The time-dependent complete-active-space self-consistent-field (TD-CASSCF) method for the description of multielectron dynamics in intense laser fields is presented, and a comprehensive description of the method is given. It introduces the concept of frozen-core (to model tightly bound electrons with no response to the field), dynamical-core (to model electrons tightly bound but responding to the field), and active (fully correlated to describe ionizing electrons) orbital subspaces, allowing compact yet accurate representation of ionization dynamics in many-electron systems. The classification into the subspaces can be done flexibly, according to simulated physical situations and desired accuracy, and the multiconfiguration time-dependent Hartree-Fock (MCTDHF) approach is included as a special case. To assess its performance, we apply the TD-CASSCF method to the ionization dynamics of one-dimensional lithium hydride (LiH) and LiH dimer models, and confirm that the present method closely reproduces rigorous MCTDHF results if active orbital space is chosen large enough to include appreciably ionizing electrons. The TD-CASSCF method will open a way to the first-principle theoretical study of intense-field induced ultrafast phenomena in realistic atoms and molecules.PACS numbers: 32.80. Rm, 42.65.Ky

We study nonlinear optical response of electron dynamics in graphene to an intense light pulse within the model of massless Dirac fermions. The time-dependent Dirac equation can be cast into a physically transparent form of extended optical Bloch equations that consistently describe the coupling of light-field-induced intraband dynamics and interband transitions. We show that the nonlinear optical response is not sufficiently described neither by pure intraband nor by pure interband dynamics but their interplay has to be taken into account. When the component of the instantaneous momentum parallel to the field changes its sign, the interband transition is strongly enhanced and considerably influences the intraband dynamics. This counteracts anharmonic response expected from purely intraband dynamics and relaxes nonlinearly. Nevertheless, graphene is still expected to exhibit nonlinear optical response in the terahertz regime such as harmonic generation.Despite its short history after the first intentional production, 1 there is rising interest in graphene over a wide spectrum of fields including materials, condensed-matter, optical, high-field, and high-energy science because of their potential application in carbon-based electronics as well as possibility to mimic and test quantum relativistic phenomena. 2,3 While unique properties such as finite conductivity at zero carrier concentration 4 and ac and dc universal conductance 5-7 are predicted and observed, the interest in the optical response of graphene is even further boosted by recent progress of terahertz ͑THz͒ radiation technology, which is another frontier research area. 8 The generation of ultrashort ͑from a few cycles even down to a single cycle͒ high-intensity ͓Ͼ100 MV/ cm at 30 THz ͑Ref. 9͒ and 70 kV/cm at 1 THz ͑Ref. 10͔͒ pulses has been reported, and even THz generation is possible from laser-irradiated graphite. 11 This will open up a new field of high-field physics in condensed matter. Along these lines the nonlinear optical response of graphene, such as induced current nonlinear in field strength and harmonic emission is becoming one of the key issues. 12-18 Using a quasiclassical kinetic approach but ignoring interband transitions, Mikhailov and Ziegler 12 predicted strong nonlinear response while Wright et al. 13 have performed Fourier analysis of the time-dependent Dirac equation ͑TDDE͒.In this Rapid Communication, we study the time-domain nonlinear dynamics of the electric current induced in graphene by an ultrashort intense optical field, starting from the ͑2+1͒-dimensional TDDE for massless fermions. Special emphasis is placed on the interplay between intraband and interband dynamics, which we describe with a set of extended optical Bloch equations ͑EBOEs͒ derived from the TDDE. The analysis using our model indicates that the induced current is dominated by the intraband dynamics but significantly affected by interband transitions. The underlying mechanism is that the latter is strongly enhanced when the instantaneous kinetic momentum of th...

Within a semiclassical description of above-threshold ionization (ATI) we identify the interplay between intracycle and intercycle interferences. The former is imprinted as a modulation envelope on the discrete multiphoton peaks formed by the latter. This allows one to unravel the complex interference pattern observed for the full solution of the time-dependent Schrödinger equation (TDSE) in terms of diffraction at a grating in the time domain. These modulations can be clearly seen in the dependence of the ATI spectra on the laser wavelength. Shifts in energy modulation result from the effect of the long Coulomb tail of the atomic potential.Tunneling ionization is a highly nonlinear quantummechanical phenomenon induced by intense laser pulses (> ∼ 10 14 W/cm 2 ). Electrons are emitted by tunneling through the potential barrier formed by the combination of the atomic potential and the external strong field. Tunneling has recently attracted increasing interest as a probe of the atomic and molecular structure [1-3]. Tunneling occurs within each optical cycle predominantly around the maxima of the absolute value of the electric field. The interference of the successive bursts of ejected electrons reaching the same final momentum gives rise to features in photoelectron energy and momentum distribution which are markedly different from typical above-threshold ionization (ATI) spectra by multicycle lasers. This temporal double-slit interference has recently been studied both experimentally [1,4] and theoretically [5]. On the other hand, the ATI peaks separated by a photon energy can be themselves viewed as an interference pattern formed by electron bursts repeated each optical cycle. Details of the interplay between these intra-and intercycle interferences have not yet been clearly identified and analyzed, to the best of our knowledge.In this Rapid Communication, we study the influence of different interference processes on ATI spectra generated by multicycle laser pulses. We clarify the underlying mechanism within a simple one-dimensional (1D) model employing classical trajectories. Within the framework of the strongfield approximation (SFA) [6] the qualitative features, the modulation of the ATI peaks akin to the modulation of Bragg peaks by the structure factor in crystal diffraction, can be unambiguously identified in the ATI spectrum determined from the full solution of the three-dimensional time-dependent Schrödinger equation (TDSE). The multicycle laser pulse thus acts as a diffraction grating in the time domain. Quantitative deviations between the SFA predictions and the full TDSE can be traced to the Coulomb tail of the atomic potential affecting this modulation. The latter opens up the opportunity to observe effects of the atomic potential in easy-to-obtain photoelectron spectra after ionization by multicycle laser pulses.Our simple semiclassical model of photoelectron spectra is based on the 1D "simple man's model (SMM)" [6][7][8]. Let us consider an atom interacting with a linearly polarized laser pulse. The...

We demonstrate the generation of a coherent water window x ray by extending the plateau region of high-order harmonics under a neutral-medium condition. The maximum harmonic photon energies attained are 300 and 450 eV in Ne and He, respectively. Our proposed generation scheme, combining a 1.6 microm laser driver and a neutral Ne gas medium, is efficient and scalable in output yields of the water window x ray. Thus, the precept of the design parameter for a single-shot live-cell imaging by contact microscopy is presented.

We investigate the dependence of the intensity of radiation due to high-harmonic generation as a function of the wavelength lambda of the fundamental driver field. Superimposed on a smooth power-law dependence observed previously, we find surprisingly strong and rapid fluctuations on a fine lambda scale. We identify the origin of these fluctuations in terms of quantum path interferences with up to five returning orbits significantly contributing.