R. M. Potvliege

Bessel-modulated Gaussian beams with quadratic radial dependence

Abstract We describe a class of closed-form solutions of the paraxial wave equation whose transverse profile is a Gaussian function multiplied by a Bessel function, the argument of which is quadratic in the distance from the axis. Their characteristics are compared to those of Bessel–Gauss beams. Unlike the latter, these Bessel-modulated Gaussian beams propagate colinearly. They have interesting non-Gaussian features for certain values of their parameters, such as concentric rings of phase dislocation outside the waist plane or a flat axial profile.

Free-space propagation of ultrashort pulses : space-time couplings in Gaussian pulse beams

The importance of space-time couplings and the validity of the paraxial approximation are discussed for the case of ultrashort freely-propagating Gaussian pulses whose spectral components all have the same frequency-independent Rayleigh length. Simple closed-form expressions describing the electric field of such pulses are derived for model spectral distributions. Space-time couplings are found to be significant only for subcycle pulses or far from the focus.

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

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.

Microwave dressing of Rydberg dark states

We study electromagnetically induced transparency (EIT) in the 5s→5p→46s ladder system of a cold 87Rb gas. We show that the resonant microwave coupling between the 46s and 45p states leads to an Autler–Townes splitting of the EIT resonance. This splitting can be employed to vary the group index by ±105 allowing independent control of the propagation of dark state polaritons. We also demonstrate that microwave dressing leads to enhanced interaction effects. In particular, we present evidence for a 1/R3 energy shift between Rydberg states resonantly coupled by the microwave field and the ensuing breakdown of the pairwise interaction approximation.

Enhanced electric field sensitivity of rf-dressed Rydberg dark states

Optical detection of Rydberg states using electromagnetically induced transparency (EIT) enables continuous measurement of electric fields in a confined geometry. In this paper, we demonstrate the formation of radio frequency (rf)-dressed EIT resonances in a thermal Rb vapour and show that such states exhibit enhanced sensitivity to dc electric fields compared to their bare counterparts. Fitting the corresponding EIT profile enables precise measurements of the dc field independent of laser frequency fluctuations. Our results suggest that space charges within the enclosed cell reduce electric field inhomogeneities within the interaction region.

Long-range Rydberg?Rydberg interactions in calcium, strontium and ytterbium

Long-range dipole–dipole and quadrupole–quadrupole interactions between pairs of Rydberg atoms are calculated perturbatively for calcium, strontium and ytterbium within the Coulomb approximation. Quantum defects, obtained by fitting existing laser spectroscopic data, are provided for all S, P, D and F series of strontium and for the 3P2 series of calcium. The results show qualitative differences with the alkali metal atoms, including isotropically attractive interactions of the strontium 1S0 states and a greater rarity of Forster resonances. Only two such resonances are identified, both in triplet series of strontium. The angular dependence of the long-range interaction is briefly discussed.

STRFLO: A program for time-independent calculations of multiphoton processes in one-electron atomic systems I. Quasienergy spectra and angular distributions

Abstract The program presented calculates quasienergy spectra of one-electron atoms or ions undergoing multiphoton ionization in an intense two-colour field or ionization in a strong static electric field. The electron is initially bound by a single Coulomb potential or by a superposition of a short-range potential and a Coulomb potential. Besides quasienergies, the program can obtain rates of photoionization in specific ATI channels and angular distributions of photoelectrons, in the approximation where the atom is represented by a single dressed state. The calculation can be perturbative or nonperturbative. The Floquet equations are solved on a basis of spherical harmonies and complex Sturmian functions.

Spatially selective loading of an optical lattice by light-shift engineering using an auxiliary laser field

We report on a method of light-shift engineering where an auxiliary laser is used to tune the atomic transition frequency. We show that the technique provides enhanced loading of laser cooled 85Rb and 87Rb atoms into deep optical traps. Furthermore, by using a blue-detuned optical molasses, light-shift engineering offers remarkable spatial selectivity in the loading of an optical lattice.

Ionization of hydrogen atoms by intense vacuum ultraviolet radiation

We study the ionization of hydrogen atoms by an intense pulsed beam of photons with energies of 17 or 50 eV. The work is motivated by the demonstration of the free-electron laser (FEL) action at the DESY Laboratory. The parameters chosen for the incident field are in the regime accessible by the FEL. Ionization yields are obtained within three different approaches, namely the strong-field approximation, the Floquet method and a numerical solution of the timedependent Schr¨ odinger equation. A marked stabilization effect for 50 eV photons is shown.

Two-colour multiphoton ionization of hydrogen by an intense laser field and one of its harmonics

New results of ab initio Floquet calculations of rates for multiphoton ionization of atomic hydrogen by a coherent superposition of an intense laser field and one of its harmonics (second, third, fifth or seventh) are presented. The authors discuss further the relationship of these processes with two-colour ionization by fields with incommensurable frequencies, as well as the range of validity of Perelomov and Popovs formula (1967).