Showing papers in "European Physical Journal D in 1998"

Two-colour operation of a Free-Electron Laser and applications in the mid-infrared

Abstract: The two-colour operation of a Free-Electron Laser (FEL) has been demonstrated with the “CLIO” infrared facility. A two section undulator allows the production of picosecond laser pulses at two different wavelengths and simultaneously, in a wavelength range from to , and with a wavelength separation of two colours of up to . The time overlap, between both colours, has been measured on a picosecond time scale and on a microsecond time scale. An initial pump-probe application experiment has been performed with the two colours: stimulated emission has been measured in a 3-level Quantum-Well system. This is the best demonstration of the stability and reliability of the two-colour laser operation.

Evidence for intrinsic Kerr bistability of high-Q microsphere resonators in superfluid helium

Abstract: Quality factors up to 109 have been obtained in the whispering gallery modes (WGMs) of fused silica microspheres immersed in a superfluid helium bath. We have observed a dispersive bistable behaviour of the WGM resonances with a threshold power of 10 μW, due to the intrinsic Kerr nonlinearity of silica. These results open the way to the realization of a thresholdless microlaser and other cavity QED projects with microspheres.

Electrons weakly bound to molecules by dipolar, quadrupolar or polarization forces

Abstract: Within the framework of a simple electrostatic model we here discuss the stability of very weakly bound molecular negative ions. In contrast with the case of conventional valence anions, the excess electron is then located in a very diffuse orbital and is mainly bound by electrostatic dipolar, quadrupolar and polarization forces at large distances from the neutral molecular core. By fitting a single repulsion parameter of the model to the available experimental data, it is possible to make quantitative predictions of the excess electron binding energies in these species. Critical values of dipole moment, quadrupole moment or polarizability required for the observation of stable dipole-bound, quadrupole-bound or polarization-bound negative ions are predicted.

A variational approach of nonlinear dissipative pulse propagation

Abstract: A variational technique to deal with nonlinear dissipative pulse propagation is established. By means of a generalization of the Kantorovitch method, suitable for non-conservative systems, we are able to cope with an extended nonlinear Schrodinger equation (NLSE) which describes pulse propagation under the influence of nonlinear loss and/or gain, in particular, in the presence of two-photon absorption (TPA). Based on the characteristics of the exact solution of the NLSE in the absence of TPA, we investigate the effects of frequency dispersion of the nonlinear susceptibility associated to the two-photon resonance, obtaining the necessary conditions for a solitary wave solution, even in the presence of a self-steepening term.

Optical properties of gold metal clusters: A time-dependent local-density-approximation investigation

Abstract: The optical response of free and matrix-embedded gold metal clusters AuN is investigated in the framework of the time-dependent local-density-approximation (TDLDA). The characteristics of the surface plasmon resonance are carefully analyzed as a function of the model parameters and the particle radius. The strong influence of the frequency-dependence of the 5d core-electron dielectric function in the vicinity of the interband threshold is emphasized. The size evolution of the Mie-frequency in free gold clusters exhibits a noticeable blue-shift trend as the particle size decreases, much stronger than in silver clusters. The width and shape of the resonance, essentially ruled by the decay via the interband transitions, are found closely correlated to the imaginary component of the core-electron dielectric function. In presence of a surrounding matrix the blue-shift trend is largely rubbed out. Agreement with recent experimental results on size-selected gold clusters embedded in an alumina matrix may be achieved by taking into account the porosity effects at the metal/matrix interface. The comparison with the predictions of classical models is also provided.

Relativistic and many-body effects in K, L, and M shell ionization energy for elements with and the determination of the 1s Lamb shift for heavy elements

Abstract: We report on a calculation of K, L and M inner-shell ionization energy in atoms with atomic numbers in the range . Many-body effects are evaluated for all n =1, 2, and 3 hole states. Those include correlation and effects due to the auto-ionizing nature of the hole states (Auger shift). For high Z we add recent corrected nuclear polarization, and several second-order vacuum polarization corrections. K and L ionization energies are compared with experimental X-ray absorption edges measurements. Excellent agreement with rare gazes and metal vapor measurements is found. We also compare our calculations with X-ray transition energies for all K and L lines that involve K, L and M holes. Finally we use K X-ray lines to deduce an hydrogenlike 1 s Lamb shift for several heavy elements, with far better accuracy than has been obtained by direct measurements of hydrogenlike ions.

Quantum state engineering using conditional measurement on a beam splitter

Abstract: State preparation via conditional output measurement on a beam splitter is studied, assuming the signal mode is mixed with a mode prepared in a Fock state and photon numbers are measured in one of the output channels. It is shown that the mode in the other output channel is prepared in either a photon-subtracted or a photon-added Jacobi polynomial state, depending upon the difference between the number of photons in the input Fock state and the number of photons in the output Fock state onto which it is projected. The properties of the conditional output states are studied for coherent and squeezed input states, and the probabilities of generating the states are calculated. Relations to other states, such as near-photon-number states and squeezed-state-excitations, are given and proposals are made for generating them by combining the scheme with others. Finally, effects of realistic photocounting and Fock-state preparation are discussed.

Molecular photodynamics involved in multi-photon excitation fluorescence microscopy

Abstract: We present a simple model for calculating the fluorescence generated by the multi-photon excitation (MPE) of molecules in solution. The model takes into account internal molecular dynamics such as ground-state depletion due to inter-system crossing (ISC), as well as external molecular dynamics associated with diffusion into and out of an excitation volume confined in 3-dimensions. Internal and external molecular dynamics are combined by using a technique of linearization of a modified diffusion equation which takes into account the possibility of concentration depletion due to photobleaching. In addition, we discuss the phenomenon of pulse saturation which effectively limits the molecular excitation rate constant in the case of short pulsed excitation. Our results are specifically applied in the context of fluorescence autocorrelation functions and single-molecule detection. In the latter case, we discuss some consequences of high-order multi-photon photobleaching. Finally, we include three appendices to rigorously define the temporal and spatial profiles of an arbitrary excitation beam, and also to discuss some properties of an exact evaluation of concentration depletion due to photobleaching.

Effect of the initial phase of the field in ionization by ultrashort laser pulses

Abstract: We report on observable new features related to ionization of atoms by laser pulses of only few cycles and some intensity. We show that for particular photo-electron energies, the angular distribution becomes asymmetric and that this asymmetry is related to the initial phase of the field.

Phase dynamics of Bose-Einstein condensates: Losses versus revivals

Abstract: In the absence of losses the phase of a Bose-Einstein condensate undergoes collapses and revivals in time due to elastic atomic interactions. As experiments necessarily involve inelastic collisions, we develop a model to describe the phase dynamics of the condensates in presence of collisional losses. We find that a few inelastic processes are sufficient to damp the revivals of the phase. For this reason the observability of phase revivals for present experimental conditions is limited to condensates with a few hundreds of atoms.

A cold atom clock in absence of gravity

Abstract: We describe the operation of a cold atom clock in reduced gravity. We have recorded the cesium hyperfine resonance signal at a frequency near 9.2 GHz in the \(\) gravity environment produced by jet plane parabolic flights. With a resonance width of 7 Hz, the device operated in a regime which is not accessible on earth. In the much lower gravity level of a satellite, our cold cesium clock would outperform the fountains with a potential accuracy of \(\). This experiment paves the way to unprecedented performance in space applications such as tests of general relativity, global time dissemination, astronomy and geodesy.

Temporal coherent control induced by wave packet interferences in one and two photon atomic transitions

Abstract: The interaction of a sequence of two identical ultrashort laser pulses with an atomic system results in quantum interferences as in Ramsey fringes experiments. These interferences allow achievement of temporal coherent control of the excitation probability. We present the results of a temporal coherent control experiment on two different atomic systems: one-photon absorption in K (4s-4p) and two-photon absorption in Cs (6s-7d). In K, the quantum interferences between the two excitation paths associated with the laser pulses are revealed through rapid oscillations of the excitation probability as a function of the time delay between the two pulses. These oscillations take place at the transition frequency (period T = 2.56 fs). The interferences are modulated by beats (at about 580 fs) resulting from the doublet structure of the excited state (4p (2 P 1/2 , 2 P 3/2 )). Three complementary interpretations of this experiment are presented: in terms of beats of quantum interferences, of variation in the spectrum intensity, and of wave packet interferences. Whenever the two laser pulses are temporally overlapped, optical interferences are superimposed on to the quantum interferences. The distinction between these two types of interference is clearly revealed in the two-photon excitation scheme performed on Cs (6s-7d (2 D 3/2 , 2 D 5/2 )) because quantum interferences occur at twice the frequency of the optical interferences.

Adiabatic population transfer in multistate chains via dressed intermediate states

Abstract: The well-known process of stimulated Raman adiabatic passage (STIRAP) provides a robust technique for achieving complete population transfer between the first and last state of a three-state chain, with little population, even transiently, in the intermediate state. The extension of STIRAP to general N-state chainwise-linked systems continues to generate interest. Recently Malinovsky and Tannor (Phys. Rev. A 56, 4929 (1997)) have shown with numerical simulation that a resonant pulse sequence, which they term “straddle STIRAP”, can produce (under appropriate conditions, including specific pulse areas) complete population transfer with very little population in intermediate states. Their proposal supplements a pair of counterintuitively ordered delayed laser pulses, driving the first and last transition of the chain and corresponding to the pump and Stokes pulses in STIRAP, with one or more additional strong pulses of longer duration which couple the intermediate transition(s) and overlap both the pump and the Stokes pulses. In this paper, we modify the “straddling” Malinovsky-Tannor pulse sequence so that the intermediate couplings are constant (and strong), at least during the times when the pump and Stokes pulses are present, and the intermediate states therefore act as a strongly coupled subsystem with constant eigenvalues. Under this condition, we show that the original N-state chain is mathematically equivalent to a system comprising N-2 parallel -transitions, in which the initial state is coupled simultaneously to N-2 dressed intermediate states, which in turn are coupled to the final state. The population transfer is optimized by suitably tuning the pump and Stokes frequencies to resonance with one of these dressed intermediate states, which effectively acts as the single intermediate state in a three-state STIRAP-like process. We show that tuning to a dressed intermediate state turns the system (for both odd N and even N) into a three-state system - with all of the properties of conventional STIRAP (complete population transfer, little transient population in the intermediate states, insensitivity to variations in the laser parameters, such as pulse area). The success of the tuning-to-dressed-state idea is explained by using simple analytic approaches and illustrated with numerical simulations for four-, five-, six- and seven-state systems.

Coherent population transfer in NO with pulsed lasers: the consequences of hyperfine structure, doppler broadening aaand electromagnetically induced absorption

Abstract: Coherent population transfer between vibrational levels of the NO molecule induced by the interaction of two delayed laser pulses, also referred to as stimulated Raman scattering involving adiabatic passage (STIRAP), is studied experimentally in a molecular beam and in the bulk. The consequences of hyperfine splitting and Doppler broadening are discussed in detail. Unlike in previous studies of this kind, transfer occurs simultaneously between more than one group of non degenerate levels. In a molecular beam or in the bulk, the transfer efficiency of STIRAP exceeds that obtained by Stimulated Emission Pumping (SEP) by a factor of 3.6 or 15, respectively. We estimate the absolute transfer efficiency T in the beam to be \(\), while \(\) is found in the bulk. In both cases, this is \(\) of the maximum value expected from numerical studies. Possible reasons for this discrepancy are discussed. Finally we show that the absorption of a pump pulse in a weakly absorbing medium is significantly enhanced by the presence of a copropagating Stokes pulse when the Rabi frequency \(\) of the latter is smaller than the width of the Doppler profile \(\). The relation of this observation to the phenomenon of Electromagnetically Induced Transparency (EIT), which is observed for \(\), is also discussed.

Isotope shifts and hyperfine structure in the transitions in calcium II

Abstract: The isotope shift and hyperfine structure in the three \(\) - transitions in Ca II have been studied by fast ion beam collinear laser spectroscopy for all stable Ca isotopes. The metastable 3d states were populated within the surface ionization source of a mass separator with a probability of about 0.1%. After resonant excitation to the 4p levels with diode laser light around 850 nm the uv photons from the \(\) transitions to the ground state were used for detection. Hyperfine structure parameters A and B for the odd isotope 43Ca, as evaluated from the splittings observed, agree well with theoretical predictions from relativistic many-body perturbation theory. Field shift constants \(\) and specific mass shift constants \(\) were extracted from the measured isotope shifts and are discussed in comparison with expectation values from theory.

Laser applications for atmospheric pollution monitoring

Abstract: In this paper, we present an overview of the capabilities of linear and non-linear spectroscopy for the 3D-analysis of gaseous pollutants and aerosols in the atmosphere. The basic technique is Lidar (Light Detection and Ranging). Results obtained in Sevilla and Lyon about the ozone smog formation and the dynamics of the transport of dust particles are presented. Finally, non-linear Mie scattering on atmospheric aerosols has been demonstrated in the laboratory, opening new perspectives for remote sensing.

Photon-number-state generation with a single two-level atom in a cavity: a proposal

Abstract: A single two-level atom can be used to prepare an arbitrary photon number state (Fock state) in a high Q cavity . The atom undergoes a controlled succession of interactions with two cavity modes. One of them contains a coherent field. The atom transfers photons one by one from this field to the initially empty second mode. The scheme can be extended to prepare a quantum superposition of the vacuum with a Fock state, a highly non-classical situation. We discuss the feasibility of the experiment with our present Rydberg-atom cavity QED set-up.

Is NaI soluble in water clusters

Abstract: \(\) clusters (solvents being \(\), \(\) or \(\)) have been studied by resonance enhanced two photons ionization, leading to the detection of \(\) clusters. When water is the solvent, large clusters up to n>50 can be observed, whereas for \(\) and \(\) no clusters larger than 10 could be evidenced. Because the first step in the ionization process is the excitation from the ground solvated (\(\)) ion pair state to a covalent excited state, the differences in the cluster size distribution for different solvent may be interpreted as a difference in cluster structures leading to a difference in the charge separation in the ground state.

Application of B-splines finite basis sets to relativistic two-photon decay rates of level in hydrogenic ions

Abstract: A theoretical study of the one- and two-photon spontaneous emission rates from the 2 s1/2 state of one-electron ions is presented. High-precision values of the relativistic emission rates for ions with nuclear charge Z up to 100 are obtained through the use of finite basis sets for the Dirac equation constructed from B-splines. Furthermore, we analyze the influence of the inclusion of quantum electrodynamics corrections in the initial and final state energies.

Frequency up-converted radiation from a cavity moving in vacuum

Abstract: We calculate the photon emission of a high finesse cavity moving in vacuum. The cavity is treated as an open system. The field initially in the vacuum state accumulates a dephasing depending on the mirrors motion when bouncing back and forth inside the cavity. The dephasing is not linearized in our calculation, so that qualitatively new effects like pulse shaping in the time domain and frequency up-conversion in the spectrum are found. Furthermore we predict the existence of a threshold above which the system should show self-sustained oscillations.

Quantum wires and quantum dots for neutral atoms

Abstract: By placing changeable nanofabricated structures (wires, dots, etc.) on an atom mirror one can design guiding and trapping potentials for atoms. These potentials are similar to the electrostatic potentials which trap and guide electrons in semiconductor quantum devices like quantum wires and quantum dots. This technique will allow the fabrication of nanoscale atom optical devices.

Temperature-dependent density profiles of trapped boson-fermion mixtures

Abstract: We present a semiclassical three-fluid model for a Bose-condensed mixture of interacting Bose and Fermi gases confined in harmonic traps at finite temperature. The model is used to characterize the experimentally relevant behaviour of the equilibrium density profile of the fermions with varying composition and temperature across the onset of degeneracy, for coupling strengths relevant to a mixture of 39K and 40K atoms.

Phase properties of a Jaynes-Cummings model with Stark shift and Kerr medium

Abstract: Phase properties of the field interacting with a two-level atom in a lossless cavity Jaynes-Cummings model, taking into account the level shifts produced by Stark effect with an additional Kerr medium for one-mode are studied using the phase formalism of Pegg and Barnett. It is shown in particular that phase properties of the field reflect the collapse and revival phenomena. The results for the time evolution of the phase probability distribution and the phase fluctuations are obtained. The effect of Stark shift on the phase properties in both the absence and presence of a Kerr medium is analyzed. Phase localization is found for certain choice of the parameters.

Memory effects in speed-changing collisions and their consequences for spectral lineshape: I. Collision regime

Abstract: A consistent approach to the lineshape in the binary collision regime accounting for speed-changing collisions based on a convenient memory function is proposed. The consequences of memory effects on the spectral lineshape, through speed-dependent line broadening and line shifting mechanisms, are studied in detail for the prototype H2-Ar system. The connection between the present memory approach and the analytical model SC + D (“speed changing” and “dephasing” collisions) based on the hard collision approximation is displayed.

Quantum and semiclassical dynamics of differential optical collisions

Abstract: We apply quantum and semiclassical theories to differential optical collisions Na(32S1/2) + Kr + Na(32P1/2,3/2) + Kr. Our results provide a basis to analyze recent experiments in which for the first time optical collisions were investigated with angular resolution under crossed-beam conditions. A characteristic feature of the differential cross sections is the pronounced oscillatory structure due to interferences of different Condon paths. These Stueckelberg oscillations form an extremely sensitive probe of the collisional dynamics and of the molecular interactions. We demonstrate perspectives to determine geometric properties of the collision complex by excitation with polarized light. By final state analysis nonadiabatic (spin-orbit, rotational) interactions can be studied with complete control of the path. In summary it is shown that the method of differential detection of optical collisions opens a variety of new accesses to atomic and molecular subcollisions.

Calculation of Wannier-Bloch and Wannier-Stark states

Abstract: The paper discusses the metastable states of a quantum particle in a periodic potential under a constant force (the model of a crystal electron in a homogeneous electric field), which are known as the Wannier-Stark ladder of resonances. An efficient procedure to find the positions and widths of resonances is suggested and illustrated by numerical calculations for a cosine potential, which are in excellent agreement with complex scaling resonance energies.

Atomic beam deflection by coherent momentum transfer and the dependence on weak magnetic fields

Abstract: The deflection of Ne atoms in the metastable state 3 P 2 by coherent transfer of the momenta of 4 or 8 photons is demonstrated, based on the technique developed for coherent population transfer with delayed pulses (STIRAP). After deflection the intensity profile of the isotope of mass 20 is fully seperated from that for the undeflected atoms of mass 22. It is furthermore shown, how the interplay of Larmor precession of the electronic magnetic moment and the sequential deflection in two spatially separated zones can be used to measure the magnetic field, integrated over the flight-path between the transfer zones.

Monte-Carlo simulations of rotating clusters

Abstract: A new scheme for estimating densities of states at non zero angular momentum is proposed, using the Monte-Carlo (MC) and multiple histogram methods. It is based on a rigorous expression of the classical density of states for a rotating system. Two features appear: the centrifugal energy ( angular momentum and I the instantaneous inertia tensor in the center of mass reference frame) is added to the potential energy and the configurational densities of states is weighted by . Comparing the MC results for the 13-atom Lennard-Jones cluster and a calculation based on molecular dynamics (MD) shows that this weight is important if the rotation induces a structural change at a finite temperature. The MC algorithm proves to be much more efficient than MD, even at finite .

Ab initio studies of stationary points of the Al O molecule

Abstract: We report a theoretical ab initio investigation on energetically low-lying stationary points of the Al2O3 molecular system. The calculations were performed at the Hartree-Fock (HF) and second-order Moller-Plesset (MP2) frozen core level of approximation using the standard 6-31G(d) basis set. Several isomeric singlet as well as higher spin states of Al2O3 which lie close to each other within an energy range of about 8 eV (at MP2) are characterised. The lowest of these stationary points is in fact a triplet state of planar symmetry. It is by 0.08 eV (MP2) lower than the often discussed linear singlet state. Atomisation energies for all species are quite large showing that the system is strongly bound. Energies, harmonic vibrational modes, and geometric parameters are compared with the results of earlier work by Solomonik and Sliznev [1], Nemukhin and Weinhold [2], Andrews et al. [3] and Desai et al. [4]. Based on our calculations we give a tentative assignment of some selected vibrational wave numbers and an interpretation of some features of the photoelectron spectrum.

Can we rationalize the structure of small silicon-carbon clusters?

Abstract: A theoretical study of \(\) clusters with \(\) using density functional theory is presented. Tests of various functionals demonstrate that local spin density approximation (LSDA) is the most adequate functional for the study of these systems. Structures, vibrational frequencies, and IR intensities of the lowest energy isomer of the studied clusters obtained using LSDA are described, and the unusual properties of the Si-C clusters are discussed. A quantitative analysis of the obtained structures was carried out, and relations between the coordinations, interatomic distances, and angles observed in the Si-C clusters were obtained through introduction of the notion of coordination. This analysis also shows that the carbon atoms mainly exhibit sp and sp2 hybridizations, and that a majority of silicon atoms do not hybridize. This study is the fi rst step of the implementation of a semi-empirical potential, which would describe the moderately small Si-C clusters.