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Register a new userA case study for terahertz-assisted single attosecond pulse generation
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We numerically investigate the use of strong THz radiation in assisting single attosecond pulse generation by few-cycle, 800 nm laser pulses. We optimize focusing conditions to generate short and powerful single attosecond pulses of high-energy photons by keeping the parameters of the THz field within the limits achieved experimentally. We show that using optimal focusing geometry isolated attosecond pulses shorter than 100 as can be obtained even in the absence of further gating or XUV compression techniques, using an 8 fs generating pulse. Furthermore, quantum path control of short- and long-trajectory components is demonstrated by varying the delay between the THz and IR pulses.
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High-order harmonic generation by few-cycle 800 nm laser pulses in neon gas in the presence of a strong terahertz (THz) field is investigated numerically with propagation effects taken into account. Our calculations show that the combination of THz fields with up to 12 fs laser pulses can be an effective gating technique to generate single attosecond pulses. We show that in the presence of the strong THz field only a single attosecond burst can be phase matched, whereas radiation emitted during other half-cycles disappears during propagation. The cutoff is extended and a wide supercontinuum appears in the near-field spectra, extending the available spectral width for isolated attosecond pulse generation from 23 to 93 eV. We demonstrate that phase matching effects are responsible for the generation of isolated attosecond pulses, even in conditions when single atom response yields an attosecond pulse train.
We examine how the generation and propagation of high-order harmonics in a partly ionized gas medium affect their strength and synchronization. The temporal properties of the resulting attosecond pulses generated in long gas targets can be significantly influenced by macroscopic effects, in particular by the intensity in the medium and the degree of ionization. Under some conditions, the use of gas targets longer than the absorption length can lead to the generation of self-compressed attosecond pulses. We show this effect experimentally, using long argon-filled gas cells as generating medium.
Attosecond science promises to reveal the most fundamental electronic dynamics occurring in matter and it can develop further by meeting two linked technological goals related to high-order harmonic sources: higher photon flux (permitting to measure low cross-section processes) and improved spectral tunability (allowing selectivity in addressing specific electronic transitions). New developments come through parametric waveform synthesis, which provides control over the shape of high-energy electric field transients, enabling the creation of highly-tunable isolated attosecond pulses via high-harmonic generation. Here we show that central energy, spectral bandwidth/shape and temporal duration of the attosecond pulses can be controlled by shaping the laser pulse waveform via two key parameters: the relative-phase between two halves of the multi-octave spanning optical spectrum, and the overall carrier-envelope phase. These results not only promise to expand the experimental possibilities in attosecond science, but also demonstrate coherent strong-field control of free-electron trajectories using tailored optical waveforms.
A new method for efficiently generating an isolated single-cycle attosecond pulse is proposed. It is shown that the ultraviolet (UV) attosecond pulse can be utilized as a robust tool to control the dynamics of electron wave packets (EWPs). By adding a UV attosecond pulse to an infrared (IR) few-cycle pulse at a proper time, only one return of the EWP to the parent ion is selected to effectively contribute to the harmonics, then an isolated two-cycle 130-as pulse with a bandwidth of 45 eV is obtained. After complementing the chirp, an isolated single-cycle attosecond pulse with a duration less than 100 as seems achievable. In addition, the contribution of the quantum trajectories can be selected by adjusting the delay between the IR and UV fields. Using this method, the harmonic and attosecond pulse yields are efficiently enhanced in contrast to the scheme [G. Sansone {it et al.}, Science {bf314}, 443 (2006)] using a few-cycle IR pulse in combination with the polarization gating technique.
A robust plasma gating to generate a single ultra-intense attosecond pulse is developed. It is a manifestation of the hole-boring effect that limits the strongest attosecond pulse emission within one laser cycle. The generated pulse is characterized by a stabilized harmonic phase $psi approx pmpi/2$ and a slowly decaying exponential spectrum bounded by $gamma$-spike scaling and CSE scaling. The phase oscillations in low-frequency region and fluctuations in high-frequency region are discussed. We also show that the phase fluctuations in high-frequency region can be reduced by including radiation reaction force.
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