Table of contents

Laser-induced filamentation in the mid-infrared gives rise to unique regimes of nonlinear wave dynamics and reveals in many ways unusual nonlinear-optical properties of materials in this frequency range. The λ² scaling of the self-focusing threshold P_cr, with radiation wavelength λ, allows the laser powers transmitted by single mid-IR filaments to be drastically increased without the loss of beam continuity and spatial coherence. When extended to the mid-infrared, laser filamentation enables new methods of pulse compression. Often working around the universal physical limitations, it helps generate few-cycle and subcycle field waveforms within an extraordinarily broad range of peak powers, from just a few up to hundreds of P_cr. As a part of a bigger picture, laser-induced filamentation in the mid-infrared offers important physical insights into the general properties of the nonlinear-optical response of matter as a function of the wavelength. Unlike their near-infrared counterparts, which can be accurately described within the framework of perturbative nonlinear optics, mid-infrared filaments often entangle perturbative and nonperturbative nonlinear-optical effects, showing clear signatures of strong-field optical physics. With the role of nonperturbative nonlinear-optical phenomena growing, as a general tendency, with the field intensity and the driver wavelength, extension of laser filamentation to even longer driver wavelengths, toward the long-wavelength infrared, promises a hic sunt dracones land.

We report a new apparatus for the study of two-species quantum degenerate mixture of ⁴¹K and ⁶Li atoms. We develop and combine several advanced cooling techniques to achieve both a large atom number and high phase space density of the two-species atom clouds. Furthermore, we build a high efficiency two-species magnetic transport system to transfer atom clouds from the 3D magneto-optical-trap chamber to a full glass science chamber of an extreme high vacuum environment and good optical access. We perform a forced radio-frequency evaporative cooling for ⁴¹K atoms while the ⁶Li atoms are sympathetically cooled in an optically plugged magnetic trap. Finally, we achieve the simultaneous quantum degeneracy for the ⁴¹K and ⁶Li atoms. The Bose–Einstein condensate of ⁴¹K has 1.4 × 10⁵ atoms with a condensate fraction of about 62%, while the degenerate Fermi gas of ⁶Li has a total atom number of 5.4 × 10⁵ at 0.25 Fermi temperature.

Methods to manipulate the individual constituents of an ultracold quantum gas mixture are essential tools for a number of applications, such as the direct quantum simulation of impurity physics. We investigate a scheme in which species-selective control is achieved using magnetic potentials dressed with multiple radiofrequencies, exploiting the different Landé ${g}_{F}$-factors of the constituent atomic species. We describe a mixture dressed with two frequencies, where atoms are confined in harmonic potentials with a controllable degree of overlap between the two atomic species. This is then extended to a four radiofrequency scheme in which a double well potential for one species is overlaid with a single well for the other. The discussion is framed with parameters that are suitable for a ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$ mixture, but is readily generalised to other combinations.

A propagation method for the scattering of a quantum wave packet from a potential surface is presented. It is used to model the quantum reflection of single atoms from a corrugated (metallic) surface. Our numerical procedure works well in two spatial dimensions requiring only reasonable amounts of memory and computing time. The effects of the surface corrugation on the reflectivity are investigated via simulations with a paradigm potential. These indicate that our approach should allow for future tests of realistic, effective potentials obtained from theory in a quantitative comparison to experimental data.

Gray molasses is a powerful tool for sub-Doppler laser cooling of atoms to low temperatures. For alkaline atoms, this technique is commonly implemented with cooling lasers which are blue-detuned from either the D₁ or D₂ line. Here we show that efficient gray molasses can be implemented on the D₂ line of ⁴⁰K with red-detuned lasers. We obtained temperatures of $48(2)\,\mu {\rm{K}}$, which enables direct loading of $9.2(3)\times {10}^{6}$ atoms from a magneto-optical trap into an optical dipole trap. We support our findings by a one-dimensional model and three-dimensional numerical simulations of the optical Bloch equations which qualitatively reproduce the experimentally observed cooling effects.

The ionization process of a hydrogen-like atom in the intense laser field is studied by the Bohmian-mechanics method. We propose a criterion for atomic ionization based on the total energy of a Bohmian particle (BP): when it is larger than or equal to zero, ionization occurs, when it is lower than zero, no ionization occurs. We find that the time-dependent ionization probabilities calculated by the Bohmian-mechanics and projection methods are almost the same under different laser parameters. Two typical Bohmian trajectories are selected to analyze the ionization process. The consistency between the time-dependent ionization probabilities calculated by the two methods is quantitatively analyzed from the aspect of the probability density of the wave-function.

Understanding the interaction of atoms and molecules with an intense laser radiation field is key for many applications such as high harmonic generation and attosecond physics. Because of the non-perturbative nature of strong field physics, some simplifications and approximation methods are often used to shed light on these processes. One of the most fruitful approaches to gain an insight into the physics of such interactions is the three-step-model, in which, the electron first tunnels out through the barrier and then propagates classically in the continuum. Despite the great success of this and other more sophisticated models there are still many ambiguities and open questions, e.g. how long it takes for the electron to tunnel through the barrier. Most of them stem from the difficulties in understanding electron trajectories in the classically 'forbidden' zone under the barrier. In this theoretical paper we show that strong field physics and the propagation of electromagnetic waves in a curved waveguide are governed by the same Schrödinger equation. We propose to fabricate a curved optical waveguide, and use this isomorphism to mimic strong field physics. Such a simulating system will allow us to directly probe the wave-function at any point, including the 'tunneling' zone.

Electron-impact single and double ionization cross sections for the W atom are calculated using a semi-relativistic distorted-wave method. The cross sections include contributions from single direct ionization, double direct ionization and excitation-autoionization. Branching ratio calculations are made to determine whether an excitation may contribute to single or double ionization. We check the accuracy of the semi-relativistic distorted-wave calculations for direct ionization of various subshells by comparison with fully-relativistic distorted-wave calculations. We also check the accuracy of the perturbative distorted-wave calculations for direct ionization of the outer most subshells by comparison with non-perturbative time-dependent close-coupling calculations.

This paper compiles our merged-beam experimental findings for the associative ionization (AI) process from charged reactants, with the aim of guiding future investigations with e.g. the double electrostatic ion storage ring DESIREE in Stockholm. A reinvestigation of the isotopic effect in H⁻(D⁻) + He⁺ collisions is presented, along with a review of ${{\rm{H}}}_{3}^{+}$ and NO⁺ production by AI involving ion pairs or excited neutrals, and put in perspective with the mutual neutralization and radiative association reactions. Critical parameters are identified and evaluated for their systematic role in controlling the magnitude of the cross section: isotopic substitution, exothermicity, electronic state density, and spin statistics.

We have studied the phenomenon of electromagnetically induced transparency (EIT) of ⁸⁷Rb vapor at room temperature in a magnetic field with an arbitrary angle to the laser propagation direction. Rather than exposing atoms to a parallelled or transverse magnetic field as usual, in our work, we apply a magnetic field (up to 45 Gauss) with an arbitrary angle to the laser propagation direction and the spectra become much more complex. More EIT dips are observed due to the Zeeman splitting on the D₂ line of ⁸⁷Rb in a ${\rm{\Lambda }}$-type configuration. With a 5 Torr N₂ buffer gas in the thermal 2 cm vapor cell, the state ${5}^{2}{P}_{3/2}$ has a very short effective lifetime, corresponding to a large energy broadening, which removes the velocity selective optical pumping effect almost completely and keeps the high resolution EIT spectrum for the energy splitting of ⁸⁷Rb in magnetic fields. The shifting of the EIT resonances with the strength of the applied magnetic field coincides well with the theory based on a full matrix Hamiltonian combined with a spectral decomposition method. Our work can be extended to measure the magnetic field vector in space. The effects of the detuning of the probe and coupling beams on the spectral lines are also investigated.

We investigate entanglement and coherence in an XXZ spin s pair immersed in a non-uniform transverse magnetic field. The ground state and thermal entanglement phase diagrams are analyzed in detail in both the ferromagnetic and antiferromagnetic cases. It is shown that a non-uniform field can control the energy levels and the entanglement of the corresponding eigenstates, making it possible to entangle the system for any value of the exchange couplings, both at zero and finite temperatures. Moreover, the limit temperature for entanglement is shown to depend only on the difference $| {h}_{1}-{h}_{2}|$ between the fields applied at each spin, leading for $T\gt 0$ to a separability stripe in the $({h}_{1},{h}_{2})$ field plane such that the system becomes entangled above a threshold value of $| {h}_{1}-{h}_{2}|$. These results are demonstrated to be rigorously valid for any spin s. On the other hand, the relative entropy of coherence in the standard basis, which coincides with the ground state entanglement entropy at T = 0 for any s, becomes non-zero for any value of the fields at $T\gt 0$, decreasing uniformly for sufficiently high T. A special critical point arising at T = 0 for non-uniform fields in the ferromagnetic case is also discussed.

The rate equations used to model plasma kinetics and spectroscopy are typically obtained from intuitive considerations. A few years ago, the authors (Csanak et al 2011 J. Phys. B: At. Mol. Opt. Phys.44 215701) have shown that the population-alignment collisional-radiative (CR) model and the magnetic sublevel to magnetic sublevel rate-equation scheme can be obtained from the Fano–Ben-Reuven quantum impact approximation (QIA). Here we provide a formal derivation of the rate-equation schemes for modeling hydrogenic plasmas and highly charged ionic plasmas with cylindrical symmetry using the QIA under certain approximations. In the case of hydrogenic plasmas the 'accidental degeneracy' (if present) leads to some coherences among the excited states of the atom (or ion) that have to be taken into account when constructing the rate equations. In the case of highly charged plasmas the Coulomb potential can be taken into account (as suggested originally by Baranger) in defining the 'bath particles', which leads to a derivation of the kinetic equations where no singularity occurs. For the case of spherically symmetric plasmas, this method also provides a derivation of the standard CR equations that have been implemented in many codes to successfully model the kinetics and spectra of highly charged ions.