N. J. Kylstra

High-Intensity Laser-Atom Physics

Publisher Summary This chapter illustrates that the development of lasers capable of delivering short pulses of very intense radiation, over a wide frequency range, has led to the discovery of new, nonperturbative multiphoton processes in laser interactions with atomic systems. In this article, we first give a survey of the main properties of multiphoton processes such as the multiphoton ionization of atoms, the emission by atoms of high-order harmonics of the exciting laser light, and laser-assisted electron-atom collisions. It then review the theory of these processes, giving particular attention to ab-initio nonperturbative methods such as the Sturmian-Floquet approach, the R-matrix-Floquet theory, and the numerical integration of the time-dependent Schrodinger equation. The chapter discusses relativistic effects that occur at ultra-high intensities. The chapter concludes by considering possible future developments of high-intensity laser-atom physics.

Transparency near a photonic band edge

We study the absorption and dispersion properties of a

Photon emission by ions interacting with short intense laser pulses: beyond the dipole approximation

{\bf \Lambda}

Relativistic effects in the time evolution of a one-dimensional model atom in an intense laser field

-type atom which decays spontaneously near the edge of a photonic band gap (PBG). Using an isotropic PBG model, we show that the atom can become transparent to a probe laser field, even when other dissipative channels are present. This transparency originates from the square root singularity of the density of modes of the PBG material at threshold.

Coherent manipulation of superconducting quantum interference devices with adiabatic passage

Abstract : Photon emission by a positive ion exposed to an intense few-cycle Ti:Sapphire laser pulse is studied within the strong field approximation. We find that the magnetic field component of the incident pulse has little effect below 10(exp 17) W sq cm peak intensity. However, for more intense pulses it significantly reduces photon emission and changes the plateau structure of the spectra, as compared to the predictions of the dipole approximation, and leads in certain parts of the spectrum to emission in the form of a single attosecond burst of X-ray photons. Results obtained by solving the time-dependent Schrodinger equation for a two dimensional model of atomic hydrogen interacting with an ultrashort high frequency laser pulse are also given for photon emission in the stabilization regime; for the case studied, the main effect of the pulses magnetic field is to produce a relatively strong emission at the second harmonic frequency.

Laser-induced continuum structure in helium: ab initio non-perturbative calculations

Using a B-spline expansion in momentum space, we have solved the time-dependent Dirac equation numerically for a model, one-dimensional atom which is subjected to an ultra-intense, high-frequency laser field. We find that for a peak electric field strength of 175 atomic units (au) and for angular frequencies of 1 and 2 au, relativistic effects start to become apparent. Even under these extreme conditions the wavefunction remains localized in a superposition of field-free bound states and very-low-energy continuum states. Comparing our results with the numerical solution of the time-dependent Schrodinger equation, we find that the Dirac wavefunction is slightly more stable against ionization. We also find that the energy distribution of the ionized electrons is strongly concentrated near threshold and that a cut-off in the high-energy spectrum occurs at the energy corresponding to the maximum momentum of a classical electron in the laser field.

Singly, doubly and triply resonant multiphoton processes involving autoionizing states in magnesium

The method of stimulated Raman adiabatic passage is applied in order to coherently manipulate a three-level superconducting quantum interference device quantum bit with two microwave pulses. Simulations indicate that this method has the potential to allow for efficient control of the system for a wide range of pulse parameters.

Atoms in intense, ultrashort laser pulses: non-dipole and relativistic effects

We present the results of non-perturbative, R-matrix Floquet calculations of laser-induced continuum structures (LICS) in helium. We consider the case in which a Nd:YAG laser is used to embed the 1s4s Rydberg state in the continuum. The resulting modification of the photoionization rate of the 1s2s state is then calculated. This LICS has been studied recently by Halfmann et al (1998 Phys. Rev. A 58 R46) and good agreement with their experimental and theoretical findings is obtained. We also investigate the effects of incoherent channels due to higher-order multiphoton processes and LICS involving the 1s4d state. The validity of the simple two-level models that are frequently used for modelling LICS is tested by comparing these results with R-matrix Floquet calculations.

Josephson spectroscopy of a dilute Bose-Einstein condensate in a double-well potential

Using the R-matrix Floquet theory we have carried out non-perturbative, ab initio one- and two-colour calculations of the multiphoton ionization of magnesium with the laser frequencies chosen such that the initial state of the atom is resonantly coupled with autoionizing resonances of the atom. Good agreement is obtained with previous calculations in the low-intensity regimes. The single-photon ionization from the 3s3p(1)P(o) excited state of magnesium has been studied in the vicinity of the 3p(2) S-1(e) autoionizing resonance at non-perturbative laser intensities. Laser-induced degenerate states (LIDS) are observed for modest laser intensities. By adding a second laser which resonantly couples the 3p2 S-1(e) and 3p3d P-1(o) autoionizing levels, we show that, due to the,small width of the 3p3d P-1(o) state, LIDS occur between this state and the 3s3p P-1(o) state at intensities of the first laser below 10(10) W cm(-2). We next investigate the case in which the first laser induces a resonant two-photon coupling between the ground state and the 3p2 S-1(e) autoionizing state, while the second laser again resonantly couples the respective 3p2 S-1(e) and 3p3d( 1)P(o) autoionizing states. At weak intensities, our calculations compare favourably with recent experimental data and calculations. We show that when the intensity of the first laser is increased, the effect of an additional autoionizing state, the 4s5s S-1(e) state, becomes significant. This state is coupled to the 3p3d P-1(o) autoionizing level by one photon, inducing a triply resonant processes. We show that LIDS occur among the three autoionizing levels and we discuss their effect on the decay rate of the ground state. We consider dressed two- and three-level atoms which can be used to model the results of our calculations.

Interaction of ultra-intense laser pulses with relativistic ions

Abstract At the high laser intensities currently available, simple descriptions of multiphoton processes based on the non-relativistic Schrödinger equation and the dipole approximation can break down. The paper first discusses how non-dipole effects, of order 1/c, due to the magnetic field component of a laser pulse, influence harmonic generation. Particular attention is devoted to photon emission by positive ions interacting with intense, fewcycle Ti: Sapphire laser pulses. It is found that for peak intensities in excess of 1017 W cm−2 the magnetic field component of the incident pulse significantly reduces photon emission and modifies the plateau structure of the spectra. It also leads, in certain parts of the spectrum, to the emission of a single attosecond burst of X-ray photons. The influence of non-dipole effects on harmonic generation in the stabilization regime are considered. Finally, the paper reviews recent progress in the theoretical study of relativistic effects arising when atoms interact with ultrastrong laser fields.