Time and space waveform optimization to extend the harmonic cutoff and to produce the water window single attosecond pulse

2021 ◽  
Vol 0 (0) ◽  
Author(s):  
Liqiang Feng ◽  
Quan Yuan
Keyword(s):  
Attosecond Pulse ◽  
Space Region ◽  
Time And Space ◽  
Harmonic Intensity ◽  
Harmonic Emission ◽  
Water Window ◽  
Waveform Optimization ◽  
High Order Harmonic ◽  
Unipolar Pulse ◽  
Time Region

Abstract Based on the three-step theory of high-order harmonic generation, the harmonic cutoff is very sensitive to the few-cycle laser waveform in both time and space regions. Therefore, in this paper, we propose the method to control the harmonic cutoff and to produce the water window attosecond pulse through the optimization of time and space waveform. It is found that, in the time region, by properly choosing the delay and phase of the few-cycle two-color pulse, not only the harmonic intensity is enhanced, but also the quantum path of the harmonic emission can be controlled. Further, with the introduction of the 3rd pulse (i.e., the infrared pulse or the unipolar pulse), the harmonic cutoff from the single harmonic emission peak can be extended, showing a water window harmonic plateau. In the space region, by using the positive spatial inhomogeneous effect, the harmonic cutoff from the basic two-color waveform can also be extended, which leads to a water window spectral continuum. Finally, by Fourier transformation of harmonics during the water window region, the ultrashort single 29 as pulses can be obtained.

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

2020 ◽  
pp. 2150022
Author(s):  
Xiaodan Jing ◽  
Liqiang Feng ◽  
Li Liu ◽  
John Mccain ◽  
Yi Li
Keyword(s):  
Molecular System ◽  
Attosecond Pulse ◽  
Resonance Ionization ◽  
Half Cycle ◽  
Harmonic Intensity ◽  
Molecular Systems ◽  
Unipolar Pulse ◽  
Energy Peak ◽  
Attosecond Pulses ◽  
Time Region

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.

Sub-waveform optimization for producing water window single-order harmonic

2020 ◽  
pp. 2150003
Author(s):  
Li-Qiang Feng ◽  
Hui Liu ◽  
Hang Liu
Keyword(s):  
Potential Method ◽  
Half Cycle ◽  
Emission Process ◽  
Harmonic Emission ◽  
Water Window ◽  
Waveform Optimization ◽  
Unipolar Pulse ◽  
Window Region ◽  
Color Field ◽  
Theoretical Analyses

Through the sub-waveform optimization of the laser field, a potential method to produce the water window single-order harmonic (SOH) has been proposed. First, by properly introducing the chirps of two-color field, the SOHs from 303th order to 616th order can be obtained. Theoretical analyses show that the folding region on the harmonic emission process, caused by the multiple accelerations, is responsible for the enhanced SOH. Moreover, the folding region is dependent on the neighbor two half-cycle profiles. Thus, through further controlling the sub-waveform of the folding region by using the unipolar pulse, the folding region on the harmonic emission process will be extended to the higher photon energy region, including the water window region. Finally, by properly choosing the combinations of chirps and unipolar pulses, the water window SOH from 446th order to 833th order (from 345 eV to 645 eV) can be obtained.

scholarly journals Attosecond source generation using polarized gating two-color field combined with unipolar pulse

2016 ◽  
Vol 94 (7) ◽  
pp. 651-658 ◽  
Author(s):  
Liqiang Feng ◽  
Hang Liu
Keyword(s):  
Harmonic Generation ◽  
Extreme Ultraviolet ◽  
Harmonic Spectrum ◽  
High Order ◽  
High Order Harmonic Generation ◽  
Emission Process ◽  
Harmonic Emission ◽  
High Order Harmonic ◽  
Unipolar Pulse ◽  
Color Field

High-order harmonic generation and attosecond extreme-ultraviolet source generation have been theoretically investigated by controlling the two-color polarized gating field combined with the unipolar pulse. The results show that by properly optimizing the polarized two-color field as well as the unipolar pulse, not only is the harmonic cutoff remarkably extended, but the single short quantum path has also been selected to contribute to the harmonic spectrum, resulting in a 313 eV less modulated supercontinuum. Classical and quantum analyses are shown to explain the harmonic emission process. Finally, the proper superposition of harmonics, a series of isolated sub-50 as pulses, can be obtained.

Generation of intense spectral continuum and isolated attosecond pulse by selecting single harmonic emission peak

2019 ◽  
Vol 33 (35) ◽  
pp. 1950444
Author(s):  
Liqiang Feng ◽  
Yi Li
Keyword(s):  
Pulse Duration ◽  
Laser Intensity ◽  
Attosecond Pulse ◽  
Resonance Ionization ◽  
Lower Intensity ◽  
Harmonic Intensity ◽  
Chirped Pulse ◽  
Harmonic Emission ◽  
Pulse Intensity ◽  
Isolated Attosecond Pulse

Generally, due to the interference of different harmonic emission peaks (HEPs), the intensity of high-order harmonic spectrum cannot keep enhancing as the pulse intensity increases. Thus, in this paper, a potential method to obtain an intense spectral continuum and isolated attosecond pulse (IAP) by selecting single HEP has been theoretically investigated. First, we choose the harmonic cutoff and the harmonic intensity from the optimal single-color laser intensity as the referenced values. Next, by properly choosing a lower-intensity negative chirped pulse, we see that the harmonic cutoff from this field is similar as that from the referenced field. Moreover, the spectral continuum is contributed by single HEP. However, the intensity of the spectral continuum from the negative chirped pulse is lower than that from the referenced field. As the pulse duration of the chirped pulse increases, the similar harmonic cutoff can also be found by using a much lower-intensity negative chirped pulse. However, the intensity of the spectral continuum is decreased compared with that from the shorter chirped pulse duration. Further, with the introduction of an IR or UV controlling pulse, the intensity of the spectral continuum can be enhanced up to the referenced value. Moreover, the longer the pulse duration of the controlling pulse is used, the lower the controlling pulse intensity is needed. Also, due to the UV resonance ionization, much lower UV intensity is needed to enhance the harmonic yield compared with that for adding IR controlling pulse case. All in all, the total laser intensity of the combined field (the fundamental pulse + the controlling pulse) is lower than that of the referenced field. Most importantly, the signal of the spectral continuum is only coming from single HEP, which can support the generations of intense IAPs with the durations of 30 as.

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

2020 ◽  
Vol 34 (17) ◽  
pp. 2050192
Author(s):  
Liqiang Feng ◽  
John Mccain
Keyword(s):  
Heavy Nucleus ◽  
Light Nucleus ◽  
Laser Intensity ◽  
Attosecond Pulse ◽  
High Order ◽  
Emission Peak ◽  
Half Cycle ◽  
Harmonic Emission ◽  
High Order Harmonic ◽  
Cycle 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.

THE CARRIER-ENVELOPE PHASE DEPENDENCE OF HIGH-ORDER HARMONIC EMISSION AND ISOLATED ATTOSECOND PULSE GENERATION

2011 ◽  
Vol 10 (02) ◽  
pp. 209-216
Author(s):  
DI YANG ◽  
SHU-LIN CONG
Keyword(s):  
Pulse Generation ◽  
Attosecond Pulse ◽  
High Order ◽  
Phase Dependence ◽  
Time Frequency ◽  
Harmonic Emission ◽  
Carrier Envelope Phase ◽  
High Order Harmonic ◽  
Isolated Attosecond Pulse ◽  
Driving Pulse

We study theoretically the high-order harmonic generation from neon atoms in a two-color laser field, which is synthesized by a 5-fs/800-nm fundamental driving pulse and a 12-fs/1600-nm subharmonic controlling pulse. Numerical simulations show that the harmonic emission is strongly sensitive to the carrier-envelope phase (CEP) of either the fundamental driving pulse or the subharmonic controlling pulse. When neon atoms are driven by such laser pulses with optimal CEP, the generated high-order harmonic spectrum is a supercontinuum that corresponds to single attosecond pulse. The calculation results are explained in terms of the semiclassical "recollison" model and the time-frequency analysis.

Effects of inhomogeneous laser field in time and space on the generation of broad spectral continuum and ultrashort attosecond pulse

2020 ◽  
Vol 34 (13) ◽  
pp. 2050131
Author(s):  
H. Liu ◽  
L. Q. Feng ◽  
R. M. Sani
Keyword(s):  
Laser Field ◽  
Ionization Probability ◽  
Time And Space ◽  
Harmonic Emission ◽  
High Order Harmonic ◽  
Resonance Enhancement ◽  
Attosecond Pulses ◽  
The Difference ◽  
He Atom ◽  
Frequency Chirping

The inhomogeneous effects of laser field in time and space on the generations of high-order harmonic spectra and attosecond pulses from He atom have been investigated with the assistance of the frequency-chirping technique and the metallic nanostructure. We find that in different combinations of chirps and spatial inhomogeneous effects, not only can the harmonic cutoff be extended, but the single harmonic emission peak (HEP) can also be selected to contribute to the spectral continuum. In detail, with the combinations of (i) symmetric chirp in frequency and positive inhomogeneous effect in space or (ii) asymmetric down chirp in frequency and negative inhomogeneous effect in space, two broad spectral continua with bandwidths of 496 eV and 480 eV can be obtained. Further, by properly adding IR or UV controlling pulse, the harmonic emission efficiency can be enhanced by several orders of magnitude. The enhancement of harmonic yield from adding UV pulse is higher than that from adding IR pulse. Moreover, the difference of harmonic enhancement between UV combined field and IR combined field is much more obvious at lower controlling laser intensity. Through analyzing the ionization probability, we find that the higher enhancement of harmonic yield from UV combined field is because of UV-resonance-enhancement-ionization between the ground state and the excited state of He atom. Finally, by properly superposing some harmonics, a number of attosecond pulses with pulse durations of 45 as can be obtained.

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