JPhys 50th anniversary: viewpoints collection

In the course of the past several years, a new level of understanding has been achieved about conditions for the existence, stability, and generation of spatiotemporal optical solitons, which are nondiffracting and nondispersing wavepackets propagating in nonlinear optical media. Experimentally, effectively two-dimensional (2D) spatiotemporal solitons that overcome diffraction in one transverse spatial dimension have been created in quadratic nonlinear media. With regard to the theory, fundamentally new features of light pulses that self-trap in one or two transverse spatial dimensions and do not spread out in time, when propagating in various optical media, were thoroughly investigated in models with various nonlinearities. Stable vorticity-carrying spatiotemporal solitons have been predicted too, in media with competing nonlinearities (quadratic–cubic or cubic–quintic). This article offers an up-to-date survey of experimental and theoretical results in this field. Both achievements and outstanding difficulties are reviewed, and open problems are highlighted. Also briefly described are recent predictions for stable 2D and 3D solitons in Bose–Einstein condensates supported by full or low-dimensional optical lattices.

A derivation is given of the exact form of the three-body Coulomb wavefunction in the asymptotic region where the separation of all particles tends to infinity. Using a modification of the method of Pluvinage (1951), an approximate three-body scattering wavefunction is derived that satisfies this boundary condition. Triply-differential cross sections (TDCS) for electron impact ionisation of atomic hydrogen calculated with this scattering wavefunction, which contains no free parameters, show excellent agreement with measurements at impact energies greater than 150 eV. The corresponding TDCS for positron impact ionisation are also presented.

For pt.I see ibid., vol.20, p.6363-78 (1987). A general description of the data requirements for opacity calculations has been given in paper I. The present paper gives a detailed description of the methods being used in a collaborative effort which is referred to as the Opacity Project. The close-coupling approximation of electron-atom collision theory is used to calculate energies and wavefunctions for bound states, oscillator strengths, photoionisation cross sections and parameters for line broadening by electron impact. The computations are made using the R-matrix method together with new codes for calculating outer-region solutions and dipole integrals. Use of these techniques provides an efficient means of calculating large amounts of accurate atomic data.

A dressed-atom approach to resonance fluorescence in intense laser fields is presented. Simple and general results are derived which include the now well known predictions concerning two-level atoms but are not restricted to such simple cases. The positions of the various components of the fluorescence and absorption spectra are given by the allowed Bohr frequencies of the total system: atom+laser mode (dressed atom). The master equation, describing spontaneous emission from the dressed atom is solved in the limit of high intensities. Simple expressions, taking into account the effect of cascades, are derived for the widths of the components.

Attosecond pulses are generated by a macroscopic number of ionizing atoms interacting with a focused laser pulse, via the process of high harmonic generation. The physics of their generation consists of an interplay between the microscopic laser–atom interaction and macroscopic effects due to ionization and phase matching in the nonlinear medium. In this review, we focus on a complete understanding of the way in which attosecond pulses arrive at a target where they can be characterized and used in an experiment. We discuss a number of results from calculations of attosecond pulse generation obtained by simultaneous solution of the time-dependent Schrödinger equation and the Maxwell wave equation. These results, which allow for a clean separation of microscopic and macroscopic factors, illustrate how macroscopic effects are used to select attosecond pulses from the radiation that is emitted by atoms interacting with a strong laser field.

High-resolution recoil-ion momentum spectroscopy (RIMS) is a novel technique to determine the charge state and the complete final momentum vector of a recoiling target ion emerging from an ionizing collision of an atom with any kind of radiation. It offers a unique combination of superior momentum resolution in all three spatial directions of with a large detection solid angle of . Recently, low-energy electron analysers based on rigorously new concepts and reaching similar specifications were successfully integrated into RIM spectrometers yielding so-called `reaction microscopes'.

Exploiting these techniques, a large variety of atomic reactions for ion, electron, photon and antiproton impact have been explored in unprecedented detail and completeness. Among them kinematically complete experiments on electron capture, single and double ionization in ion - atom collisions at projectile energies between 5 keV and 1.4 GeV have been carried out. Double photoionization of He has been investigated at energies close to the threshold up to . At the contributions to double ionization after photoabsorption and Compton scattering were separated kinematically for the first time. These and many other results will be reviewed in this paper. In addition, the experimental technique is described in some detail and emphasis is given to envisaging the rich future potential of the method in various fields of atomic collision physics with atoms, molecules and clusters.

Applying a space translation operation, the Schrodinger equation for an atom in an electromagnetic field is solved with sufficient accuracy to obtain probabilities for multiple absorption of photons from a monochromatic laser beam of arbitrary intensity or frequency. It is shown that the derived expression for the N-photon T-matrix contains the usual single photon matrix elements given by the perturbation theory and that the perturbative result is obtained in the limit of low intensity. Other explicit examples are considered. The conditions of applicability of the method are specified.

We have performed a number of experiments with a Bose-Einstein condensate (BEC) in a one-dimensional optical lattice. Making use of the small momentum spread of a BEC and standard atom optics techniques, a high level of coherent control over an artificial solid-state system is demonstrated. We are able to load the BEC into the lattice ground state with a very high efficiency by adiabatically turning on the optical lattice. We coherently transfer population between lattice states and observe their evolution. Methods are developed and used to perform band spectroscopy. We use these techniques to build a BEC accelerator and a novel, coherent, large-momentum-transfer beam-splitter.

The authors report the observation of very-high-order odd harmonics of Nd:YAG laser radiation in rare gases at an intensity of about 10¹³ W cm^-2. Harmonic light as high as the 33rd harmonic in the XUV range (32.2 nm) is generated in argon. The key point is that the harmonic intensity falls slowly beyond the fifth harmonic as the order increases. Finally, a UV continuum, beginning at 350 nm and extending down towards the short wavelength region is apparent in xenon.