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...

The candidate has participated in the ongoing Nordforsk Network "Time-domain quantum processes studied by ultrafast radiation pulses" and in the Nordita workshop "Studying Quantum Mechanics in the Time Domain".iii not to forget, the vast amount of bread. We may not be colleagues in the future, but we will always be friends.I am greatful to Prof. Pablo Fainstein at Centro Atómico Bariloche for facilitating our stay in Argentina, and Daniel and Juan for making life outside work very pleasant. Thank you on behalf of myself and my family. We hope to visit you again someday. Prof. Alain Dubois at Université Pierre et Marie Curie in Paris also deserves thanks for welcoming me there, and of course Stéphane and Nicolas for their help and patience.The thesis would not have been if not for my colleagues, but I could not have finished it without the daily support of Kristian and Edvard. You are the loves of my life. To old friends and new friends, Marit and Joy-Loi, Maria and Magnhild in particular, thank you for all good times that have been and will be. You always make me see the world from new perspectives, which indeed is very useful in this profession, but most of all help keep me sane. My sisters, Ingeborg and Ragnhild, and the rest of my family, thank you for being the best cheerleading troop ever. I feel really privileged that you are my family.And finally, thanks to my parents, who taught us the joy of learning and experiencing from an early age. This thesis is for you. v

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.

We demonstrate a feature of the Rydberg blockade mechanism which occurs between two initially excited circular Rydberg atoms. When both atoms are exposed to weak time-dependent electric fields, it is shown that the intrashell dynamics of each atom is strongly modified by the presence of the other. Three characteristic dynamical regimes are identified with separating radii which both scale linearly with principal quantum number n for otherwise constant field parameters. A region of conditional entangled electron dynamics is separated from the outer asymptotic region of independent atom dynamics through a conditional radius, Rc. An inner region, where both atoms becomes locked in their initial state, is again separated from the conditional region by a smaller blocking radius, Rb. About 10 years ago it was discovered that the large dipole moment of Rydberg states of interacting atoms can induce a detuning which effectively prohibits more than a single atom to become optically excited within a given volume [1,2]. Thus, in an atomic cloud exposed to a driving optical excitation scheme, the dipole-dipole interaction sets up an entangled multiparticle state with special correlation properties. Recently, the dipole blockade mechanism has been measured in controlled two-atom experiments with high-lying Rydberg states of rubidium [3,4] as well as in cold gases [5][6][7]. In addition to the fascinating exploration of exotic quantum dynamics in mesoscopic systems, the dipole blockade opens for applications within quantum information [8]. Here a number of quantum gates based on single-atom gates and conditional two-atom gates and protocols involving a large number of atoms have been proposed [9].Isolated Rydberg atoms can be experimentally prepared in almost any given linear combinations of spherical l,m states of a given principal quantum number n, including circular states (magnetic quantum number m = ±(n − 1)), coherent elliptical states (corresponding to classical states of fixed eccentricity [10]), or strongly polarized Stark states (Stark quantum number k ∼ n) [11][12][13]. Experiments where recent progress has realized trapping and probing of conditional dynamics of two single atoms may therefore also probe intrashell dynamics of two initially excited Rydberg atoms. From the point of view of optical driving frequencies between ground-state atoms |g and a single Rydberg level |e , the dynamics of this setup seems at first sight only to amount to a trivial phase development, as the combined initial state in fact is the dipole blocked dark state |ee of the optical excitation scheme. However, when considering the response of Rydberg atoms to weak, time-dependent electric and magnetic fields, it is clear that for these interactions the initial state couple effectively to a manifold of intrashell states |e i ,e j . In fact, the isolated atom intrashell dynamics can be completely controlled and driven between certain initial and final states with 100% transition probability for any n level [14].In this Brief Report we explor...

We calculate, based on first-order perturbation theory, the total and differential ionization probabilities from a dynamic periodic Rydberg wave packet of a given n-shell exposed to a train of femtosecond laser pulses. The total probability is shown to depend crucially on the laser repetition rate: For certain frequencies the ionization probability vanishes, while for others it becomes very large. The origin of this effect is the strong dependence of the ionization probability on the Stark quantum number. Correspondingly, the angular electronic distribution also changes significantly with the increasing number of pulses for certain repetition rates.