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Single ionization of H$_2$ molecules exposed to strong and short laser pulses is investigated by a semi-classical method. Three laser characteristics are considered: i) The carrier-wave frequency corresponds to wavelengths covering and bridging the two ionization regimes: From tunnel ionization (TI) at 800 nm to multiphoton ionization (MPI) at 266 nm. ii) Values of the peak intensity are chosen within a window to eliminate competing double ionization processes. iii) Particular attention is paid to the polarization of the laser field, which can be linearly or circularly polarized. The results and their interpretation concern two observables, namely the end-of-pulse total ionization probability and vibrational distribution generated in the cation H$_2^+$. The most prominent findings are an increased ionization efficiency in circular polarization and a vibrational distribution of the cation that favors lower-lying levels than those that would be populated in a vertical (Franck-Condon) ionization, leading to non Franck-Condon distributions, both in linear and circular polarizations.

International audience; In contrast to light, matter-wave optics of quantum gases deals with interactions even in free space and for ensembles comprising millions of atoms. We exploit these interactions in a quantum degenerate gas as an adjustable lens for coherent atom optics. By combining an interaction-driven quadrupole-mode excitation of a Bose-Einstein condensate (BEC) with a magnetic lens, we form a time-domain matter-wave lens system. The focus is tuned by the strength of the lensing potential and the oscillatory phase of the quadrupole mode. By placing the focus at infinity, we lower the total internal kinetic energy of a BEC comprising 101(37) thousand atoms in three dimensions to 3/2 kB⋅38+6−7 pK. Our method paves the way for free-fall experiments lasting ten or more seconds as envisioned for tests of fundamental physics and high-precision BEC interferometry, as well as opens up a new kinetic energy regime.

The stability of matter-wave sensors benefits from interrogating large-particle-number atomic ensembles at high cycle rates. The use of quantum-degenerate gases with their low effective temperatures allows constraining systematic errors towards highest accuracy, but their production by evaporative cooling is costly with regard to both atom number and cycle rate. In this work, we report on the creation of cold matter-waves using a crossed optical dipole trap and shaping it by means of an all-optical matter-wave lens. We demonstrate the trade off between residual kinetic energy and atom number by short-cutting evaporative cooling and estimate the corresponding performance gain in matter-wave sensors. Our method is implemented using time-averaged optical potentials and hence easily applicable in optical dipole trapping setups., Comment: 10 pages, 5 figures

We describe the so-called “Transient Grating Method” which is a non-invasive experimental technique well suited to measure the dispersion properties of ordered or disordered nanostructures and thin films, at frequencies up to a few GHz. This pump-and-probe technique involves the interference pattern produced by two in-coming IR optical pulses to set a standing elastic wave on the surface of the sample through photoelastic processes. The wave vector of this elastic wave can be easily tuned by adjusting the angle between the two incident beams. Two continuous visible laser beams in a heterodyne detection scheme are used to detect the vibrations on the surface and in turn the dispersion of the related elastic modes. The achievements of the technique are illustrated by the measurement of the dispersion of surface acoustic waves in piezogenerators consisting of GaN nanowires embedded into a dielectric matrix (HSQ). We then report on the analysis of the experimental data that allowed extracting the elastic parameters of this composite medium.

Space offers virtually unlimited free-fall in gravity. Bose-Einstein condensation (BEC) enables ineffable low kinetic energies corresponding to pico- or even femtokelvins. The combination of both features makes atom interferometers with unprecedented sensitivity for inertial forces possible and opens a new era for quantum gas experiments. On January 23, 2017, we created Bose-Einstein condensates in space on the sounding rocket mission MAIUS-1 and conducted 110 experiments central to matter-wave interferometry. In particular, we have explored laser cooling and trapping in the presence of large accelerations as experienced during launch, and have studied the evolution, manipulation and interferometry employing Bragg scattering of BECs during the six-minute space flight. In this letter, we focus on the phase transition and the collective dynamics of BECs, whose impact is magnified by the extended free-fall time. Our experiments demonstrate a high reproducibility of the manipulation of BECs on the atom chip reflecting the exquisite control features and the robustness of our experiment. These properties are crucial to novel protocols for creating quantum matter with designed collective excitations at the lowest kinetic energy scales close to femtokelvins., 6 pages, 4 figures

International audience; We report on the ultrafast vibrational response of single Ge-Si core-shell nanowires obtained by epitaxial growth and investigated by femtosecond transient reflectivity and coherent x-ray diffraction measurements. The oscillations of the sample reflectivity are correlated with the fundamental breathing mode for wires with a diameter ranging from 150 to 350 nm and compared with solutions of the Navier equation. Taking advantage of a free standing geometry, we are able to get a mechanical quality factor of higher than 80. Coupling electron microscopy and pump and probe investigations with a very high spectral resolution performed on the same wire, we demonstrate that both shell and core diameter fluctuations are revealed and quantified. X-ray coherent diffraction measurements on individual nanowires evidence changes in the Ge-core diameter and different strain states along a single structure.

The fringe pattern that allows geometrical and orbital structure information to be extracted from LIED spectra of symmetric molecules is shown to reflect a symmetry conservation principle. We show that under a field polarization which preserves certain symmetry elements of the molecule, the symmetry character of the initial wave function is conserved during its time-evolution. We present a symmetry analysis of a deviation from a perfect alignment by decomposing the field into a major, symmetry-determining part, and a minor, symmetry breaking, part. This decomposition leads to a corresponding factorization of the time-evolution operator. The formalism is applied to the analysis of the robustness of LIED readings and inversions with respect to deviations from a perfect perpendicular and parallel alignment of a symmetric ABA triatomic molecule. The results indicate a particularly strong stability of the type of LIED spectra associated with the perpendicular alignment situation.

In this paper, we discuss the possibility of imaging molecular orbitals from photoelectron spectra obtained via Laser Induced Electron Diffraction (LIED) in linear molecules. This is an extension of our work published recently in Physical Review A \textbf{94}, 023421 (2016) to the case of the HOMO-1 orbital of the carbon dioxide molecule. We show that such an imaging technique has the potential to image molecular orbitals at different internuclear distances in a sub-femtosecond time scale and with a resolution of a fraction of an Angstr\"om.

Coherence among rotational ion channels during photoionization is exploited to control the anisotropy of the resulting photoelectron angular distributions at specific photoelectron energies. The strategy refers to a robust and single parameter control using two ultrashort light pulses delayed in time. The first pulse prepares a superposition of a few ion rotational states, whereas the second pulse serves as a probe that gives access to a control of the molecular asymmetry parameter $\ensuremath{\beta}$ for individual rotational channels. This is achieved by tuning the time delay between the pulses, leading to channel interferences that can be turned from constructive to destructive. The illustrative example is the ionization of the $E{(}^{1}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}$) state of ${\mathrm{Li}}_{2}$. Quantum wave-packet evolutions are conducted including both electronic and nuclear degrees of freedom to reach angle-resolved photoelectron spectra. A simple interference model based on coherent phase accumulation during the field-free dynamics between the two pulses is precisely exploited to control the photoelectron angular distributions from almost isotropic to marked anisotropic.

International audience; The present research aims at studying the elastic properties of III-Nitride nanowires (NWs) embedded in HSQ matrix, capable to efficiently convert the mechanical energy into electricity. We report on elastic properties of such novel nano-engineered functional materials studied by using "transient grating" acoustic experiment.

Using a quantum wave packet simulation including the nuclear and electronic degrees of freedom, we investigate the femtosecond and picosecond energy- and angle-resolved photoelectron spectra of the E($^1\Sigma_g^+$) electronic state of Li$_2$. We find that the angular distributions of the emitted photoelectrons depend strongly on the pulse duration in the regime of ultrashort laser pulses. This effect is illustrated by the extraction of a time-dependent asymmetry parameter whose variation with pulse duration can be explained by an incoherent average over different ion rotational quantum numbers. We then derive for the variation of the asymmetry parameter a simple analytical formula, which can be used to extract the asymptotic CW asymmetry parameters of individual transitions from measurements performed with ultra-short pulses., Comment: 8 pages, 8 figures

We extend the model of exciton-plasmon materials to include a ro-vibrational structure of molecules using wave-packet propagations on electronic potential energy surfaces. The new model replaces conventional two-level emitters with more complex molecules allowing to examine the influence of alignment and vibrational dynamics on strong coupling with surface plasmon-polaritons. We apply the model to a hybrid system comprising a thin layer of molecules placed on top of a periodic array of slits. Rigorous simulations are performed for two types of molecular systems described by vibrational bound-bound and bound-continuum electronic transitions. Calculations reveal new features in transmission, reflection and absorption spectra including the observation of significantly higher values of the Rabi splitting and vibrational patterns clearly seen in the corresponding spectra. We also examine the influence of anisotropic initial conditions on optical properties of hybrid materials demonstrating that the optical response of the system is significantly affected by an initial pre-alignment of the molecules. Our work demonstrates that pre-aligned molecules could serve as an efficient probe for the sub-diffraction characterization of the near-field near metal interfaces., 14 pages, 4 figures, submitted to PRB

Imaging structures at the molecular level is a fast developing interdisciplinary research field that spans across the boundaries of physics and chemistry. High spatial resolution images of molecules can be obtained with photons or ultrafast electrons. In addition, images of valence molecular orbitals can be extracted via tomographic techniques based on the coherent XUV radiation emitted by a molecular gas exposed to an intense ultra-short infrared laser pulse. In this paper, we demonstrate that similar information can be obtained by inverting energy resolved photoelectron spectra using a simplified analytical model.

We introduce a non-Hermitian Schrodinger-type approximation of optical Bloch equations for two-level systems. This approximation provides a complete and accurate description of the coherence and decoherence dynamics in both weak and strong laser fields at the cost of losing accuracy in the description of populations. In this approach, it is sufficient to propagate the wave function of the quantum system instead of the density matrix, providing that relaxation and dephasing are taken into account via automatically adjusted time-dependent gain and decay rates. The developed formalism is applied to the problem of scattering and absorption of electromagnetic radiation by a thin layer comprised of interacting two-level emitters.

International audience; We report the analysis of elastic properties of a composite medium consisting of GaN nanowires embedded into a dielectric matrix, which constitutes the active material of nanowire-based piezogenerators. A non-invasive technique, Transient Grating Method, is used to generate and analyze the propagation of acoustic waves in the frequency range from a few tens of MHz to several GHz. The wave propagation is simulated using the time-efficient computer model based on the explicit integral and asymptotic representations for laser-generated surface acoustic waves in layered anisotropic waveguides. Effective parameters of the multilayered model are obtained via the minimization of the discrepancy between the measured and calculated wave characteristics. A comparison with a reference composed of a dielectric matrix without nanowires shows that the mechanical behavior is strongly influenced by the presence of the nanostructures.

Brillouin light scattering, Raman light scattering and visible–infrared reflectometry techniques have been used to investigate, respectively, the elastic properties, the phonons and the optical properties of bulk textured polycrystalline yttrium–aluminum garnet doped with 2 at% neodymium obtained by the sintering of commercial oxides. From the analysis of the observed bulk longitudinal and transverse acoustic modes with the knowledge of the refractive index 1.81 inferred from the visible reflectometry, the two independent effective elastic constants of the isotropic polycrystal C11 = 362 GPa and (C11 − C12)/2 = 121 GPa are determined leading to the value of the bulk modulus B =( C11 +2 C12)/3 = 200 GPa. The ratio e0/e∞ = 3.1 and the optic permittivity e∞ = 3.46 are derived from the infrared reflectivity data. Pair potential calculations of the three single crystal elastic constants c11 = 340, c12 = 127 and c44 = 112 GPa, of the bulk modulus B =( c11 +2 c12)/3 = 198 GPa, of the zone-center (� ) phonons and of the permittivity function provide good comparison with our experimental results. © 2007 Elsevier Ltd. All rights reserved.

International audience; From radio-electronics signal analysis to biological sample actuation, surface acoustic waves (SAWs) are involved in a multitude of modern devices. However, only the most simple standing or progressive waves such as plane and focused waves have been explored so far. In this paper, we expand the SAW toolbox with a wave family named "swirling surface acoustic waves" which are the 2D anisotropic analogue of bulk acoustic vortices. Similarly to their 3D counterpart, they appear as concentric structures of bright rings with a phase singularity in their center resulting in a central dark spot. After the rigorous mathematical definition of these waves, we synthesize them experimentally through the inverse filtering technique revisited for surface waves. For this purpose, we design a setup combining arrays of interdigitated transducers and a multichannel electronic that enables one to synthesize any prescribed wave field compatible with the anisotropy of the substrate in a region called the “acoustic scene.” This work opens prospects for the design of integrated acoustic vortex generators for on-chip selective acoustic tweezing.

In this reply we show that the criticisms raised by J. Noronha are based on a misapplication of the model we have proposed in [A. Jaouadi, M. Telmini, E. Charron, Phys. Rev. A 83, 023616 (2011)]. Here we explicitly discuss the range of validity of the approximations underlying our analytical model. We also show that the discrepancies pointed out for very small atom numbers and for very anisotropic traps are not surprising since these conditions exceed the range of validity of the model., Comment: Physical Review A, American Physical Society, 2015, pp.Not available yet

We present a detailed, realistic analysis of the implementation of a proposal for dipole-induced electromagnetic transparency (DIET) [R. Puthumpally-Joseph, M. Sukharev, O. Atabek, and E. Charron, Phys. Rev. Lett. 113, 163603 (2014)] using an ensemble of cold atoms at high density. Using both direct numerical simulations and simple analytical models, we show how, in a realistic $N$-level quantum system, narrow transparency windows can appear at large densities. The existence of such windows is attributed to quantum interference effects in overlapping resonances. Our analysis is applied to the ${D}_{1}$ transition of Rb atoms, and we show that, at high densities, Rb can behave like a simple three-level emitter exhibiting all the properties of DIET. Some interesting effects such as slow light are also presented, and their limits in the context of DIET are discussed

International audience; We report on thermal investigations performed in a time resolved experimental scheme. The time domain thermoreflectance (TDTR) is applied in an unusual geometry where the pump and probe beams are not superimposed but focused and shifted. In this way, the determination of the in-plane thermal diffusivity is achieved from temperature snapshots at different time delays. In the first part, taking into account the specific generation process and the detection inherent to the time domain thermoreflectance approach, an analytical solution for the temperature field is obtained for bulk samples, and compared to experimental data. A comparison with the frequency domain thermoreflectance microscopy is also outlined. In Part II section, the lateral heat diffusion in a layered structure is investigated. The comparison of the heat diffusion spreading in case of a highly conductive layer deposited on an insulator substrate and the reverse situation are carefully studied. Finally, we show how the time dependence is efficient to probe and identify material thermal properties or thermal interfacial resistance. (C) 2015 AIP Publishing LLC.

International audience; The morphological structure and optical properties of the wings of 14 species of Morphidae have been investigated. Most of the scales of the iridescent species of Morphidae (Lepidoptera) present a very particular structure. The ground scales, responsible for the major part of the optical properties, are covered by a very regular set of longitudinal ridges. The ridges themselves are constituted by a superposition of lamellae that act locally as a multilayered structure. This very specific morphology leads to both interferences and diffraction effects. The first one is responsible of the brilliant blue coloration of the males, while the second one diffracts this colored light at a very large angle. These two phenomena give to the butterfly a very effective long-range communication system. The morphological characteristics of the scales of the various species are presented in detail.Two types of optical measurement were performed on the iridescent wings of 14 different species of Morphidae: spectroscopic measurements under various incidences and gonioscopic measurements for a given incidence angle and wavelength. The first allows a determination of the index of refraction of the cuticular material. The second leads to the drawing of spatial diffraction maps. It shows that most of the reflected light is diffracted laterally over a very large angle (90° < θ < 120°, according to the different species) and that this repartition depends of the polarization of incident light. As predicted by previous calculations, the dissymmetric structure of the ridge is responsible for the separation of the polarization modes in the various diffraction orders.

We investigate the range of applicability of the approximations of adiabatic and semiadiabatic elimination of the continuum in the numerical simulation of strong field photodissociation processes. By comparing exact and approximate calculations of the fragmentation of the H2 molecular ion when excited by a short and intense laser pulse, we show that these approximations do not hold when a few excited vibrational states are transiently populated. This failure mainly arises from the fast variation of the phases of the excited-state time-dependent amplitudes. © 2004 Wiley

Non-perturbative time-dependent calculations of single and double ionization of a one-dimensional model of atomic calcium by short and intense laser pulses were performed at various wavelengths. The comparison of the probabilities calculated within a two-active electron (TAE) approach with those obtained using a single-active electron (SAE) approximation clearly demonstrates the crucial role played by the electronic correlation and by doubly excited states within this model, even for the formation of Ca+.

Butterflies of the Morphidae family probably constitute the most popular example of iridescent colours in nature. They present, many fundamental interests in biomimetic processes and can be used as models or sources of inspiration for many industrial applications. The microstructures of the scales are responsible for the coloration, with a major part attributed to interference effects. Their actual optical properties have been described in detail, but the refraction index of the cuticular material is not known with precision. In this paper, we investigate the absolute diffuse reflectivity of the dorsal side of the wings of the species Morpho menelaus. An effective medium theory applied to a multilayer film permits the determination of the anisotropic index of refraction of the cuticle material in the 350–800 nm range. 2003 Elsevier B.V. All rights reserved.

Recent experiments have investigated the dissociative ionization of NaI and CsI, each aggregated with a molecule of acetonitrile CH3CN, using two femtosecond laser pulses. The first, the pump pulse, excites the NaI or CsI diatom to a dissociative state. The second, the probe pulse, then ionizes the sodium or cesium atom after a variable delay time, and the resulting ion is detected. In the case of NaI, the ion signal is characterized by a single maximum. For CsI, however, an oscillatory signal is observed. By performing two-dimensional wave packet propagations, we are able to reproduce this behavior, which can be explained using simple physical arguments.

We determine the optical response of a thin and dense layer of interacting quantum emitters. We show that, in such a dense system, the Lorentz redshift and the associated interaction broadening can be used to control the transmission and reflection spectra. In the presence of overlapping resonances, a dipole-induced electromagnetic transparency (DIET) regime, similar to electromagnetically induced transparency (EIT), may be achieved. DIET relies on destructive interference between the electromagnetic waves emitted by quantum emitters. Carefully tuning material parameters allows us to achieve narrow transmission windows in, otherwise, completely opaque media. We analyze in detail this coherent and collective effect using a generalized Lorentz model and show how it can be controlled. Several potential applications of the phenomenon, such as slow light, are proposed.

We present a theoretical treatment for the single and double ionization of molecules in strong laser pulses, including the effect of electron correlations. Applied to a model N2 molecule, our simulations show that double ionization occurs via a direct two-electron mechanism at moderate laser intensities , while a step-by-step sequential ionization process dominates at higher intensity . At intermediate intensity, these two mechanisms have a comparable contribution to the total double-ionization yield. This phenomenon is directly reflected in the spatial distributions of N2+ and N22+ in the focal volume of the laser pulse. In addition we show that at high intensity the singly and doubly charged molecular ions are created in well separated regions of space.

The femtosecond dynamics of NaI two-body (NaI+hν→NaI++e−) and three-body (NaI+hν→Na++I+e−) break-up is investigated by quantum mechanical simulations. A first femtosecond laser pulse creates an excited molecular wave packet, and an additional ultrafast photoionization step monitors its subsequent time evolution. The theoretical results are compared with available experimental data. The branching ratio between these two competing fragmentation channels is extracted and an experimental scenario is proposed to control the outcome of this reactive half-collision.

International audience; The transient acoustic pulse emitted by a single InAs quantum dots layer was measured depending on the pump wavelength. By tuning the pump wavelength through the transition energies of the GaAs barrier, the wetting layer and the quantum dots themselves, the acoustic phonon emission is shown to be strongly correlated to the electronic structure of the quantum dots layer system. The contributions of the wetting layer and the quantum dots to the acoustic signal were clearly identified and quantified by comparing different regions of the same sample containing or not quantum dots. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4789872]

International audience; We report on a complete elastic characterization by picosecond ultrasonics technique of lithium phosphorous oxynitride (LiPON) thin films, which are nowadays, considered as promising solid-state electrolytes for microbatteries technology. Longitudinal sound velocity is deduced from Brillouin oscillation detection at variable incidence angles. The density is estimated through an interfacial energy analysis related to the echo amplitude decay. Transverse wave velocity is obtained across the dispersion curve of Rayleigh surface waves. Combining these different approaches, Young's modulus and Poisson's ratio of LiPON are finally found to be 79 GPa and 0.27, respectively. (C) 2013 Elsevier B. V. All rights reserved.

The fragmentation dynamics of the hydrogen molecular ion H2+ and of its isotopic derivate HD+subjected to an intense pulsed laser radiation are studied using quantum wave packet propagations. It is shown that bichromatic optical excitations are subject to a high degree of control through the variation of the relative phase between the two fields. A phase‐locked (ω,2ω) laser pulse is used to induce asymmetry in the angular distribution of the emitted fragments. In addition, an appreciable isotope separation in the fragmentation of HD+ is predicted. The critical role of quantum molecular interferences in such phase‐controllable processes is demonstrated.

The photodissociation of the ${\mathrm{HD}}^{+}$ molecular ion in intense short-pulsed linearly polarized laser fields is studied using a time-dependent wave-packet approach where molecular rotation is fully included. We show that applying a coherent superposition of the fundamental radiation with its second harmonic can lead to asymmetries in the fragment angular distributions, with significant differences between the hydrogen and deuterium distributions in the long wavelength domain where the permanent dipole is most efficient. This effect is used to induce an appreciable isotope separation.

A number of unexpected features of small molecules subjected to intense laser fields, with wavelengths ranging from infrared to ultraviolet, have been observed or predicted in the past few years: above-threshold dissociation, molecular bond softening, vibrational population trapping. We review these processes for the case of the molecular ion H2+ and discuss the experimental and theoretical tools that are used to study this system. Both electron and proton energy distributions are used to interpret the experimental results. Theoretically, the fragmentation dynamics can be described equivalently as a laser-assisted half-collision process, using solutions of the time-independent Floquet theory, or as the evolution of a wavepacket subjected to a classical radiation held with a given pulse shape, using solutions of the time-dependent Schrodinger equation. A broad range of laser intensity and pulsewidth has been explored, with the short-pulse results (analysed in terms of 'dressed' potential curves) offering the best interface between theory and experiment. We finally report on a promising new avenue for coherent control of fragmentation dynamics, through the use of two-colour phase-locked radiation.

International audience; We explore the laser-induced ionization dynamics of N2 and CO2 molecules subjected to a few-cycle, linearly polarized, 800\,nm laser pulse using effective two-dimensional single active electron time-dependent quantum simulations. We show that the electron recollision process taking place after an initial tunnel ionization stage results in quantum interference patterns in the energy resolved photo-electron signals. If the molecule is initially aligned perpendicular to the field polarization, the position and relative heights of the associated fringes can be related to the molecular geometrical and orbital structure, using a simple inversion algorithm which takes into account the symmetry of the initial molecular orbital from which the ionized electron is produced. We show that it is possible to extract inter-atomic distances in the molecule from an averaged photon-electron signal with an accuracy of a few percents.

International audience; We address the feasibility of imaging geometric and orbital structure of a polyatomic molecule on an attosecond time-scale using the laser induced electron diffraction (LIED) technique. We present numerical results for the highest molecular orbitals of the CO2 molecule excited by a near infrared few-cycle laser pulse. The molecular geometry (bond-lengths) is determined within 3% of accuracy from a diffraction pattern which also reflects the nodal properties of the initial molecular orbital. Robustness of the structure determination is discussed with respect to vibrational and rotational motions with a complete interpretation of the laser-induced mechanisms.

International audience; We investigate theoretically an original route to achieve Bose-Einstein condensation using dark power-law laser traps. We propose to create such traps with two crossing blue-detuned Laguerre-Gaussian optical beams. Controlling their azimuthal order $\ell$ allows for the exploration of a multitude of power-law trapping situations in one, two and three dimensions, ranging from the usual harmonic trap to an almost square-well potential, in which a quasi-homogeneous Bose gas can be formed. The usual cigar-shaped and disk-shaped Bose-Einstein condensates obtained in a 1D or 2D harmonic trap take the generic form of a "finger" or of a "hockey puck'' in such Laguerre-Gaussian traps. In addition, for a fixed atom number, higher transition temperatures are obtained in such configurations when compared with a harmonic trap of same volume. This effect, which results in a substantial acceleration of the condensation dynamics, requires a better but still reasonable focusing of the Laguerre-Gaussian beams.

Bose-Einstein condensation (BEC) of an ideal gas is investigated, beyond the thermodynamic limit, for a finite number $N$ of particles trapped in a generic three-dimensional power-law potential. We derive an analytical expression for the condensation temperature ${T}_{c}$ in terms of a power series in ${x}_{0}={\ensuremath{\varepsilon}}_{0}/{k}_{B}{T}_{c}$, where ${\ensuremath{\varepsilon}}_{0}$ denotes the zero-point energy of the trapping potential. This expression, which applies in Cartesian, cylindrical, and spherical power-law traps, is given analytically at infinite order. It is also given numerically for specific potential shapes as an expansion in powers of ${x}_{0}$ up to the second order. We show that, for a harmonic trap, the well-known first-order shift of the critical temperature $\ensuremath{\Delta}{T}_{c}/{T}_{c}\ensuremath{\propto}{N}^{\ensuremath{-}1/3}$ is inaccurate when $N\ensuremath{\leqslant}{10}^{5}$, the next order (proportional to ${N}^{\ensuremath{-}1/2}$) being significant. We also show that finite-size effects on the condensation temperature cancel out in a cubic trapping potential, e.g., $V(r)\ensuremath{\propto}{r}^{3}$. Finally, we show that in a generic power-law potential of higher order, e.g., $V(r)\ensuremath{\propto}{r}^{\ensuremath{\alpha}}$ with $\ensuremath{\alpha}g3$, the shift of the critical temperature becomes positive. This effect provides a large increase of ${T}_{c}$ for relatively small atom numbers. For instance, an increase of about $+$40% is expected with ${10}^{4}$ atoms in a $V(r)\ensuremath{\propto}{r}^{12}$ trapping potential.

The extension of the discrete dipole approximation (DDA) model to the calculation of the ultrafast transient optical response of metal nanoparticles, whatever their shape, is presented. For this, the DDA is combined with the resolution of the Boltzmann equation for the conduction electrons within the low-perturbation limit and the Rosei model for the metal band structure. This approach thus takes into account the athermal regime for the electron gas. It is then illustrated in the cases of gold nanospheres and nanopeanuts. The spectral dependence of the ultrafast response is shown to be very sensitive to the particle morphology, the surface plasmon resonance enhancing the transient modification of the interband transitions. These results are used to interpret spectrally resolved pump-probe measurements performed on a dense nanoparticle assembly, demonstrating selective-excitation and selective-detection processes.

The temporal dynamics of double ionization of ${\text{H}}_{2}$ has been investigated both experimentally and theoretically with few-cycle laser pulses. The main observables are the proton spectra associated with the ${\text{H}}^{+}+{\text{H}}^{+}$ fragmentation channel. The model is based on the time-dependent Schr\"odinger equation and treats the electronic and nuclear coordinates on the same level. Therefore it allows the ultrafast nuclear dynamics to be followed as a function of the laser pulse duration, carrier-envelope phase offset, and peak intensity. We mainly report results in the sequential double-ionization regime above $2\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.3em}{0ex}}\text{W}\phantom{\rule{0.2em}{0ex}}{\text{cm}}^{\ensuremath{-}2}$. The proton spectra are shifted to higher energies as the pulse duration is reduced from 40 down to 10 fs. The good agreement between the model predictions and the experimental data at 10 fs permits a theoretical study with pulse durations down to a few femtoseconds. We demonstrate the very fast nuclear dynamics of the ${\text{H}}_{2}{}_{+}$ ion for a pulse duration as short as 1 fs between the two ionization events, giving $\text{H}_{2}{}^{+}$ from ${\text{H}}_{2}$ and ${\text{H}}^{+}+{\text{H}}^{+}$ from $\text{H}_{2}{}^{+}$. The carrier-envelope phase offset plays a significant role only for pulse durations shorter than 4 fs. At 10 fs, the laser intensity dependence of the proton spectra is fairly well reproduced by the model.

We detail and extend the results of [Milman {\it et al.}, Phys. Rev. Lett. {\bf 99}, 130405 (2007)] on Bell-type inequalities based on correlations between measurements of continuous observables performed on trapped molecular systems. We show that for some observables with a continuous spectrum which is bounded, one is able to construct non-locality tests sharing common properties with those for two-level systems. The specific observable studied here is molecular spatial orientation, and it can be experimentally measured for single molecules, as required in our protocol. We also provide some useful general properties of the derived inequalities and study their robustness to noise. Finally, we detail possible experimental scenarii and analyze the role played by different experimental parameters., Comment: 10 pages and 5 figures

We introduce Bell-type inequalities allowing for non-locality and entanglement tests with two cold heteronuclear molecules. The proposed inequalities are based on correlations between each molecule spatial orientation, an observable which can be experimentally measured with present day technology. Orientation measurements are performed on each subsystem at diferent times. These times play the role of the polarizer angles in Bell tests realized with photons. We discuss the experimental implementations of the proposed tests, which could also be adapted to other high dimensional quantum angular momenta systems., 4 pages

The manipulation of cold atoms with optical fields is a very promising technique for a variety of applications ranging from laser cooling and trapping to coherent atom transport and matter wave interferometry. Optical fields have also been proposed as interesting tools for quantum information processing with cold atoms. In this paper, we present a theoretical study of the dynamics of a cold 87Rb atomic cloud falling in the gravity field in the presence of two crossing dipole guides. The cloud is either deflected or split between the two branches of this guide. We explore the possibilities of optimization of this device and present preliminary results obtained in the case of zero-temperature dilute Bose-Einstein condensates., Comment: Proceedings of the International Spectroscopy Conference ISC-2007, Sousse, Tunisia

We show how quantum optimal control theory can help achieve high-fidelity quantum gates in real experimental settings. We discuss several optimization methods (from iterative algorithms to optimization by interference and to impulsive control) and different physical scenarios (from optical lattices to atom chips and to Rydberg atoms).