Single Isolated Attosecond Pulses Research Articles

Attosecond pulse isolation via intense laser field synthesis

Precisely synthesized driving fields for relativistic laser-plasma interactions are shown, using particle in cell simulations, to allow for the absolute control over the temporal structure, and efficiency, of emitted attosecond pulse trains. Using a two-color driving field with an off-harmonic frequency as the second color exploits the sensitivity of the generation mechanism to the exact waveform seen by the plasma electrons. Fine control over the relative phase between the two colors, combined with the incommensurate nature of the driving field, breaks the symmetry of the input wave and allows for the selective enhancement of individual pulses in the resultant attosecond pulse train. Our results show that, by suppression of the surrounding pulses in the train, it is possible to use this technique to generate a single isolated attosecond pulse without compromising the resultant intensity as is common in other temporal gating schemes and how it can be combined with the noncollinear gating mechanism to further improve pulse isolation. Published by the American Physical Society 2024

Generation of single isolated attosecond pulse from a monolayer hexagonal boron nitride using a polarization gating technique

An ab initio approach is presented to generate an isolated attosecond pulse (IAP) by gating high-order harmonic emission from a monolayer hexagonal boron nitride (h-BN), where in this case the ellipticity of the fields are varying rapidly with time. The high harmonic generation from a monolayer h-BN lattice is studied when it is exposed to the single circularly- and bicircularly-polarized laser fields. The effect of positive and negative chirped laser pulses on the high harmonics are considered. The role of ellipticity in the HHG to create single IAP from 2D h-BN under counter-rotating bicircularly polarized laser pulses is investigated in mid-infrared wavelength region. In order to generate single IAP, the polarization gating (PG) technique is implemented. By adjusting properly positive chirped parameters and implementing the conventional polarization gating to an input intensity of 20 TW cm−2, an isolated 80 as pulse from the superposition of several harmonics is generated.

Electron‐Nuclear Dynamics in Molecular Harmonic Generation Driven by a Plasmonic Nonhomogeneous Field

Electron (z)‐nuclear (R) dynamics in the molecular high‐order harmonic generation (MHHG) from H2+ driven by the plasmonic nonhomogeneous field, generated by the surface plasmon polaritons in the bowtie‐shaped nanostructure, have been theoretically investigated through solving the two dimensional time‐dependent Schrödinger equation with the Non‐Bohn‐Oppenheimer approximation. It is found that (i) due to the plasmonic enhancement of the laser intensity, the harmonic cutoff can be extended when the spatial position of H2+ is away from the gap center of the nanostructure. However, due to the limit of the gap size, the threshold value of the harmonic cutoff can be obtained at a given position of H2+. (ii) Due to the asymmetric enhancement of the laser intensity in space, the extended higher harmonics are respectively from E(t) > 0 a.u. or E(t) < 0 a.u. for the cases of the positive and the negative spatial position of H2+. As a result, the intensities of the extended higher harmonics are different and can be controlled by changing the carrier‐envelope phase and the pulse duration of the laser field. (iii) In the few‐cycle pulse duration, the MHHG mainly comes from the multi‐photon resonance ionization (MPRI), while as the pulse duration increases, the MPRI, the charge‐resonance enhanced ionization (CREI) and even the dissociative ionization (DI) are contributed to the MHHG. Moreover, as the spatial position of H2+ moves, the contributions of the MHHG from the MPRI, the CERI and the DI can be controlled. (iv) The contributions of the MHHG from the two‐H nuclei have been investigated and found that when E(t) > 0 a.u., the intensities of the harmonics from the negative‐H is higher than those from the positive‐H; while when E(t) < 0 a.u., the intensities of the harmonics from the positive‐H plays the main role in the MHHG. Moreover, the multi‐minima, caused by the two‐center interference can also be found. (v) Finally, by superposing a properly selected harmonics, a single isolated attosecond pulse (SIAP) with the full width at half maximum (FWHM) of 34 as can be obtained.

High-Order Harmonic Extension and Generation of Single Isolated Attosecond Pulse in Hydrogen Gas by Using Plasmonic Field Enhancement

High-Order Harmonic Extension and Generation of Single Isolated Attosecond Pulse in Hydrogen Gas by Using Plasmonic Field Enhancement

Single isolated attosecond pulse generated by helium atom exposed to the two laser pulses with the same color and midinfrared intense laser pulse in the plasmon

We have investigated the characteristics of high harmonic and attosecond pulse generated by helium atom exposed to the combined field of two laser pulses with the same color and a midinfrared laser pulse in nano-structure plasmon by solving the one-dimensional time-dependent Schrödinger equation. It is shown that the cut-off position of the harmonics is obviously extended in the combined field, however, the plasmon structure determined effect of absorbing the electron in the continuum states has an important influence on the cut-off position of the harmonics. It is found that the contribution from some single trajectory of electron is enhanced and other trajectories are suppressed by changing the plasmon position, so a single isolated attosecond pulse is obtained. Comparison with the scenario of atoms driven in homogeneous field, the width of the pulse decreases greatly, and an isolated pulse is achieved to be as short as 28 attoseconds.

New routes towards even shorter attosecond soft X‐ray pulses

New routes towards even shorter attosecond soft X‐ray pulses

Tailoring a 67 attosecond pulse through advantageous phase-mismatch

A single isolated attosecond pulse of 67 as was composed from an extreme UV supercontinuum covering 55-130 eV generated by the double optical gating technique. Phase mismatch was used to exclude the single-atom cutoff of the spectrum that possesses unfavorable attochirp, allowing the positive attochirp of the remaining spectrum to be compensated by the negative dispersion of a zirconium foil. Two algorithms, PROOF and FROG-CRAB, were employed to retrieve the pulse from the experimental spectrogram, yielding nearly identical results.

Generation of a broadband xuv continuum in high-order-harmonic generation by spatially inhomogeneous fields

We address an efficient scheme to generate a broadband extreme-ultraviolet (xuv) continuum from high-order harmonic generation emerging from the concept of plasmonic field enhancement in the vicinity of metallic nanostructures [Kim et al., Nature (London) 453, 757 (2008)]. Based on the numerical solution of a time-dependent Schr\"odinger equation, for moderate field intensities and depending on the inhomogeneity of the field, we are able to increase the plateau region roughly by a factor of two and generate a broadband xuv continuum. The underlying physics of the plasmon enhancement in harmonic generation is investigated in terms of the semiclassical trajectories of strong field-electron dynamics, and perfect consistency is found between quantum mechanical simulations. It is found that the field inhomogeneity plays a critical role in quantum path selection. After a critical value, we observe a systematic suppression in the long trajectories, suggesting the generation of a single isolated attosecond pulse. Finally, we investigate the dependence of cutoff position on the order of field inhomogeneity and find a ${\ensuremath{\beta}}^{2.3\ensuremath{\mp}0.2}$ scaling.

A single isolated sub-50 attosecond pulse generation with a two-color laser field by a frequency-chirping technique

We discuss the possibility of using the frequency-chirping technique to shorten the duration of the generated single attosecond pulse (SAP) by a two-color laser field of 800 and 1600nm with few-cycle pulses. By adopting various combinations of the two frequency-chirped laser fields in our numerical simulation of ionizing He atom, we demonstrate that the best possible condition to obtain the shortest SAP is using the same chirping in both the fundamental and the half-harmonic laser fields without any phase effect and any delay time. There is a maximum increment of about 40eV in the bandwidth of the XUV super-continuum in the cutoff (the second plateau) region. A single isolated attosecond pulse of 48 as can be generated that is further reduced to 9.7 as by phase compensation.

Isolated Attosecond Pulse Generation without the Need to Stabilize the Carrier-Envelope Phase of Driving Lasers

Single isolated attosecond pulses can be extracted from a pulse train with an ultrafast gate in the generation target. By setting the gate width sufficiently narrow with the generalized double optical gating, we demonstrate that single isolated attosecond pulses can be generated with any arbitrary carrier-envelope phase value of the driving laser. The carrier-envelope phase only affects the photon flux, not the pulse duration or contrast. Our results show that isolated attosecond pulses can be generated using carrier-envelope phase unstabilized 23 fs pulses directly from chirped pulse amplifiers.

Isolated attosecond pulse generation using multicycle pulses directly from a laser amplifier

Using a generalized version of double optical gating, we produced single isolated attosecond pulses with 2-mJ, 25-fs driving lasers. Temporal characterization revealed that the 160-as pulses are accompanied with very weak pre- and postpulses. The dependence of the extreme-ultraviolet spectrum on the carrier-envelope phase of the 25-fs laser exhibited a unique $2\ensuremath{\pi}$ periodicity, indicating the robustness of the subcycle gating.

Generation of an intense single isolated attosecond pulse by use of two-colour waveform control

We theoretically demonstrate the generation of an intense single attosecond pulse by superposing a weak sub-harmonic pulse upon a sine-waveform few-cycle driving pulse. By use of a sine-waveform few-cycle pulse instead of its traditionally used cosine waveform counterpart, we show that efficient tunnel ionization for generating electrons which can revisit their parent ion with high kinetic energy can occur only once in the few-cycle laser field, leading to an increase of efficiency by nearly two orders of magnitude in single attosecond pulse generation as compared with the use of a cosine-waveform field.

Retrieval of satellite pulses of single isolated attosecond pulses

When isolated attosecond pulses are reconstructed from an ideal streaked photoelectron spectrogram, the relative amplitude of accompanying satellite pulses can be identified from their interference. However, the interference pattern in the spectrogram can be smeared by variation of the streaking laser intensity in the focal volume and by interpolation when using large delay steps acquiring the spectrogram, making accurate measurement of the relative intensity of the satellite pulses difficult. We investigate these effects on the reconstruction of satellite pulses with full- and half-cycle separations. Our simulations show that satellite pulses with full-cycle separation are largely unaffected by these issues.

Single isolated attosecond pulse from multicycle lasers

We propose a new experimental scheme to produce clean isolated pulses lasting a few hundreds of attoseconds. It is based on high harmonics generation and uses the polarization gating technique combined with the ionization dynamics and the spatial filtering provided by the three-dimensional field propagation. The proposed method is easy to implement, robust against laser parameter fluctuations, and shows to be effective up to a 25 fs pulse duration.

Effects of laser pulse duration on extreme ultraviolet spectra from double optical gating

Previously a two-color field was combined with a polarization gating to allow for the generation of single isolated attosecond pulses from multicycle lasers. Here, the scaling of energy for the extreme ultraviolet pulses corresponding to single attosecond pulses as a function of input laser pulse duration was investigated for argon, neon, and helium gas. Laser pulses as long as 12 fs were able to generate extreme ultraviolet supercontinua with high photon flux. The spectra profile depended strongly on the carrier envelope phase of the pump laser.

Optimizing the photon flux of double optical gated high-order harmonic spectra

The combination of polarization gating and two-color gating is a promising approach to generate single isolated attosecond pulses with multicycle pump lasers. We searched the phase-matching conditions for high-order harmonic generation with such a double optical gating using $0.85\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}$, $8\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ laser pulses centered at $790\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. At the optimized gas target location and pressure, the single extreme ultraviolet pulse energy is $6.5\phantom{\rule{0.3em}{0ex}}\mathrm{nJ}$ for argon and 170 pJ for neon, which are much higher then those generated with conventional polarization gating. In addition, the carrier-envelope phase effects with a $2\ensuremath{\pi}$ periodicity were attained when the highest extreme ultraviolet photon flux was produced, indicating the robustness of the double optical gating.

Tunable high-order harmonic generation and the role of the folded quantum path

We theoretically demonstrate the selective enhancement of high-order harmonic generation (HHG) in two-color laser fields consisting of a single-cycle fundamental wave ($800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ wavelength) and a multicycle subharmonic wave ($2400\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ wavelength). By performing time-frequency analyses based on a single-active-electron model, we reveal that such an enhancement is a result of the modified electron trajectories in the two-color field. Furthermore, we show that selectively enhanced HHG gives rise to a bandwidth-controllable extreme ultraviolet supercontinuum in the plateau region, facilitating the generation of intense single isolated attosecond pulses.

A low-loss, robust setup for double optical gating of high harmonic generation

Previously, a second harmonic field was added to a polarization gating field by a Mach–Zehnder interferometer to gate the high harmonic generation process in argon gas. To reduce the losses of the interferometer, we developed a collinear setup consisting of only two quartz plates and a barium borate crystal. The high intensity at the focus allowed the double optical gating to be performed on neon gas. As a result, a supercontinuous spectrum was produced capable of supporting 130as. There is no delay jitter between the second harmonic field and the polarization gating field associated in this setup, which is necessary for generating stable single isolated attosecond pulses.

Theoretical investigation of single attosecond pulse generation in an orthogonally polarized two-color laser field

We theoretically demonstrate the generation of extreme ultraviolet supercontinua in an orthogonally polarized two-color few-cycle laser field. We show that the ionized electrons can be driven back to their parent ion after traveling along curved trajectories in a plane perpendicular to the beam propagation direction, giving rise to a train of attosecond pulses at different polarization angles. A single isolated attosecond pulse can be obtained by blocking the low-order high harmonics, which contribute to the formation of the satellite pulses.

Controlling attosecond pulse generation with a double optical gating

Numerical simulations have revealed that by adding a second harmonic field to a laser field with a time-dependent ellipticity, single isolated attosecond pulses can be generated as a result of the combined power of the two-color gating and the polarization gating. The duration of the pump laser applicable to this double optical gating scheme is a factor of 2 longer than that for the conventional polarization gating. Pulses with 200 attosecond duration can be generated from argon gas even with $10\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ lasers. It was discovered that the number of attosecond pulses and their intensities could be controlled by either the relative phase between the two color fields or the carrier-envelope phase.