We explore high-order harmonic generation (HHG) from a graphene sheet exposed to intense femtosecond laser pulses based on the Lewenstein model. It is demonstrated that the HHG cutoff frequency increases with graphene size up to the classical limit for distant diatomic systems. In contrast to two-center systems, the cutoff frequency remains constant with increasing power of the harmonics as the graphene diameter extends beyond maximal electron excursion. It is shown that the extended nature of the graphene sheet allows for strong HHG signals at maximum cutoff for linearly as well as circularly polarized laser pulses, the latter opening for generation of strong circularly polarized attosecond pulses. High-order harmonic generation (HHG) refers to the nonlinear process of creation of very high overtones of an intense laser pulse with central frequency ω 0 , which interacts with a dilute gas of atoms or molecules. The realization of laser intensities beyond 10 14 W/cm 2 paved the way for theoretical studies [1,2] and experiments [3][4][5] on HHG from a gas of atoms in the early 1990s. Now, after about 20 years of intense HHG research, the three-step model [6] describing HHG within a single-atom picture is well established: The atom (i) ionizes, (ii) gains energy when accelerated by the electric field in the continuum, and (iii) eventually recombines with the ion emitting a photon at odd multiples of the driving-field frequency. Since a single excursion and recombination of an electron takes place within one-half optical cycle, the generated HHG photons define a coherent attosecond highfrequency laser pulse which is a unique tool for probing and imaging of ultrafast dynamics [7][8][9]. In laser-based imaging the HHG spectra have been used for tomographic reconstruction of molecular orbitals withångström spatial resolution [10][11][12].In recent years HHG following interaction with molecules has received particular attention. First, it has been shown that ionization at one molecular center and recombination at another allows for larger maximum harmonic frequencies [13,14]. Second, the two-center structure allows for the generation of attosecond pulses with elliptical polarization as well as even harmonics if the inversion symmetry is broken [15,16]. It has been shown theoretically that a preprepared molecular medium can be used to produce controlled secondary attosecond pulses, when exposed to a seed attosecond XUV pulse [17]. In addition, the study of HHG has been advancing towards molecules of increasing complexity such as benzene rings [18], fullerenes [19], and carbon nanotubes [20], including the investigation of symmetry properties essential for the selective generation of high-order harmonics.The realization of graphene [21], a two-dimensional monolayer of carbon atoms, has received explosive interest in the last decade due to its extraordinary physical properties such * stian.sorngard@ift.uib.no † sigrid.simonsen@ift.uib.no as its superior strength and electronic conductivity. What was for years believed to b...

We have studied the process of direct (nonsequential) two-photon double ionization of molecular hydrogen (H2). Solving the time-dependent Schrödinger equation by an ab initio method, total (generalized) and singledifferential cross sections are obtained at photon energies from 26 to 33 eV. Both parallel and perpendicular orientation of the molecule with respect to the laser polarization direction are considered, and the results are compared with previously calculated cross sections at 30 eV, as well as the predictions of a simple model.

The ionization dynamics of circular Rydberg states in strong circularly polarized infrared (800 nm) laser fields is studied by means of numerical simulations with the time-dependent Schrödinger equation. We find that at certain intensities, related to the radius of the Rydberg states, atomic stabilization sets in, and the ionization probability decreases as the intensity is further increased. Moreover, there is a strong dependence of the ionization probability on the rotational direction of the applied laser field, which can be understood from a simple classical analogy.

We calculate high-order harmonic spectra from graphene based on the strong-field approximation using circularly polarized infrared laser pulses. We allow for the plane of polarization to be tilted with respect to the two-dimensional graphene sheet, demonstrating that the structure of the harmonic spectra strongly depends on the tilt angle.

Quantitative modeling of spin relaxation in quantum dotsHansen, J.P.; Sorngård, S.A.; Forre, M.; Räsänen, Esa Hansen, J. P., Sorngård, S. A., Forre, M., & Räsänen, E. (2012). Quantitative modeling of spin relaxation in quantum dots. Physical Review B, 85 (35326). 2012Publisher's PDF PHYSICAL REVIEW B 85, 035326 (2012) Quantitative modeling of spin relaxation in quantum dots We use numerically exact diagonalization to calculate the spin-orbit-and phonon-induced triplet-singlet relaxation rate in a two-electron quantum dot exposed to a tilted magnetic field. Our scheme includes a threedimensional description of the quantum dot, the Rashba and the linear and cubic Dresselhaus spin-orbit coupling, the ellipticity of the quantum dot, and a full angular description of the magnetic field. We are able to find reasonable agreement with the experimental results of Meunier et al. [Phys. Rev. Lett. 98, 126601 (2007)] in terms of the singlet-triplet energy splitting and the spin relaxation rate, respectively. We analyze in detail the effects of the spin-orbit factors, magnetic-field angles, and dimensionality and discuss the origins of the remaining deviations from the experimental data.

Multiphoton ionization of helium is investigated in the superintense field regime, with particular emphasis on the role of the electron-electron interaction in the ionization and stabilization dynamics. To accomplish this, we solve ab initio the time-dependent Schrödinger equation with the full electron-electron interaction included. By comparing the ionization yields obtained from the full calculations with the corresponding results of an independent-electron model, we come to the somewhat counterintuitive conclusion that the single-particle picture breaks down at superstrong field strengths. We explain this finding from the perspective of the so-called Kramers-Henneberger frame, the reference frame of a free (classical) electron moving in the field. The breakdown is tied to the fact that shake-up and shake-off processes cannot be properly accounted for in commonly used independent-electron models. In addition, we see evidence of a change from the multiphoton to the shake-off ionization regime in the energy distributions of the electrons. From the angular distribution, it is apparent that the correlation is an important factor even in this regime.

We calculate the angular emission characteristics of phonons from parabolic confined quantum dots containing a single or two interacting electrons. The emission spectra are shown to be generally characterized by a narrow polar angle giving a phonon propagation direction explicitly characterized by the energy difference of the transition. In addition, the phonon emission spectra contain a given number of azimuthally oriented lobes which reflect the quantum structure of the initial excited state. This implies that measurements of angular resolved phonon emission spectra can give detailed information on the electronic charge distribution and energy spectra of excited quantum states. When such a structure is known, a large-scale ordering of identical quantum dots with respect to the emission angles may realize phonon amplification by stimulated emission. Contents

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