Special Issue "Attosecond Science and Technology: Principles and Applications"

A special issue of Applied Sciences (ISSN 2076-3417). This special issue belongs to the section "Optics and Lasers".

Deadline for manuscript submissions: closed (30 April 2019).

Special Issue Editors

Prof. Dr. Mauro Nisoli
Website
Guest Editor
Physics of Matter, Politecnico di Milano, Piazza Leonardo da Vinci, 32, 20133 Milano MI, Italy
Interests: ultrafast science; attosecond physics; ultrashort light pulse generation and applications; ultrafast phenomena in the matter
Dr. Matteo Lucchini

Guest Editor
Department of Physics, Politecnico di Milano, Piazza Leonardo da Vinci, 32, 20133 Milano MI, Italy
Interests: attosecond science and technology; optics and photonics

Special Issue Information

Dear Colleagues,

Since the first demonstration of attosecond pulses in 2001, the field of attosecond science and technology has grown exponentially, and new attosecond laboratories have emerged throughout the world. Impressive progress in laser technology (in terms of achievable pulse duration and intensity, wavelength tunability and repetition rate) and the introduction of novel experimental techniques (such as attosecond photoelectron spectroscopy, attosecond all-optical investigation methods, high-order harmonic spectroscopy, etc.) have opened the way to the investigation and control of ultrafast electron dynamics in atoms, molecule, and solids.

This Special Issue aims to analyze recent developments and future trends in “Attosecond Science and Technology”. Topics of interest include, but are not limited to, the following areas:

  • Advanced laser technology for attosecond science
  • High-order harmonic generation in gases and solids
  • Attosecond pulse generation and characterization
  • Attosecond measurement techniques
  • Ultrafast phenomena on attosecond/few-femtosecond timescales in atoms, molecules, nanostructures and condensed phase
  • New sources of ultrafast XUV and X-rays
Prof. Dr. Mauro Nisoli
Dr. Matteo Lucchini
Guest Editors

Manuscript Submission Information

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Keywords

  • Advanced laser technology for attosecond science 
  • High-order harmonic generation in gases and solids 
  • Attosecond pulse generation and characterization 
  • Attosecond measurement techniques 
  • Ultrafast phenomena on attosecond/few-femtosecond timescales in atoms, molecules, nanostructures and condensed phase 
  • New sources of ultrafast XUV and X-rays

Published Papers (19 papers)

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Research

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Open AccessArticle
Interference-Induced Phenomena in High-Order Harmonic Generation from Bulk Solids
Appl. Sci. 2019, 9(8), 1572; https://doi.org/10.3390/app9081572 - 16 Apr 2019
Abstract
We consider a quantum mechanical model for the high-order harmonic generation in bulk solids. The bandgap is assumed to be considerably larger than the exciting photon energy. Using dipole approximation, the dynamical equations for different initial Bloch states are decoupled in the velocity [...] Read more.
We consider a quantum mechanical model for the high-order harmonic generation in bulk solids. The bandgap is assumed to be considerably larger than the exciting photon energy. Using dipole approximation, the dynamical equations for different initial Bloch states are decoupled in the velocity gauge. Although there is no quantum mechanical interference between the time evolution of different initial states, the complete harmonic radiation results from the interference of fields emitted by all the initial (valence band) states. In particular, the suppression of the even-order harmonics can also be viewed as a consequence of this interference. The number of the observable harmonics (essentially the cutoff) is also determined by interference phenomena. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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Open AccessArticle
Analytical Theory of Attosecond Transient Absorption Spectroscopy of Perturbatively Dressed Systems
Appl. Sci. 2019, 9(7), 1350; https://doi.org/10.3390/app9071350 - 30 Mar 2019
Abstract
A theoretical description of attosecond transient absorption spectroscopy for temporally and spatially overlapping XUV and optical pulses is developed, explaining the signals one can obtain in such an experiment. To this end, we employ a two-stage approach based on perturbation theory, which allows [...] Read more.
A theoretical description of attosecond transient absorption spectroscopy for temporally and spatially overlapping XUV and optical pulses is developed, explaining the signals one can obtain in such an experiment. To this end, we employ a two-stage approach based on perturbation theory, which allows us to give an analytical expression for the transient absorption signal. We focus on the situation in which the attosecond XUV pulse is used to create a coherent superposition of electronic states. As we explain, the resulting dynamics can be detected in the spectrum of the transmitted XUV pulse by manipulating the electronic wave packet using a carrier-envelope-phase-stabilized optical dressing pulse. In addition to coherent electron dynamics triggered by the attosecond pulse, the transmitted XUV spectrum encodes information on electronic states made accessible by the optical dressing pulse. We illustrate these concepts through calculations performed for a few-level model. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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Open AccessArticle
From Symmetry Breaking via Charge Migration to Symmetry Restoration in Electronic Ground and Excited States: Quantum Control on the Attosecond Time Scale
Appl. Sci. 2019, 9(5), 953; https://doi.org/10.3390/app9050953 - 06 Mar 2019
Cited by 4
Abstract
This article starts with an introductory survey of previous work on breaking and restoring the electronic structure symmetry of atoms and molecules by means of two laser pulses. Accordingly, the first pulse breaks the symmetry of the system in its ground state with [...] Read more.
This article starts with an introductory survey of previous work on breaking and restoring the electronic structure symmetry of atoms and molecules by means of two laser pulses. Accordingly, the first pulse breaks the symmetry of the system in its ground state with irreducible representation I R R E P g by exciting it to a superposition of the ground state and an excited state with different I R R E P e . The superposition state is non-stationary, representing charge migration with period T in the sub- to few femtosecond time domains. The second pulse stops charge migration and restores symmetry by de-exciting the superposition state back to the ground state. Here, we present a new strategy for symmetry restoration: The second laser pulse excites the superposition state to the excited state, which has the same symmetry as the ground state, but different I R R E P e . The success depends on perfect time delay between the laser pulses, with precision of few attoseconds. The new strategy is demonstrated by quantum dynamics simulation for an oriented model system, benzene. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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Open AccessArticle
Equivalence of RABBITT and Streaking Delays in Attosecond-Time-Resolved Photoemission Spectroscopy at Solid Surfaces
Appl. Sci. 2019, 9(3), 592; https://doi.org/10.3390/app9030592 - 11 Feb 2019
Abstract
The dynamics of the photoelectric effect in solid-state systems can be investigated via attosecond-time-resolved photoelectron spectroscopy. This article provides a comparison of delay information accessible by the two most important techniques, attosecond streaking spectroscopy and reconstruction of attosecond beating by interference of two-photon [...] Read more.
The dynamics of the photoelectric effect in solid-state systems can be investigated via attosecond-time-resolved photoelectron spectroscopy. This article provides a comparison of delay information accessible by the two most important techniques, attosecond streaking spectroscopy and reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) at solid surfaces, respectively. The analysis is based on simulated time-resolved photoemission spectra obtained by solving the time-dependent Schrödinger equation in a single-active-electron approximation. We show a continuous transition from the few-cycle RABBITT regime to the streaking regime as two special cases of laser-assisted photoemission. The absolute delay times obtained by both methods agree with each other, within the uncertainty limits for kinetic energies >10 eV. Moreover, for kinetic energies >10 eV, both streaking delay time and RABBITT delay time coincide with the classical time of flight for an electron propagating from the emitter atom to the bulk-vacuum interface, with only small deviations of less than 4 as due to quantum mechanical interference effects. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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Open AccessArticle
Attosecond Streaking Time Delays: Finite-Range Interpretation and Applications
Appl. Sci. 2019, 9(3), 492; https://doi.org/10.3390/app9030492 - 31 Jan 2019
Cited by 1
Abstract
We review theoretical studies of the attosecond streaking time delay concept in photoionization via the investigation of the electron dynamics in the streaking field after the transition of the photoelectron into the continuum upon absorption of an extreme ultraviolet photon. Based on the [...] Read more.
We review theoretical studies of the attosecond streaking time delay concept in photoionization via the investigation of the electron dynamics in the streaking field after the transition of the photoelectron into the continuum upon absorption of an extreme ultraviolet photon. Based on the results, a so-called finite range interpretation was introduced, which highlighted that the delay is accumulated until the streaking pulse ends and, hence, over a finite range of the potential of the parent ion. Following a discussion of the analysis leading to this interpretation, we summarize a few applications which provide insights into different aspects of the streaking time delay concept in photoionization. Besides a review of previously presented results, we give an analysis of the relevance of the first half-cycle of the streaking field and an outlook regarding the perspective of using the streaking method to resolve dynamical changes in the potential that the photoelectron explores during its propagation in the continuum. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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Open AccessArticle
The Role of Electron Trajectories in XUV-Initiated High-Harmonic Generation
Appl. Sci. 2019, 9(3), 378; https://doi.org/10.3390/app9030378 - 22 Jan 2019
Cited by 2
Abstract
High-harmonic generation spectroscopy is a powerful tool for ultrafast spectroscopy with intrinsic attosecond time resolution. Its major limitation—the fact that a strong infrared driving pulse is governing the entire generation process—is lifted by extreme ultraviolet (XUV)-initiated high-harmonic generation (HHG). Tunneling ionization is replaced [...] Read more.
High-harmonic generation spectroscopy is a powerful tool for ultrafast spectroscopy with intrinsic attosecond time resolution. Its major limitation—the fact that a strong infrared driving pulse is governing the entire generation process—is lifted by extreme ultraviolet (XUV)-initiated high-harmonic generation (HHG). Tunneling ionization is replaced by XUV photoionization, which decouples ionization from recollision. Here we probe the intensity dependence of XUV-initiated HHG and observe strong spectral frequency shifts of the high harmonics. We are able to tune the shift by controlling the instantaneous intensity of the infrared field. We directly access the reciprocal intensity parameter associated with the electron trajectories and identify short and long trajectories. Our findings are supported and analyzed by ab initio calculations and a semiclassical trajectory model. The ability to isolate and control long trajectories in XUV-initiated HHG increases the range of the intrinsic attosecond clock for spectroscopic applications. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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Open AccessArticle
Direct Classic Route of Generating Mono-Color EM-Pulse with Attosecond-Level Duration
Appl. Sci. 2019, 9(3), 362; https://doi.org/10.3390/app9030362 - 22 Jan 2019
Cited by 2
Abstract
Current conception of attosecond pulse is based on Fourier optics and refers to an electromagnetic pulse with a broad, homogeneous weight Fourier spectrum. Its preparation/generation is along an indirect route in which the output of commercial available μm-level wavelength laser is “processed” [...] Read more.
Current conception of attosecond pulse is based on Fourier optics and refers to an electromagnetic pulse with a broad, homogeneous weight Fourier spectrum. Its preparation/generation is along an indirect route in which the output of commercial available μ m-level wavelength laser is “processed” by elaborately designed optics medium allowing high-order harmonics effect to change its Fourier spectrum to be of a flat high-frequency tail. Such an indirect, quantum scheme is limited by its efficiency in high-order harmonics generation. For higher efficiency, other routes for the same goal, i.e., light pulse with an attosecond-level duration, deserve to be tried. The method proposed is a direct, classic scheme. It is to directly control the time duration of classic electrons doing acceleration/deceleration in a feasible, elaborately-designed driving DC fields configuration. The duration can be adjusted by initial electrons velocity, geometric dimension of driving field configuration. The maximum strength of a generated pulse is controlled by the number of electrons. The frequency of a generated pulse is controlled by initial electrons position in the configuration. The shortest duration of single pulse can be down to sub-attosecond-level according to currently available minimum geometric dimension of driving field and suitable gesture of electrons entering into the driving field configuration. This work displays a feasible, direct, classic route of achieving EM pulse with an attosecond-level duration. In particular, the pulse is mono-color, rather than a superposition of Fourier components with nearly-equal weight. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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Open AccessArticle
Enhancement of High-Order Harmonic Generation due to the Large Gradient of the Electric Field Amplitude
Appl. Sci. 2019, 9(2), 282; https://doi.org/10.3390/app9020282 - 14 Jan 2019
Cited by 1
Abstract
The influence of the waveform of circularly polarized laser field on high-order harmonic (HH) generation from atoms is investigated by solving the time-dependent Schrödinger equation (TDSE) and by classical trajectory analysis, without assuming an initial transverse velocity. Both the HH simulation and the [...] Read more.
The influence of the waveform of circularly polarized laser field on high-order harmonic (HH) generation from atoms is investigated by solving the time-dependent Schrödinger equation (TDSE) and by classical trajectory analysis, without assuming an initial transverse velocity. Both the HH simulation and the classical trajectory calculation demonstrate that the positive temporal gradient of the electric field amplitude is a key factor that makes the electron return to the parent ion possible. Moreover, the larger the temporal gradient of the field amplitude is, the more the electron trajectories will revisit the parent ion. Correspondingly, the enhancement of HH is observed. This is confirmed by the pulse-duration dependence of the harmonic yield driven by a circularly polarized laser field. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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Open AccessArticle
Refined Ptychographic Reconstruction of Attosecond Pulses
Appl. Sci. 2018, 8(12), 2563; https://doi.org/10.3390/app8122563 - 10 Dec 2018
Cited by 3
Abstract
Advanced applications of attosecond pulses require the implementation of experimental techniques for a complete and accurate characterization of the pulse temporal characteristics. The method of choice is the frequency resolved optical gating for the complete reconstruction of attosecond bursts (FROG-CRAB), which requires the [...] Read more.
Advanced applications of attosecond pulses require the implementation of experimental techniques for a complete and accurate characterization of the pulse temporal characteristics. The method of choice is the frequency resolved optical gating for the complete reconstruction of attosecond bursts (FROG-CRAB), which requires the development of suitable reconstruction algorithms. In the last few years, various numerical techniques have been proposed and implemented, characterized by different levels of accuracy, robustness, and computational load. Many of them are based on the central momentum approximation (CMA), which may pose severe limits in the reconstruction accuracy. Alternative techniques have been successfully developed, based on the implementation of reconstruction algorithms which do not rely on this approximation, such as the Volkov-transform generalized projection algorithm (VTGPA). The main drawback is a notable increase of the computational load. We propose a new method, called refined iterative ptychographic engine (rePIE), which combines the advantages of a robust algorithm based on CMA, characterized by a fast convergence, with the accuracy of advanced algorithms not based on such approximation. The main idea is to perform a first fast iterative ptychographic engine (ePIE) reconstruction and then refine the result with just a few iterations of the VTGPA in order to correct for the error introduced by the CMA. We analyse the accuracy of the novel reconstruction method by comparing the residual error (i.e., the difference between the reconstructed and the simulated original spectrograms) when VTGPA, ePIE, and rePIE reconstructions are employed. We show that the rePIE approach is particularly useful in the case of short attosecond pulses characterized by a broad spectrum in the vacuum-ultraviolet (VUV)–extreme-ultraviolet (XUV) region. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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Open AccessArticle
Towards GW-Scale Isolated Attosecond Pulse Far beyond Carbon K-Edge Driven by Mid-Infrared Waveform Synthesizer
Appl. Sci. 2018, 8(12), 2451; https://doi.org/10.3390/app8122451 - 01 Dec 2018
Cited by 4
Abstract
We discuss the efficient generation of intense “water window” (0.28–0.54 keV) isolated attosecond pulses (IAPs) using a mid-infrared (MIR) waveform synthesizer. Our numerical simulations clearly indicate that not only a longer-wavelength driving laser but also a weak control pulse in the waveform synthesizer [...] Read more.
We discuss the efficient generation of intense “water window” (0.28–0.54 keV) isolated attosecond pulses (IAPs) using a mid-infrared (MIR) waveform synthesizer. Our numerical simulations clearly indicate that not only a longer-wavelength driving laser but also a weak control pulse in the waveform synthesizer helps extend the continuum cutoff region and reduce the temporal chirp of IAPs in high-order harmonic generation (HHG). This insight indicates that a single-cycle laser field is not an optimum waveform for generating the shortest IAP from the veiwpoints of reducing the attochirp and increasing the efficiency of HHG. By combining a waveform synthesizer technology and a 100 mJ MIR femtosecond pulse based on a dual-chirped optical parametric amplification (DC-OPA) method, a gigawatt-scale IAP (55 as with 10 nJ order) in the water window region can be generated even without attochirp compensation. The MIR waveform synthesizer is highly beneficial for generating a shorter IAP duration in the soft X-ray region because there are no suitable transparent dispersive materials that can be used for compressing the attochirp. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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Open AccessArticle
Artificial Neural Network Trained to Predict High-Harmonic Flux
Appl. Sci. 2018, 8(11), 2106; https://doi.org/10.3390/app8112106 - 01 Nov 2018
Cited by 1
Abstract
In this work we present the results obtained with an artificial neural network (ANN) which we trained to predict the expected output of high-order harmonic generation (HHG) process, while exploring a multi-dimensional parameter space. We argue on the utility and efficiency of the [...] Read more.
In this work we present the results obtained with an artificial neural network (ANN) which we trained to predict the expected output of high-order harmonic generation (HHG) process, while exploring a multi-dimensional parameter space. We argue on the utility and efficiency of the ANN model and demonstrate its ability to predict the outcome of HHG simulations. In this case study we present the results for a loose focusing HHG beamline, where the changing parameters are: the laser pulse energy, gas pressure, gas cell position relative to focus and medium length. The physical quantity which we predict here using ANN is directly related to the total harmonic yield in a specified spectral domain (20–40 eV). We discuss the versatility and adaptability of the presented method. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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Open AccessArticle
Ab Initio Simulation of Attosecond Transient Absorption Spectroscopy in Two-Dimensional Materials
Appl. Sci. 2018, 8(10), 1777; https://doi.org/10.3390/app8101777 - 30 Sep 2018
Cited by 5
Abstract
We extend the first-principles analysis of attosecond transient absorption spectroscopy to two-dimensional materials. As an example of two-dimensional materials, we apply the analysis to monolayer hexagonal boron nitride (h-BN) and compute its transient optical properties under intense few-cycle infrared laser pulses. [...] Read more.
We extend the first-principles analysis of attosecond transient absorption spectroscopy to two-dimensional materials. As an example of two-dimensional materials, we apply the analysis to monolayer hexagonal boron nitride (h-BN) and compute its transient optical properties under intense few-cycle infrared laser pulses. Nonadiabatic features are observed in the computed transient absorption spectra. To elucidate the microscopic origin of these features, we analyze the electronic structure of h-BN with density functional theory and investigate the dynamics of specific energy bands with a simple two-band model. Finally, we find that laser-induced intraband transitions play a significant role in the transient absorption even for the two-dimensional material and that the nonadiabatic features are induced by the dynamical Franz–Keldysh effect with an anomalous band dispersion. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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Open AccessArticle
Bright High-Order Harmonic Generation around 30 nm Using Hundred-Terawatt-Level Laser System for Seeding Full Coherent XFEL
Appl. Sci. 2018, 8(9), 1446; https://doi.org/10.3390/app8091446 - 24 Aug 2018
Cited by 1
Abstract
In the past few years, the laser wakefield acceleration (LWFA) electron is a hot topic. One of its applications is to produce soft X-ray free-electron laser (XFEL). During this process, high harmonic generation (HHG) is a potential seed. To decrease the timing jitter [...] Read more.
In the past few years, the laser wakefield acceleration (LWFA) electron is a hot topic. One of its applications is to produce soft X-ray free-electron laser (XFEL). During this process, high harmonic generation (HHG) is a potential seed. To decrease the timing jitter between LWFA and HHG, it is better for them to come from the same laser source. We have experimentally investigated bright high-order harmonic generation with a 200-terawatt (TW)/1-Hz Ti: Sapphire laser system. By using the loosely focused method and optimizing the phase-matching conditions, we have obtained bright high-order harmonics around 30 nm. Output energy of the 29th harmonic (27.6 nm) reaches as high as 100 nJ per pulse, and the harmonic beam divergence is estimated to be 0.3 mrad in a full width at half maximum (FWHM). Although the hundred-TW-level laser system has the problems of poor beam quality and shot-to-shot energy fluctuation for HHG, the generated soft X-ray (~30 nm) sources can also have good stability by carefully optimizing the laser system. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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Open AccessArticle
Static Coherent States Method: One- and Two-Electron Laser-Induced Systems with Classical Nuclear Dynamics
Appl. Sci. 2018, 8(8), 1252; https://doi.org/10.3390/app8081252 - 29 Jul 2018
Abstract
In this report, we introduce the static coherent states (SCS) method for investigating quantum electron dynamics in a one- or two-electron laser-induced system. The SCS method solves the time-dependent Schrödinger equation (TDSE) both in imaginary and real times on the basis of a [...] Read more.
In this report, we introduce the static coherent states (SCS) method for investigating quantum electron dynamics in a one- or two-electron laser-induced system. The SCS method solves the time-dependent Schrödinger equation (TDSE) both in imaginary and real times on the basis of a static grid of coherent states (CSs). Moreover, we consider classical dynamics for the nuclei by solving their Newtonian equations of motion. By implementing classical nuclear dynamics, we compute the electronic-state potential energy curves of H2+ in the absence and presence of an ultra-short intense laser field. We used this method to investigate charge migration in H2+. In particular, we found that the charge migration time increased exponentially with inter-nuclear distance. We also observed substantial charge localization for sufficiently long molecular bonds. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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Open AccessArticle
Full Characterization of a Molecular Cooper Minimum Using High-Harmonic Spectroscopy
Appl. Sci. 2018, 8(7), 1129; https://doi.org/10.3390/app8071129 - 12 Jul 2018
Cited by 5
Abstract
High-harmonic generation was used to probe the spectral intensity and phase of the recombination-dipole matrix element of methyl chloride (CH3Cl), revealing a Cooper minimum (CM) analogous to the 3p CM previously reported in argon. The CM structure altered the spectral [...] Read more.
High-harmonic generation was used to probe the spectral intensity and phase of the recombination-dipole matrix element of methyl chloride (CH3Cl), revealing a Cooper minimum (CM) analogous to the 3p CM previously reported in argon. The CM structure altered the spectral response and group delay (GD) of the emitted harmonics, and was revealed only through careful removal of all additional contributors to the GD. In characterizing the GD dispersion, also known as the “attochirp” we additionally present the most complete validation to date of the commonly used strong-field approximation for calculating the GD, demonstrating the correct intensity scaling and extending its usefulness to simple molecules. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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Open AccessArticle
Gauge-Invariant Formulation of Time-Dependent Configuration Interaction Singles Method
Appl. Sci. 2018, 8(3), 433; https://doi.org/10.3390/app8030433 - 13 Mar 2018
Cited by 5
Abstract
We propose a gauge-invariant formulation of the channel orbital-based time-dependent configuration interaction singles (TDCIS) method [Phys. Rev. A, 74, 043420 (2006)], one of the powerful ab initio methods to investigate electron dynamics in atoms and molecules subject to an external laser field. In [...] Read more.
We propose a gauge-invariant formulation of the channel orbital-based time-dependent configuration interaction singles (TDCIS) method [Phys. Rev. A, 74, 043420 (2006)], one of the powerful ab initio methods to investigate electron dynamics in atoms and molecules subject to an external laser field. In the present formulation, we derive the equations of motion (EOMs) in the velocity gauge using gauge-transformed time-dependent, not fixed, orbitals that are equivalent to the conventional EOMs in the length gauge using fixed orbitals. The new velocity-gauge EOMs avoid the use of the length-gauge dipole operator, which diverges at large distance, and allows us to exploit computational advantages of the velocity-gauge treatment over the length-gauge one, e.g., a faster convergence in simulations with intense and long-wavelength lasers, and the feasibility of exterior complex scaling as an absorbing boundary. The reformulated TDCIS method is applied to an exactly solvable model of one-dimensional helium atom in an intense laser field to numerically demonstrate the gauge invariance. We also discuss the consistent method for evaluating the time derivative of an observable, which is relevant, e.g., in simulating high-harmonic generation. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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Review

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Open AccessReview
High-Harmonic and Terahertz Spectroscopy (HATS): Methods and Applications
Appl. Sci. 2019, 9(5), 853; https://doi.org/10.3390/app9050853 - 27 Feb 2019
Cited by 2
Abstract
Electrons driven from atom or molecule by intense dual-color laser fields can coherently radiate high harmonics from extreme ultraviolet to soft X-ray, as well as an intense terahertz (THz) wave from millimeter to sub-millimeter wavelength. The joint measurement of high-harmonic and terahertz spectroscopy [...] Read more.
Electrons driven from atom or molecule by intense dual-color laser fields can coherently radiate high harmonics from extreme ultraviolet to soft X-ray, as well as an intense terahertz (THz) wave from millimeter to sub-millimeter wavelength. The joint measurement of high-harmonic and terahertz spectroscopy (HATS) was established and further developed as a unique tool for monitoring electron dynamics of argon from picoseconds to attoseconds and for studying the molecular structures of nitrogen. More insights on the rescattering process could be gained by correlating the fast and slow electron motions via observing and manipulating the HATS from atoms and molecules. We also propose the potential investigations of HATS of polar molecules, and solid and liquid sources. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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Open AccessReview
Laser-Control of Ultrafast π-Electron Ring Currents in Aromatic Molecules: Roles of Molecular Symmetry and Light Polarization
Appl. Sci. 2018, 8(12), 2347; https://doi.org/10.3390/app8122347 - 22 Nov 2018
Cited by 2
Abstract
Being motivated by the recent progress in attosecond laser technology, we theoretically explore the strategy of inducing ultrafast electron dynamics inherent to aromatic molecules, i.e., ring currents by means of polarized laser pulses. The main topic of discussion is how to control the [...] Read more.
Being motivated by the recent progress in attosecond laser technology, we theoretically explore the strategy of inducing ultrafast electron dynamics inherent to aromatic molecules, i.e., ring currents by means of polarized laser pulses. The main topic of discussion is how to control the direction of ring currents in an aromatic molecule of low symmetry, for which the design of an efficient control pulse cannot be achieved intuitively. We first consider a system with a single aromatic ring and show that coherent π-electron angular momentum, which oscillates with time, can be produced and controlled by a polarized laser pulse with its ellipticity and orientation properly chosen. Nonadiabatic couplings with molecular vibration gradually weaken the angular momentum, while the vibrational amplitude strongly depends on the polarization of incident light. This suggests the conversion of the polarization dependence of ring current into that of subsequent vibration, which may open a way to detect laser-driven ultrafast electron dynamics by vibrational spectroscopy. The laser-control scheme for the ring current is then extended to a molecule with two aromatic rings, which exhibits characteristic phenomena absent in that with a single ring. We demonstrate that two-dimensional switching of the direction of angular momentum is possible in such molecules. In addition, ring current can be localized at a specific ring by tailored lasers. The application of the present control method to polycyclic aromatic hydrocarbons will lead to the development of next-generation organic optical switching devices. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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Open AccessReview
Toward the Generation of an Isolated TW-Attosecond X-ray Pulse in XFEL
Appl. Sci. 2018, 8(9), 1588; https://doi.org/10.3390/app8091588 - 07 Sep 2018
Cited by 2
Abstract
The isolated terawatt (TW) attosecond (as) hard X-ray pulse will expand the scope of ultrafast science, including the examination of phenomena that have not been studied before, such as the dynamics of electron clouds in atoms, single-molecule imaging, and examining the dynamics of [...] Read more.
The isolated terawatt (TW) attosecond (as) hard X-ray pulse will expand the scope of ultrafast science, including the examination of phenomena that have not been studied before, such as the dynamics of electron clouds in atoms, single-molecule imaging, and examining the dynamics of hollow atoms. Therefore, several schemes for the generation of an isolated TW-as X-ray pulse in X-ray free electron laser (XFEL) facilities have been proposed with the manipulation of electron properties such as emittance or current. In a multi-spike scheme, a series of current spikes were employed to amplify the X-ray pulse. A single-spike scheme in which a TW-as X-ray pulse can be generated by a single current spike was investigated for ideal parameters for the XFEL machine. This paper reviews the proposed schemes and assesses the feasibility of each scheme. Full article
(This article belongs to the Special Issue Attosecond Science and Technology: Principles and Applications)
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