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Recent reports on sub‐terahertz (THz) generation from a laser‐irradiated wire might have evaded the most essential contents of the wire radiation dynamics. Here, the origin of terahertz generation from a metal wire is revisited and a comprehensive diagnosis of the terahertz radiation from a 100 µm‐diameter tungsten wire irradiated by an intense femtosecond laser is implemented. For the first time, the long‐neglected but more efficient high‐frequency terahertz radiation is experimentally observed of which the spectra, polarization, tunability, and wire‐length‐dependent intensification are investigated comprehensively. A new picture of the wire radiation dynamics is presented to reveal the origin of the extreme terahertz surface wave, its evolution, and radiation mechanism. This extremely intense and ultrashort half‐cycle surface wave is spontaneously induced by a laser‐driven transient charge‐separation field and evolves into a multi‐cycle surface wakefield structure as it propagates along the wire owing to the self‐interaction between the surface wave and its carrier, the surface current. By manipulating the coupling and transport dynamics of the surface wave on the wire, tunable and intensified THz radiation covering a wide range from 0.1 to 20 THz has been realized, paving the way for broad applications such as terahertz acceleration, bio‐medicine, nonlinear THz science and beyond. [ABSTRACT FROM AUTHOR]

A systematic directionality coherent control of total proton momentum distributions in the dissociative‐ionization of H2+ subjected to a strong field of six‐cycle laser pulses in a full range of carrier‐envelope phase ϑ is studied by solving a non‐Born‐Oppenheimer time‐dependent Schrödinger equation numerically. The trend of distributions involves insightful investigation into the spatial‐temporal overlap between nuclear wave packets evolving on the coupled field‐dressed electronic potentials of H2+ associated with the n‐photon potential crossings. This leads to new quantum images for the nonlinear nonperturbative interaction of H2+ with a strong field. It turns out that the symmetry of the ϑ‐dependent momentum distribution begins to break after undergoing interaction with one‐photon field‐dressed potentials. At this point, the most probable proton momentum distribution tends to move in a forward direction indicating also that the three‐photon field‐dressed potentials strongly govern the dissociative‐ionization pathway. [ABSTRACT FROM AUTHOR]

Progress in high-energy physics has been closely tied to the development of high-performance electromagnetic calorimeters. Recent experiments have demonstrated the possibility to significantly accelerate the development of electromagnetic showers inside scintillating crystals typically used in homogeneous calorimeters based on scintillating crystals when the incident beam is aligned with a crystallographic axis to within a few mrad. In particular, a reduction of the radiation length has been measured when ultrarelativistic electron and photon beams were incident on a high-Z scintillator crystal along one of its main axes. Here, we propose the possibility to exploit this physical effect for the design of a new type of compact e.m. calorimeter, based on oriented ultra-fast lead tungstate (PWO-UF) crystals, with a significant reduction in the depth needed to contain electromagnetic showers produced by high-energy particles with respect to the state-of-the-art. We report results from tests of the crystallographic quality of PWO-UF samples via high-resolution X-ray diffraction and photoelastic analysis. We then describe a proof-of-concept calorimeter geometry defined with a Geant4 model including the shower development in oriented crystals. Finally, we discuss the experimental techniques needed for the realization of a matrix of scintillator crystals oriented along a specific crystallographic direction. Since the angular acceptance for e.m. shower acceleration depends little on the particle energy, while the decrease of the shower length remains pronounced at very high energy, an oriented crystal calorimeter will open the way for applications at the maximum energies achievable in current and future experiments. Such applications span from forward calorimeters, to compact beam dumps for the search for light dark matter, to source-pointing space-borne γ-ray telescopes, to decrease the size and the cost of the calorimeter needed to fully contain e.m. showers initiated by GeV to TeV particles.

We present a joint experimental and theoretical study of non-sequential double ionization (NSDI) in argon driven by a 3100 nm laser source. The correlated photoelectron momentum distribution (PMD) shows a strong dependence on the pulse duration, and the evolution of the PMD can be explained by an envelope-induced intensity effect. Determined by the time difference between tunneling and rescattering, the laser vector potential at the ionization time of the bound electron will be influenced by the pulse duration, leading to different drift momenta. Such a mechanism is extracted through a classical trajectory Monte Carlo-based model and it can be further confirmed by quantum mechanical simulations. This work sheds light on the importance of the pulse duration in NSDI and improves our understanding of the strong field tunnel-recollision dynamics under mid-IR laser fields. [ABSTRACT FROM AUTHOR]

We use a numerically solved time-dependent Schrödinger equation for calculating the photoelectron momentum distribution of ground-state hydrogen atoms in the presence of superintense ultrashort high-frequency pulses. It is demonstrated that the dynamic interference effect within a superintense XUV laser beam has the ability to significantly alter the photoelectron momentum distribution. In our work, a clearly visible dynamic interference pattern is observed when hydrogen atoms are exposed to a superintense circularly polarized laser pulse with a photon energy of ℏω = 53.605 eV, which has previously been found for linearly polarized pulses or the weakly bounded model H− system for circularly polarized pulses. Angular-distorted interference arises for linear superintense XUV pulses of similar intensity. The significant differences in photoelectron momentum distributions that have been seen by linearly and circularly polarized XUV pulses are caused by the Coulomb rescattering phenomenon.

We report on the spatial and spectral variation of high-order harmonics generation from noble gases when driven by high-repetition rate femtosecond laser with different peak intensity and polarization states. Ar, Xe, and Kr were chosen for the generation of coherent extreme ultraviolet radiation. We observe that increasing the intensity of the driving pulses (DPs) leads to disappearance of harmonics yield in the on-axis part of the spatial distribution of the harmonics in Xe target, contrary to Ar and Kr. This observation is attributed to the higher ionization potential of the latter gases. Spectral depletion of harmonics generated in Ar and Xe using radially and azimuthally polarized Gaussianâ€"Laguerre two-color DPs is also observed. The significant depletion in the spectral distribution of harmonics is obtained when two-color pump (TCP) scheme is applied. We demonstrate how TCP scheme with azimuthally polarized DPs can be used as a tool to control the spectral distribution of harmonics generated from gas atoms during strong field by utilizing the presence of second harmonic electromagnetic field. [ABSTRACT FROM AUTHOR]

We present a joint experimental and theoretical study of non-sequential double ionization (NSDI) in argon driven by a 3100 nm laser source. The correlated photoelectron momentum distribution (PMD) shows a strong dependence on the pulse duration, and the evolution of the PMD can be explained by an envelope-induced intensity effect. Determined by the time difference between tunneling and rescattering, the laser vector potential at the ionization time of the bound electron will be influenced by the pulse duration, leading to different drift momenta. Such a mechanism is extracted through a classical trajectory Monte Carlo-based model and it can be further confirmed by quantum mechanical simulations. This work sheds light on the importance of the pulse duration in NSDI and improves our understanding of the strong field tunnel-recollision dynamics under mid-IR laser fields.

We present an efficient and powerful method to optimize the production of high-order harmonic generation (HHG) and the synthesis of an isolated attosecond pulse. The system under investigation is the helium atoms that are exposed to the combination of chirped two-color mid-IR laser field and its 1sâ€"2p transition frequency. An ab initio study is conducted by solving the three dimensional time-dependent Schrödinger equation accurately and efficiently by means of the time-dependent generalized pseudospectral method. We applied Bayesian optimization to obtain an optimal laser waveform to generated a HHG spectrum with enhanced harmonics and extended cutoff frequency. Harmonic phases are analysed by the wavelet transform. We demonstrated that an isolated 10 attosecond pulse can be synthesized using a sequence of consecutive and phase-locked harmonics in the plateau region. [ABSTRACT FROM AUTHOR]

Free‐space super‐strong terahertz (THz) electromagnetic fields offer multifaceted capabilities for reaching extreme nonlinear THz optics. However, the lack of powerful solid‐state THz sources with single pulse energy >1 mJ is impeding the proliferation of extreme THz applications. The fundamental challenge lies in hard to achieve high efficiency due to high intensity pumping caused crystal damage, linear absorption, and nonlinear distortion induced short effective interaction length, and so on. Here, through cryogenically cooling the crystals, tailoring the pump laser spectra, chirping the pump pulses, and magnifying the laser energies, 1.4‐mJ THz pulses are successfully realized in lithium niobates under the excitation of 214‐mJ femtosecond laser pulses via tilted pulse front technique. The 800 nm‐to‐THz energy conversion efficiency reaches 0.7%, and a free‐space THz peak electric and magnetic field reaches 6.3 MV cm−1 and 2.1 Tesla. Numerical simulations reproduce the experimental optimization processes. To show the capability of this super‐strong THz source, nonlinear absorption in high conductive silicon induced by strong THz electric field is demonstrated. Such a high‐energy THz source with a relatively low peak frequency is very appropriate not only for electron acceleration toward table‐top X‐ray sources but also for extreme THz science and nonlinear applications. [ABSTRACT FROM AUTHOR]

The influence of an external strong electrostatic field on the statistical characteristics of equilibrium free charge carriers in a quasi-two-dimensional structure is considered using an example of an InAs semiconductor quantum film. The analysis is carried out for the case when a strong quantization regime for charge carriers is realized in the film and only the first and second film sub-bands of their quantized motion are occupied. The general analytic expressions for the energy of the quantized motion of charge carriers in a film in the presence of a strong electrostatic field are given. An explicit dependence of the width of the band gap of the sample on the value of the external field is obtained. It is shown that with the increase in the external field, the width of band gap of the sample increases. The calculations and corresponding analytical expressions for the chemical potential, concentration, energy and heat capacity of free charge carriers of an InAs quantum film in the presence of a strong electrostatic field are also presented. It is shown that with an increase in the external electrostatic field, the chemical potential of the system retains its negative sign, but increases by the magnitude. For a given value of the external field, the chemical potential of the system increases linearly with the increase in temperature. It is shown that at a given temperature the carriers’ density decreases with the increase in the external field. At a fixed value of the field, the concentration increases with the increase in temperature. The behavior of the energy and heat capacity of the charge carriers is also determined by the same dependence on the temperature of the system and the magnitude of the external field. It is also shown that the main contribution to the indicated characteristics of the sample is made namely by the first filled sub-band of quantized motion of charge carriers. If the second sub-band is also filled, its contribution to the determination of the statistical characteristics of the sample is exponentially smaller than the contribution of the first filled sub-band.

Many predictions of new phenomena given by strong field quantum electrodynamics (SFQED) will be tested on next generation multi-petawatt laser facilities in the near future. These new phenomena are basis to understand physics in extremely strong electromagnetic fields therefore have attracted wide research interest. Here we discuss a new SFQED phenomenon that is named as vacuum radiation. In vacuum radiation, a virtual electron loop obtain energy from time dependent external electric field and radiate an entangled photon pair. Features of vacuum radiation in a locally time dependent electric field including spectrum, characteristic temperature, production rate and power are given.

Interaction of intense lasers with solid materials offers an alternative way to achieve high-order harmonic generation (HHG). Since the underlying mechanisms of the harmonic emission remain uncertain, a large number of theoretical studies have been proposed to explain the experimental findings in this fast-growing field. Here, we review the primary numerical methods and present a brief perspective of HHG in solids.

Abstract The variation of probe absorption with the intensity of coupling and probe field in a cascade three energy-level atomic systems is observed based on the density matrix equation. It shows that in the case of weak probe field, probe absorption presents electromagnetically induced transparency (EIT) near the resonant frequency. The depth of transparent window increases with the increase of the intensity of the coupling field until complete transparent. After that, the width of EIT window increases and changes to Aulter-Townes double peak finally. While with the increase of the probe intensity, the contrast of EIT declines rapidly, and EIT will disappear when Rabi frequency of the probe field exceeds the decay rate of the transverse relaxation. If the coupling field is off-resonance, the double peak of probe absorption is no longer symmetric to the resonant point and the intensity of two peaks is no equal also. With the increase of the probe field, the intensity of two peaks converts each other, and at last forms one wide peak by overlapping together. At room temperature, EIT phenomenon can still be observed if adopting Doppler-free configuration and weak probe field. But if the probe field is strong enough, besides the absorption curve is broadened, a small peak which comes from Doppler effects will appear at the bottom of the EIT window also. Thus if we engage in the research work related to EIT, we ought to adopt Doppler-free configuration and weak probe field. [ABSTRACT FROM AUTHOR]

A mechanism pertaining to an anomalous optical anisotropy under a strong field in semiconductors having cubic structure is proposed. The valence-conduction band transitions of valence electrons located around atomic bonds re-distribute electrons between bands and, consequently, produce symmetry defects in local electronic density. The optical anisotropy is due to these symmetry defects, which are the sources for localized and non-compensated dipole moments randomly distributed within the crystal. The quantum time-dependent probability amplitude method, combined with classical electrodynamics, demonstrate that the non-compensated dipole moments destroy the optical isotropy of a crystal. To compare the scheme with an experiment, the theory is applied to $GaAs$ single crystal. Within a strong-field IR regime, the predicted four-fold symmetry of optical anisotropy, along with its amplitude, correlate well with an experiment.

The trajectory-based method for calculating the probabilities of transitions in the quantum system developed in our previous works and tested for atoms is applied to calculating the photoionization probability for the simplest molecule – hydrogen molecular ion. In a weak field it is established a good agreement between our photoionization cross section and the data obtained by other theoretical methods for photon energy in the range from one-photon ionization threshold up to 25 a.u. Photoionization cross section in the range 25 < ω ≤ 100 a.u. was calculated for the first time judging by the literature known to us. It is also confirmed that the trajectory method works in a wide range of the field magnitudes including superatomic values up to relativistic intensity. [ABSTRACT FROM AUTHOR]

The influence of an external strong electrostatic field on the statistical characteristics of equilibrium free charge carriers in a quasi-two-dimensional structure is considered using an example of an InAs semiconductor quantum film. The analysis is carried out for the case when a strong quantization regime for charge carriers is realized in the film and only the first and second film subbands of their quantized motion are occupied. The general analytic expressions for the energy of the quantized motion of charge carriers in a film in the presence of a strong electrostatic field are given. An explicit dependence of the width of the band gap of the sample on the value of the external field is obtained. It is shown that with the increase in the external field, the width of band gap of the sample increases. The calculations and corresponding analytical expressions for the chemical potential, concentration, energy and heat capacity of free charge carriers of an InAs quantum film in the presence of a strong electrostatic field are also presented. It is shown that with an increase in the external electrostatic field, the chemical potential of the system retains its negative sign, but increases by the magnitude. For a given value of the external field, the chemical potential of the system increases linearly with the increase in temperature. It is shown that at a given temperature the carriers' density decreases with the increase in the external field. At a fixed value of the field, the concentration increases with the increase in temperature. The behavior of the energy and heat capacity of the charge carriers is also determined by the same dependence on the temperature of the system and the magnitude of the external field. It is also shown that the main contribution to the indicated characteristics of the sample is made namely by the first filled sub-band of quantized motion of charge carriers. If the second sub-band is also filled, its contribution to the determination of the statistical characteristics of the sample is exponentially smaller than the contribution of the first filled sub-band. [ABSTRACT FROM AUTHOR]

Electron rescattering has been well studied and simulated for cases with ponderomotive energies of the quasi-free electrons, derived from laser−gas and laser−surface interactions, lower than 50 eV. However, with advents in longer wavelengths and laser field enhancement metallic surfaces, previous simulations no longer suffice to describe more recent strong field and high yield experiments. We present a brief introduction to and some of the theoretical and empirical background of electron rescattering emissions from a metal. We set upon using the Jellium potential with a shielded atomic surface potential to model the metal. We then explore how the electron energy spectra are obtained in the quantum simulation, which is performed using a custom computationally intensive time-dependent Schrödinger equation solver via the Crank−Nicolson method. Finally, we discuss the results of the simulation and examine the effects of the incident laser’s wavelength, peak electric field strength, and field penetration on electron spectra and yields. Future simulations will investigate a more accurate density functional theory metallic model with a system of several non-interacting electrons. Eventually, we will move to a full time-dependent density functional theory approach.

Many predictions of new phenomena given by strong field quantum electrodynamics (SFQED) will be tested on next generation multi-petawatt laser facilities in the near future. These new phenomena are basis to understand physics in extremely strong electromagnetic fields therefore have attracted wide research interest. Here we discuss a new SFQED phenomenon that is named as vacuum radiation. In vacuum radiation, a virtual electron loop obtain energy from time dependent external electric field and radiate an entangled photon pair. Features of vacuum radiation in a locally time dependent electric field including spectrum, characteristic temperature, production rate and power are given. [ABSTRACT FROM AUTHOR]

We study the special features of vacuum particle creation in an external classical field for two simple external field models in standard QED. Our investigation is based on a kinetic equation that is a nonperturbative consequence of the fundamental QED equations of motion. We identify the special features of system evolution that apply qualitatively also for other systems and are therefore rather general. The common basis for a description of these systems is formed by kinetic equations for vacuum particle creation belonging to the class of integro-differential equations of non-Markovian type with fastly oscillating kernel. This allows us to characterize the processes of this type as belonging to the class of field-induced phase transitions. Examples range from condensed matter physics to cosmology. [ABSTRACT FROM AUTHOR]

In recent years the laser-induced interatomic Coulombic decay (ICD) process in paired quantum dots has been predicted (Bande, 2013). In this work we target the enhancement of ICD by scanning over a range of strong-field laser intensities. The GaAs quantum dots are modeled by a one-dimensional double-well potential in which simulations are done with the space-resolved multi-configuration time-dependent Hartree method including antisymmetrization to account for the fermions. As a novelty a complementary state-resolved ansatz is developed to consolidate the interpretation of transient state populations, widths obtained for the ICD and the competing direct ionization channel, and Fano peak profiles in the photoelectron spectra. The major results are that multi-photon processes are unimportant even for the strongest fields. Further, below- π to π pulses display the highest ICD efficiency while the direct ionization becomes less dominant. [ABSTRACT FROM AUTHOR]

We employ an extreme ultraviolet (XUV) pulse to impulsively excite dipole polarization in atoms or molecules, which corresponds to coherently prepared superposition of excited states. A delayed near infrared (NIR) pulse then perturbs the fast evolving polarization, and the resultant absorbance change is monitored in dilute helium, dense helium, and sulfur hexafluoride (SF6) molecules. We observe and quantify the time-dependence of various transient phenomena in helium atoms, including laser-induced phase (LIP), time-varying (AC) Stark shift, quantum path interference, and laser-induced continuum structure. In the case of dense helium targets, we discuss nonlinear macroscopic propagation effects pertaining to LIP and resonant pulse propagation, which account for the appearance of new spectral features in transient lineshapes. We then use tunable NIR photons to demonstrate the wavelength dependence of the transient laser induced effects. In the case of molecular polarization experiment in SF6, we show suppression of XUV photoabsorption corresponding to inter-valence transitions in the presence of a strong NIR field. In each case, the temporal evolution of transient absorption spectra allows us to observe and understand the transient laser induced modifications of the electronic structure of atoms and molecules. [ABSTRACT FROM AUTHOR]

Controlling spatial–temporal overlap between nuclear wave packets propagating on the coupled electronic states with opposite parity by intense laser field leads to a π periodic modulation in the symmetry breaking of the coupled forward–backward electron and proton motions. The most probable directionality of electron and proton moving simultaneously in opposite directions with total electron–proton asymmetry parameter A e − p tot = 0. 62 occurs at fixed carrier-envelope phases of ϑ = 15 ° and 195 °. Analysis of the time evolution of the nuclear and the electronic-nuclear wave packets on the field-dressed potentials gives rise to newly insightful images of the nonlinear nonperturbative processes for the asymmetrical localization. [Display omitted] • Coherent control of electron–proton localization in the dissociative-ionization of H 2 + subjected to strong field six-cycle laser pulses (I = 4 × 10 W/cm , λ = 800 nm) is studied by solving numerically a non-Born–Oppenheimer time-dependent Schrödinger equation. • Controlling spatial–temporal overlap between nuclear wave packets with a full range of the carrier-envelope phase of laser pulse propagating on the coupled electronic states with opposite parity leads to a π periodic modulation in the symmetry breaking of the coupled forward–backward electron and proton motions. • The most probable directionality of electron and proton moving simultaneously in opposite directions with total electron–proton asymmetry parameter A e − p tot = 0. 62 occurs at fixed carrier-envelope phases of ϑ = 15 ° and 195 °. • Furthermore, it is found that the moving direction of the electron-nuclear wave packet is dominated by the electron wave packet by pulling the entire H 2 + molecule to the right or left. • Analysis of the time evolution of the H 2 + nuclear wave packets using Floquet formalism on two lowest field-dressed diabatic potentials combined with the electronic-nuclear wave packets on the field-induced Coulomb potential give rise to newly insightful images of the nonlinear nonperturbative processes for the asymmetrical localization. [ABSTRACT FROM AUTHOR]

A technique for broadband spectral amplitude control of light pulses produced in high-order harmonic generation (HHG) is presented. It has been shown elsewhere that broadband spectral phase control in HHG is achievable using a computerized feedback loop scheme by coherently adding a filtered region of the HHG emission to the intense IR driving pulse with optimal attenuation and time delay parameters. In the present study, further computational evidence of the capabilities of this control scheme is provided by considering the spectral amplitude in a broadband region of the HHG spectrum as the control target for the production of isolated attosecond pulses. Different spectral widths and central photon energies are examined, such as a spectral width of 30 eV centered at 36 eV, well in the plateau, and a width of 20 eV centered at 60 eV in the cutoff region. An iterative procedure of the method is implemented and optimal isolated single cycle pulses at a central photon energy of 36 eV are obtained. This control scheme is a fundamental tool that can be implemented for amplitude and phase shaping of any suitable spectral region in HHG.