependent density- functional theory (TDDFT)is considered to explore broad harmonic plateau with high efficiency. We show that an attosecond pulse with a pulse duration of 349as from the superposition of several optimum harmonics is generated. Furthermore, in order to study of the quantum trajectory of electrons, time-frequency analysis is utilized. This work is proved to be a potential way for applications in coherent imaging and photoelectron spectroscopy.

A laser synthesis scheme via the superposition of single-color and hyper-Gaussian pulses is proposed, and the synthesized laser pulse is adopted to interact with pre-aligned molecules for the production of an isolated attosecond pulse (IAP). The numerical results based on strong-field approximation disclose that the proposed scheme is efficient for the generation of an IAP under suitable hyper-Gaussian parameters, and the intensity of an IAP is sensitive to molecular pre-alignment angles, and thus an optimal set of hyper-Gaussian parameters and pre-alignment angles can support the strongest IAP. [ABSTRACT FROM AUTHOR]

We propose a new theoretical method of producing a single attosecond pulse by high-order harmonic generation in hydrogen gaseous atoms above the conical plasmonic nanostructure (CPN) with a radially polarized laser field. The results show that the maximum amplification of the laser field above the structure apex about 4879 times can be provided by adjusting several physical characteristics of the structure and the incident light. Numerical studies show that the distribution of the amplification field above the CPN apex satisfies high-order harmonics generation (HHG) and attosecond pulse generation (APG) conditions by various incident laser beam intensities. Spectral analysis shows that this scheme can modulate the electron quantum paths. Selecting one quantum path, a broadband supercontinuum spectrum can be produced, which without phase compensation, directly generates a sharp isolated sub-20-attosecond pulse with a high signal-to-noise ratio. • High-order harmonic generation in gaseous atoms above the conical plasmonic nanostructure (CPN) with a radially polarized laser field. • Huge amplification of the laser field above the structure apex about 4879 times. • Generation of short sub-20as pulse without phase compensation at 10 10 W/cm 2 incident pulse. • Generation of intense high-order harmonics and attosecond pulses. [ABSTRACT FROM AUTHOR]

We theoretically demonstrate a scheme for efficiently generating near-circularly polarized isolated attosecond pulses from mixed gases driven by non-integer frequency ratio two-color multicycle laser fields. It is shown that isolated attosecond pulses with a duration of 310 as can be generated, exhibiting non-zero ellipticity with values reaching as high as 0.9. Further analysis reveals that the ellipticities of these isolated attosecond pulses are attributed to the interference of high-order harmonics generated from different components of the mixture. When the components in the mixture have opposite orbital parity, constructive and destructive interference will occur for left- and right-circularly polarized harmonic components, or vice versa. Additionally, the temporal profile of the isolated attosecond pulse depends solely on the driving fields, while the polarization is determined by the gas mixture. The obtained elliptically polarized isolated attosecond pulses may serve as a promising tool for manipulating ultrafast dynamics in magnetic materials and chiral media. • Highly elliptically polarized attosecond pulses are generated from mixed gases. • We apply the non-integer frequency ratio two-color fields for generating the IAPs. • An IAP with ellipticity up to 0.9 is produced, and its duration is 310 as. [ABSTRACT FROM AUTHOR]

We present the theory and simulation of attosecond transient absorption in helium atoms under the single-active-electron approximation. This study investigates the attosecond dynamics of intrinsic atomic states that interact with a field comprising vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) fields. The absorption spectrum of the helium atom is obtained from the response function, which is constructed by numerically solving the three-dimensional time-dependent Schrödinger equation. We observe a fine structure near the intrinsic atomic level, which is modulated with a 0.2 fs period. Based on high-order time-dependent perturbation theory, the frequency-dependent phase of the dipole response induced by the VUV and XUV fields is analytically obtained, and the fine structure is well explained by the phase difference. In addition, the absorption fringes are dependent on the chirp of the VUV field. This study investigates the features of the attosecond transient absorption in the VUV region, which may have valuable applications in the study of ultrafast phenomena in atoms, molecules, and solids.

In this study, the attosecond transient absorption (ATA) spectrum below the excited states of the helium atom was investigated by numerically solving the fully three-dimensional time-dependent Schrödinger equation. Under single-active electron approximation, the helium atom was illuminated by a combined field comprising of extreme ultraviolet (XUV) and delayed infrared (IR) fields. The response function demonstrates that the absorption near the central frequency (ωX) of the XUV field is periodically modulated during the overlapping between the XUV and IR pulses. Using the time-dependent perturbation, the absorption near ωX is attributed to the wavepacket excited by the XUV pulse. The wave function oscillating at the frequency of the XUV pulse was obtained. Furthermore, the chirp-dependent absorption spectrum near ωX potentially provides an all-optical method for characterizing the attosecond pulse duration. Finally, these results can extend to other systems, such as solids or liquids, indicating a potential for application in photonic devices, and they may be meaningful for quantum manipulation.

Copyright of Laser Technology is the property of Gai Kan Bian Wei Hui and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

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. [ABSTRACT FROM AUTHOR]

Copyright of Laser Technology is the property of Gai Kan Bian Wei Hui and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

We theoretically investigate the elliptically polarized (EP) high-order harmonic generation (HHG) from the mixed gases driven by the combined laser field, which is combined with an EP laser field and a linearly polarized (LP) laser field. The numerical results show that the harmonic intensity is enhanced, and the cutoff is extended compared with that in the EP laser field. Benefiting from the interference between the harmonics generated from the two single gases, we obtain the bright broadband EP HHG with large ellipticity from the mixed gases. The origin of the harmonic ellipticity is illustrated by the trajectories of the electrons. The result indicates that the harmonic ellipticity is large if the angle between the velocity directions at the initial time and the final time is close to 90°. In addition, the analysis shows that the left-handed components of the harmonics from two single gases produce destructive interference, and the right-handed components of that produce constructive interference, which results in the enhancement of the intensity difference of the left- and right-handed harmonics from the mixed gases. Thus, the harmonic ellipticity is large from the mixed gases. Our results are conducive to obtaining near circularly polarized (CP) XUV and attosecond pulse radiation from mixed gases in a broad spectral range. [ABSTRACT FROM AUTHOR]