Attosecond Extreme Ultraviolet Pulses Research Articles

Control of Attosecond Entanglement and Coherence.

Calculations are presented of vibrational wave packet dynamics in H_{2}^{+} ions formed by ionization of neutral H_{2} by a pair of attosecond extreme ultraviolet laser pulses, using time-delayed dissociation of the cation by an ultraviolet probe pulse. The strength of experimentally observable two-level quantum beats as a function of the attosecond two-pulse delay can be related to ion+photoelectron entanglement resulting from the ionization process. This conclusion is supported by an evaluation of the purity of the reduced ion and photoelectron density matrices.

Method for high precision measurement of decaying dynamics using attosecond wave-mixing spectroscopy.

Ultrafast wave-mixing spectroscopies involving extreme ultraviolet (EUV) attosecond pulses provide unprecedented insight into electronic dynamics. Here, we proposed a versatile lifetime-detection method for doubly excited states with odd or even parities by mixing an attosecond EUV pulse with two few-cycle near infrared (NIR) pulses in atomic helium under a noncollinear geometry. By properly choosing the time order of the pulse sequence, the spatially resolved nonlinear signals carry significant information of the decaying dynamics of excited states, which can be utilized to retrieve the lifetimes of states with different parities in a single measurement. The validity and robustness of the method has been verified by numerical simulations based on a few-level model of helium including the spatial distribution of atoms. The accuracy of the lifetime measurement method is better than a few hundred attoseconds. It provides a powerful tool for probing decaying dynamics of the electronic wave packet with superb resolution.

The Coulomb effects in laser-assisted single photoionization

Attosecond photoionization time delays reveal information about an outgoing electron getting away from the Coulomb potential. In this study, we improve the eikonal approximation model that overestimates the laser-Coulomb coupling effect in the laser-assisted photoionization of atoms. The improved eikonal approximation model can be accurate on about the 2 asec level relative to the time-dependent Schrödinger equation results. The improvement of photoionization time delays demonstrates that the electrons ionized by extreme ultraviolet attosecond pulse possesses momentum larger than the asymptotical momentum due to the attractive Coulomb potential of the parent ion. These results will give a deeper insight into understanding the laser-assisted single photoionization.

极紫外阿秒脉冲的色散控制及束线设计

极紫外阿秒脉冲的色散控制及束线设计

The effects of plasma density-gradient on laser-driven transmitted emission

High-order harmonic generation provides an opportunity for generating intense extreme ultraviolet attosecond pulses, which is useful to explore the property of materials. Transmitted radiation emitted by an ultrathin target illuminated by intense lasers is a promising way to generate such pulses. We show that the preplasma density gradient has an important influence on the transmitted radiation. An optimal scale length exists to support ultrathin nanobunches to generate coherent synchrotron emission and thus enormously enhance the radiation. Particle in cell simulations suggest that the optimal scale length depends on the laser intensity.

Near-circularly-polarized attosecond pulse generation from carbon monoxide molecules with a combination of linearly and circularly polarized fields

We investigate the polarization properties of high harmonics generated from oriented carbon monoxide molecules driven by a linearly polarized laser field combined with a weaker circularly polarized high-frequency assistant laser field (LPCP). The results show that the harmonics with higher ellipticities are obtained close to the cutoff region. Further analysis shows that the results are due to the manipulation of the electron trajectory by the LPCP field and the molecular asymmetric structure. By taking advantage of the properties above, we propose and theoretically demonstrate a method to generate attosecond extreme ultraviolet pulses with large ellipticity. Because of the difference of harmonic ellipticities contributed from the long and short trajectories, an even higher harmonic ellipticity can be obtained by filtering the long trajectory. In addition, by adjusting the relative phase between linearly polarized and circularly polarized fields, the attosecond XUV pulse varies from being left-elliptically polarized to right-elliptically polarized.

Self-heterodyned detection of dressed state coherences in helium by noncollinear extreme ultraviolet wave mixing with attosecond pulses

Noncollinear wave-mixing spectroscopies with attosecond extreme ultraviolet (XUV) pulses provide unprecedented insight into electronic dynamics. In infrared and visible regimes, heterodyne detection techniques utilize a reference field to amplify wave-mixing signals while simultaneously allowing for phase-sensitive measurements. Here, we implement a self-heterodyned detection scheme in noncollinear wave-mixing measurements with a short attosecond XUV pulse train and two few-cycle near infrared (NIR) pulses. The initial spatiotemporally overlapped XUV and NIR pulses generate a coherence of both odd (1snp) and even (1sns and 1snd) parity states within gaseous helium. A variably delayed noncollinear NIR pulse generates angularly-dependent four-wave mixing signals that report on the evolution of this coherence. The diffuse angular structure of the XUV harmonics underlying these emission signals is used as a reference field for heterodyne detection, leading to cycle oscillations in the transient wave-mixing spectra. With this detection scheme, wave-mixing signals emitting from at least eight distinct light-induced, or dressed, states can be observed, in contrast to only one light induced state identified in a similar homodyne wave-mixing measurement. In conjunction with the self-heterodyned detection scheme, the noncollinear geometry permits the conclusive identification and angular separation of distinct wave-mixing pathways, reducing the complexity of transient spectra. These results demonstrate that the application of heterodyne detection schemes can provide signal amplification and phase-sensitivity, while maintaining the versatility and selectivity of noncollinear attosecond XUV wave-mixing spectroscopies. These techniques will be important tools in the study of ultrafast dynamics within complex chemical systems in the XUV regime.

Generation of an Attosecond Pulse in Helium Excited by Half-Cycle X-Ray Pulses

Conventionally, attosecond pulses are obtained using generation of high-order optical harmonics upon excitation of various media by femtosecond laser pulses with the central frequency in the infrared range. Here, we consider theoretically an alternative possibility of obtaining an isolated extreme ultraviolet (XUV) attosecond pulse in a thin layer of helium atoms excited by a pair of half-cycle (quasi-unipolar) X-ray pulses. The approach is based on the free polarization decay of an atomic medium. An analogy of the processes under consideration with the phenomenon of superradiance is discussed.

High harmonic generation-2ω attosecond stereo-photoionization interferometry in N2

We have investigated photoionization delays in N2 by combining an extreme ultraviolet (XUV) attosecond pulse train generated by high harmonic generation (HHG) and a second harmonic femtosecond pulse with angularly resolved photoelectron spectroscopy. While photoionization delay measurements are usually performed by using a standard XUV-infrared scheme, here we show that the present approach allows us to separate electronic states that otherwise would overlap, thus avoiding the spectral congestion found in most molecules. We have found a relative delay between the X and A ionic molecular states as a function of the photon energy of up to 40 attoseconds, which is due to the presence of a shape resonance in the X channel. This approach can be applied to other small quantum systems with few active electronic states.

Noise effects and the impact of detector responses on the characterization of extreme ultraviolet attosecond pulses.

We employ numerical simulations to study the effects of noise on the reconstruction of the duration and satellite intensity ratio for transform-limited single and double pulses of 200 as duration. The forms of noise we implement are delay jitters between the attosecond pulse and the near-IR laser field, energy resolution of the photoelectron detector, and Poisson noise in streaking spectrograms with different count levels. We use the streaking method to characterize the pulse and the extended ptychographic iterative engine retrieval algorithm to reconstruct the pulse from the simulated streaking spectrogram. We found that, for practical purposes, when implementing a combination of all three mentioned noise contributions, the attosecond pulse duration will be overestimated when the photoelectron count level is low. Furthermore, the satellite pulse amplitude of the attosecond double pulse can be retrieved within 10% accuracy.

Perturbed ac Stark Effect for Attosecond Optical-Waveform Sampling

The temporal structures of ultrashort optical pulses are key to the study of ultrafast phenomena. The authors demonstrate an all-optical method for time-domain characterization of an ultrashort optical pulse. Utilizing nonionizing lasers to induce the ac Stark effect in a helium atom, by interrogating the quasienergies of the laser-dressed atom using extreme-ultraviolet attosecond pulses, the waveform of an optical pulse can be precisely diagnosed. Using a nonionizing laser minimizes plasma-induced pulse distortion, and provides a complementary detection scheme for an effective and reliable ``optical oscilloscope''.

All-optical frequency-resolved optical gating for isolated attosecond pulse reconstruction

We demonstrate an all-optical approach for precise characterization of attosecond extreme ultraviolet pulses. An isolated attosecond pulse is produced from high-order harmonics using an intense driving pulse with a proper gating technique. When a weak field is synchronized with the driver, it perturbs the harmonics generation process via altering the accumulated phase of the electron trajectories. The perturbed harmonic spectrum can be formulated as a convolution of the unperturbed dipole and a phase gate, implying the validity of complete reconstruction of isolated attosecond pulses using a conventional frequency-resolved optical gating method. This in situ measurement avoids the central momentum approximation assumed in the widely used attosecond streaking measurement, providing a simple and reliable metrology for an isolated attosecond pulse.

Attosecond-pulse metrology based on high-order harmonic generation

An all-optical method to retrieve the temporal intensity profile of an extreme ultraviolet (XUV) attosecond pulse is proposed based on XUV-assisted high-order harmonic generation (HHG) by an intense infrared (IR) pulse. For a harmonic located on the XUV-induced high-energy plateau (beyond the IR HHG plateau), the measured harmonic yield as a function of the time delay between the XUV and IR pulses is shown to accurately map the temporal intensity profile of the XUV pulse. Single-color and two-color orthogonal, linearly polarized IR pulses are used to demonstrate the method.

Many-body calculations of two-photon, two color matrix elements for attosecond delays

SynopsisInterferometric measurements based on the combination of extreme ultraviolet (XUV) attosecond pulse trains with weak infrared laser fields, have been successfully used to study time-resolved and recently angle-resolved photoemission in various atomic systems. These measurements rely on quantum interference between electron wavepackets created by absorption of XUV radiation, with further absorption or emission of laser photons. Here we present many-body calculations of the full two-photon process.

Autoionization dynamics of (2P1/2)ns/d states in krypton probed by noncollinear wave mixing with attosecond extreme ultraviolet and few-cycle near infrared pulses.

The autoionization dynamics of the (2P1/2)ns/d Rydberg states in krypton are investigated using spatially isolated wave-mixing signals generated with a short train of subfemtosecond extreme ultraviolet (XUV) pulses and noncollinear, few-cycle near infrared pulses. Despite ubiquitous quantum beat oscillations from XUV-induced coherences within the excited-state manifold, these wave-mixing spectra allow for the simultaneous evaluation of autoionization lifetimes from a series of Rydberg states above the first ionization potential. Experimentally measured lifetimes of 22 ± 8 fs, 33 ± 6 fs, and 49 ± 6 fs for the wave-mixing signals emitting from the (2P1/2)6d/8s, (2P1/2)7d/9s, and (2P1/2)8d/10s resonances compare favorably with lifetimes for the (2P1/2)6d, 7d, and 8d Rydberg states determined from spectral linewidths. Analysis of the quantum beats reveals that the enhancement of wave-mixing pathways that couple the (2P1/2)nd states to themselves leads to individual reporter state-dependent decays in the wave-mixing signals. The results demonstrate the promise of wave-mixing spectroscopies with subfemtosecond XUV pulses to provide valuable insights into processes governed by electronic dynamics.

Multipass-cell-based post-compression of radially and azimuthally polarized pulses to the sub-two-cycle regime

Nonlinear spectral broadening of radially and azimuthally polarized pulses by using the multipass cell technique is proposed and investigated with numerical simulations. The scheme features post-compression of optical pulses from 30 fs down to sub-5 fs centered around 800 nm wavelength with negligible beam distortion and high spectral broadening homogeneity. The performance of the multipass cell is studied with solid-state and gaseous media at μJ and mJ energy levels, respectively. The simulations show that dispersion management in the cell is crucial to obtain sub-two-cycle pulses, while dispersive coatings are considered. It is also shown that multipass cells with silver mirrors provide an alternative option that eases the requirement for dispersion control with an acceptable sacrifice on the net transmission. The sub-two-cycle radially/azimuthally polarized laser pulses are of great interest for generation of radially/azimuthally polarized isolated attosecond extreme ultraviolet pulses and for novel schemes of electron acceleration.

Charge migration in photo-ionized aromatic amino acids.

Attosecond pump-probe spectroscopy is a unique tool for the direct observation of the light-activated electronic motion in molecules and it offers the possibility to capture the first instants of a chemical reaction. Recently, advances in attosecond technology allowed the charge migration processes to be revealed in biochemically relevant molecules. Although this purely electronic process might be key for a future chemistry at the electron time scale, the influence of this ultrafast charge flow on the reactivity of a molecule is still debated. In this work, we exploit extreme ultraviolet attosecond pulses to activate charge migration in two aromatic amino acids, namely phenylalanine and tryptophan. Advanced numerical calculations are performed to interpret the experimental data and to discuss the effects of the nuclear dynamics on the activated quantum coherences. By comparing the experimental results obtained in the two molecules, we show that the presence of different functional groups strongly affects the fragmentation pathways, as well as the charge rearrangement. The observed charge dynamics indeed present peculiar aspects, including characteristic periodicities and decoherence times. Numerical results indicate that, even for a very large molecule such as tryptophan, the quantum coherences can survive the nuclear dynamics for several femtoseconds. These results open new and important perspectives for a deeper understanding of the photo-induced charge dynamics, as a promising tool to control the reactivity of bio-relevant molecules via photo-excitation. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

Interferometric attosecond lock-in measurement of extreme-ultraviolet circular dichroism

Probing the vectorial properties of light–matter interactions inherently requires control over the polarization state of light. The generation of extreme-ultraviolet attosecond pulses has opened new perspectives in measurements of chiral phenomena. However, limited polarization control in this regime prevents the development of advanced vectorial measurement schemes. Here, we establish an extreme-ultraviolet lock-in detection scheme, allowing the isolation and amplification of extremely weak chiral signals, by achieving dynamical polarization control. We demonstrate a time-domain approach to control and modulate the polarization state, and perform its characterization via an in situ measurement. Our approach is based on the collinear superposition of two independent, phase-locked, orthogonally polarized extreme-ultraviolet sources and the control of their relative delay with sub-cycle accuracy. We achieve lock-in detection of magnetic circular dichroism, transferring weak amplitude variations into a phase modulation. This approach holds the potential to significantly extend the scope of vectorial measurements to the attosecond and nanometre frontiers. A time-domain approach that can continuously tune the polarization state of extreme-ultraviolet attosecond pulses allows the isolation and amplification of extremely weak chiral signals, extending vectorial measurements to the attosecond and nanometre scales.

High-flux ultrafast extreme-ultraviolet photoemission spectroscopy at 18.4\u2009MHz pulse repetition rate

Laser-dressed photoelectron spectroscopy, employing extreme-ultraviolet attosecond pulses obtained by femtosecond-laser-driven high-order harmonic generation, grants access to atomic-scale electron dynamics. Limited by space charge effects determining the admissible number of photoelectrons ejected during each laser pulse, multidimensional (i.e. spatially or angle-resolved) attosecond photoelectron spectroscopy of solids and nanostructures requires high-photon-energy, broadband high harmonic sources operating at high repetition rates. Here, we present a high-conversion-efficiency, 18.4-MHz-repetition-rate cavity-enhanced high harmonic source emitting 5 × 105 photons per pulse in the 25-to-60-eV range, releasing 1 × 1010 photoelectrons per second from a 10-µm-diameter spot on tungsten, at space charge distortions of only a few tens of meV. Broadband, time-of-flight photoelectron detection with nearly 100% temporal duty cycle evidences a count rate improvement between two and three orders of magnitude over state-of-the-art attosecond photoelectron spectroscopy experiments under identical space charge conditions. The measurement time reduction and the photon energy scalability render this technology viable for next-generation, high-repetition-rate, multidimensional attosecond metrology.

Electron correlation\xa0driven non-adiabatic relaxation in molecules excited by an ultrashort extreme ultraviolet pulse

The many-body quantum nature of molecules determines their static and dynamic properties, but remains the main obstacle in their accurate description. Ultrashort extreme ultraviolet pulses offer a means to reveal molecular dynamics at ultrashort timescales. Here, we report the use of time-resolved electron-momentum imaging combined with extreme ultraviolet attosecond pulses to study highly excited organic molecules. We measure relaxation timescales that increase with the state energy. High-level quantum calculations show these dynamics are intrinsic to the time-dependent many-body molecular wavefunction, in which multi-electronic and non-Born−Oppenheimer effects are fully entangled. Hints of coherent vibronic dynamics, which persist despite the molecular complexity and high-energy excitation, are also observed. These results offer opportunities to understand the molecular dynamics of highly excited species involved in radiation damage and astrochemistry, and the role of quantum mechanical effects in these contexts.