A. Lyras

Quantum Hall physics with cold atoms in cylindrical optical lattices

We propose and study various realizations of a Hofstadter-Hubbard model on a cylinder geometry with fermionic cold atoms in optical lattices. The cylindrical optical lattice is created by copropagating Laguerre-Gauss beams, i.e.~light beams carrying orbital angular momentum. By strong focusing of the light beams we create a real space optical lattice in the form of rings, which are offset in energy. A second set of Laguerre-Gauss beams then induces a Raman-hopping between these rings, imprinting phases corresponding to a synthetic magnetic field (artificial gauge field). In addition, by rotating the lattice potential, we achieve a slowly varying flux through the hole of the cylinder, which allows us to probe the Hall response of the system as a realization of Laughlins thought experiment. We study how in the presence of interactions fractional quantum Hall physics could be observed in this setup.

Electronic correlation effects in a model of endohedral Mg (Mg@C60)

We have studied theoretically the atomic structure and ground-state photoionization cross-section in a model of endohedral Mg (Mg@C60) with particular emphasis on doubly excited bound and autoionizing (AI) states. We modelled Mg@C60 as a system of two active electrons outside a frozen, doubly charged ionic core, confined to a spherically symmetric potential well of finite depth. We employed a suitable B-spline basis set within a CI approach to describe the two-electron states of the confined system. In addition, we have calculated the ground-state photoionization cross-section in a limited energy range above the first ionization threshold. We found that doubly excited perturbers appear in each regular singly excited Rydberg series of bound states. They are composed of two-electron configurations built with those one-electron orbitals mostly affected by the confining potential. Moreover, the low-lying part of the AI spectrum for each symmetry analysed is perturbed by states novel to the confined system, whose configuration expansion is again dominated by the one-electron orbitals mostly affected by the confining potential and whose AI widths vary over a wide range of values. Finally, the ground-state photoionization cross-section in a limited energy range (a few eV) above the first ionization threshold is structured by the AI resonances in a way markedly different from the corresponding Mg atom cross-section.

Eigenchannel R-matrix study of two-photon processes including above-threshold ionization in magnesium

The approach based on the eigenchannel R-matrix method and multichannel quantum-defect theory, introduced by Robicheaux and Gao to calculate two-photon processes in light alkaline-earth atoms, has been implemented in jj-coupling introducing explicitly spin - orbit effects and employing both the length and velocity forms of the electric dipole transition operator. Emphasis is placed on the identification of the intermediate- and final-state resonances appearing in the cross sections. The efficiency and potential of the method are demonstrated in magnesium, by calculating two-photon ionization cross sections, branching ratios and photoelectron angular distributions for an extended photon energy range, including above-threshold ionization. The contributions of electronic correlation, core-polarization and spin - orbit effects are investigated by a systematic comparison with experimental results and earlier theoretical calculations.

Saturation and population transfer of a two-photon excited four-level potassium atom

A theoretical study of the nonlinearity reduction in a four-level system of a potassium atom in the presence of a strong nanosecond laser field, which excites the transition |4S1/2↔|6S1/2 with two photons, is presented. It is shown that the destructive quantum interference between the laser field and the internally generated radiations results in a linear response of the atomic path-1 (|4S1/2↔|6S1/2↔|5P3/2↔|4S1/2) emitted parametric fields. For sufficiently high laser intensities and/or atomic densities, the path-1 emitted fields are driven into saturation, a regime accompanied by a substantial population redistribution among the states. Reasonable agreement between earlier experimental results and the theoretical ones is obtained. It is also shown that upon saturation of path-1, for low atomic density, the path-2 (|4S1/2↔|6S1/2↔|4P3/2↔|4S1/2) is activated. A subtle interplay between laser intensity and atomic density may determine the activation of path-2. Also, it is shown that the path-2 emission |6S1/2↔|4P3/2 is an amplified spontaneous emission process which induces a |4P3/2↔|4S1/2 emission, without population inversion, in a cascade scheme.

Amplified spontaneous emission without population inversion in potassium vapour by internally generated fields

We report the experimental observation of the amplified spontaneous emission (ASE) generated at the 4P3/2–4S1/2 transition of potassium atoms following the excitation of the 4S1/2–6S1/2 two-photon transition. We propose that the amplification of the observed radiation is inversionless, due to driving fields on neighbouring transitions that are not imposed externally, but are internally generated in the medium. A theoretical model based on the density operator formalism is presented, which confirms that the radiation observed in the 4P3/2–4S1/2 transition is generated without population inversion for a broad range of the parameters used. It seems that a self-generated ladder scheme is the dominant mechanism, but other schemes are also examined and compared with the experimental results in order to assess their impact on the generation of the inversionless 4P3/2–4S1/2 emission.

High-order above threshold ionization spectrum of hydrogen and photoelectron angular distributions

The multiphonon ionization of the hydrogen atom resulting from its interaction with an intense and short laser pulse has been investigated theoretically, by solving the time-dependent Schrodinger equation (TDSE) via the state-specific expansion of the time-dependent wavefunction. We report above threshold ionization (ATI) spectra and photoelectron angular distributions (PADS) for photon energies ranging between 2 and 4 eV and for laser intensities up to 2*1014 W cm-2. In agreement with recent experimental observations, for h(cross) omega =2 eV and 3 eV a plateau-like structure was found in the ATI spectra. Furthermore, for high-order ATI peaks the PADS show a variety of side-lobes, including emission at 90 degrees off the laser polarization axis. These results are interpreted in terms of tunnelling, multiphonon absorption and interference phenomena.

On the possibility of radiation amplification through multiphoton pumping of autoionizing states

Abstract We study the possibility of amplified emission at wavelengths corresponding to transitions between multiphoton-pumped autoionizing states and initially unpopulated excited bound states of atomic or ionic systems. We test our ideas in He and Be 2+ and find that amplification is possible, although the gain turns out to be extremely small for the cases studied so far. Variations of the amplification scheme that could lead to gain enhancement are discussed.

Temporal dynamics of the internally generated radiations in a two-photon excited four-level potassium atom

A numerical study, stimulated by existing experimental results concerning the temporal dynamics of internally generated radiations, is presented for a four-level system of the potassium atom. An intense nanosecond duration laser pulse excites the two-photon transition and initiates the generation of internal radiations from quantum noise. It is shown that the temporal profiles of the generated radiations along the atomic path-1, (             ) and path-2, (             ), evolve differently as a function of certain system parameters. It is also shown that the excitation laser intensity affects the parametric emissions along path-1 and the population of the states. The path-2 emissions, which also depend on the excitation laser intensity, are most probably due to amplified spontaneous emission (ASE), without population inversion, in the case of strong excitation. We finally examine the radiation dynamics under the effect of the elastic dephasing collisions between the potassium atoms with the buffer gas atoms.

Energy dependence of photoelectron angular distributions from two- and four-photon ionization of Mg in the vicinity of the 3p2 1S0 doubly excited state

We report on measurements concerning the variation of photoelectron angular distributions (PADs) from two- and four-photon ionization of ground state Mg, as the laser wavelength is scanned across the 3p2 1S0 resonance. The energy dependence of the asymmetry parameters determined from the PADs and corresponding to two-photon ionization is found to be in good agreement with prior theoretical predictions. For the four-photon PADs there are at present no theoretical results. Nevertheless, the resonant character is found considerably red-shifted in accordance with earlier experimental and theoretical studies of the single and double ionization yields. In either case, the PADs show a dominant s-wave on-resonance contribution, while the d-wave contribution apparently dominates under non-resonant excitation within the scanned wavelength range. The PADs offer detailed wavelength- and channel-dependent information on the dynamics of the autoionization resonances under pulsed laser excitation, corroborating and elucidating the conclusions reached from ionization yield studies.

The many faces of resonances in multiphoton ionization

We present a theoretical analysis of multiphoton ionization through resonant intermediate states shifted through resonance during a short, intense pulse. The resonant structure in the photoelectron spectra is compared with what it would be if the peak intensity or the frequency of the laser were scanned, and the various differences are discussed in detail. The dynamical behavior is described in terms of a density matrix evolving under a realistic temporal and spatial pulse shape. Our results, on atoms with realistic parameters, show structure similar to that observed in recent experiments. Results for circularly polarized light are also presented. The limitations and advantages of the model are discussed.