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The mutual neutralization reaction A⁺ + B⁻ → A⁰ + B⁰ plays an important role whenever an anion populated plasma is involved (interstellar molecular clouds, ionospheres, stellar atmospheres, etc.), and modifies its radiative emissions which are used for estimating stars metallicity. Additionally, the measurement of mutual neutralization can serve as a benchmark for the numerical models estimating the inelastic collisions used in these models of reaction dynamics. In this manuscript, we present results for several low energy mutual neutralization reactions measured with the UCLouvain single pass merged beam apparatus: He⁺/Li⁺ + H⁻/D⁻, C⁺/N⁺/O⁺ + D⁻/C⁻/O⁻/Si⁻/P⁻/S⁻, Na⁺ + O⁻/S⁻, and Ne⁺ + C⁻/O⁻/S⁻. We detail how a pair of time and position sensitive detectors allows us to obtain accurate branching ratios and partial differential cross sections. In order to rationalize the branching ratios we have measured, we developed a multi-channel Landau-Zener model combined with an updated version of the anion centered asymptotic (analytical) model (ACAM). The overall agreement between theory and experiment allows us to use this model to describe the mutual neutralization process. The ACAM validity domain is also studied, revealing additional processes responsible for the discrepancies between theory and experiment, such as the production of Rydberg series of the neutral atoms. The differential cross section was also computed using two models: the interrupted Rutherford scattering model and the impact parameter method. The former is analytical and describes well the collision energy dependency of the differential cross section while the latter gives quantitative results. It is also shown how differential cross section analysis provides a better insight into the mutual neutralization process, e.g. which molecular states are temporarily populated. As a conclusion, this manuscript suggests several ideas for improving both the theoretical models and the experimental setup., (SC - Sciences) -- UCL, 2022

We consider the interaction of atomic hydrogen, in its ground state, with an electromagnetic pulse whose duration is fixed in terms of the number of optical cycles. We study the probability of excitation of the atom in the static field limit i.e. for field frequencies going to zero. Despite the fact that the well-known Born–Fock adiabatic theorem is valid only for a system whose energy spectrum is discrete, we show that it is still possible to use this theorem to derive, in the low frequency limit, an analytical formula which gives the probability of transition to any excited state of the atom as a function of the field intensity, the carrier envelope phase and the number of optical cycles within the pulse. The results for the probability of excitation to lowlying excited states, obtained with this formula, agree with those we get by solving the timedependent Schrödinger equation. The domain of validity is discussed in detail.

We present and discuss a new computationally inexpensive method to study, within the single active electron approximation, the interaction of a complex system with an intense ultrashort laser pulse. As a first application, we consider the one photon single ionisation of the highest occupied molecular orbital of the water molecule by a laser pulse. The ionisation yield is calculated for different orientations of the molecule with respect to the field polarization axis and compared against predictions of another single active electron approach.

Among the plethora of nano-objects, those conducting electricity attracted a great deal of attention with interesting transport properties such as electrical conductance quantization, electron interferences, single charge effects etc. This thesis aims to study puzzling electron transport signatures emerging as electrons flow through nanoscale electrically-conducting networks patterned within two-dimensional electron systems (2DES), where charge transport occurs in the ballistic and coherent regime at low temperature. These networks, constituted from connected nanowires, exhibit peculiar electrical conductance variations that we managed to associate with electron interferences within individual one-dimensional wires and two-dimensional ballistic electron scattering. Local-scale data on electronic transport were collected with scanning gate microscopy (SGM) - a near-field technique where an electrostatically biased nanoscale conductive tip, scanned in the vicinity of the network, is used to tune the electrostatic potential felt by electrons crossing the network. SGM allowed us to decrypt electron transport mechanisms at the local scale, to demonstrate the presence of an unidimensional electron interferometer in the network and to pinpoint a charge injection mechanism analogous to the two-dimensional Rutherford scattering both experimentally and in simulations. In turn, this work provides useful tools in the perspective of building ‘electron optics’ devices, where a local modulation of the electrostatic potential enables, on one hand, to control Fabry-Pérot electron resonances and, on the other hand, to redirect the electron flow in a similar way as an optical lens curves light rays, allowing to study Rutherford-like ballistic scattering mechanisms., (FSA - Sciences de l'ingénieur) -- UCL, 2018

The strong dependence of Raman intensities with respect to the laser frequency is investigated thanks to the development of a first-principles methodology that relies on finite differences of the calculated dielectric function. This methodology is applied to the computation of the first-order frequency-dependent Raman intensity of silicon, gallium arsenide and silicon carbide, with excitonic effects described by the Bethe-Salpeter equation. We found these to be crucial for the accurate description of the experimental enhancement for laser photon energies around the gap. We also present a study on transition-metal dichalcogenides that reveals an atypical exciton-phonon interaction for WS2 and WSe2. This approach is generalized to the more complex second-order Raman intensity, with phonon losses coming from the entire Brillouin zone. Temperature effects are also important for optical properties. Indeed, vibrations of the atoms induce temperature-dependent modifications of the band structure that, in turn influence optical properties. In this work, we present a methodology for first-principles calculations of temperature-dependent band structure and optical properties, that is validated with respect to frequency-dependent and temperature-dependent Raman intensities of silicon. Finally, all the techniques presented in this work are combined to analyze the more difficult case of multiferroic BiFeO3. Recent experiments have shown strong laser-frequency dependence effects in Raman intensities but a small temperature dependence on first-order Raman. However, second-order Raman intensities show strong temperature dependence. Frequency-dependent Raman intensities and temperature-dependent band structures are computed and discussed., (FSA - Sciences de l'ingénieur) -- UCL, 2017

We consider the ionisation of atomic hydrogen by a strong infrared field. We extend and study in more depth an existing semi-analytical model. Starting from the time-dependent Schrödinger equation in momentum space and in the velocity gauge we substitute the kernel of the non-local Coulomb potential by a sum of N separable potentials, each of them supporting one hydrogen bound state. This leads to a set of N coupled one-dimensional linear Volterra integral equations to solve. We analyze the gauge problem for the model, the different ways of generating the separable potentials and establish a clear link with the strong field approximation which turns out to be a limiting case of the present model. We calculate electron energy spectra as well as the time evolution of electron wave packets in momentum space. We compare and discuss the results obtained with the model and with the strong field approximation and examine in this context the role of excited states.

We consider the interaction of atomic hydrogen with an intense laser field within the strong-field approximation. By using a Faddeev-like formalism, we introduce a new perturbative series in the binding potential of the atom. As a first test of this new approach, we calculate the electron energy spectrum in the very simple case of a photon energy higher than the ionisation potential. We show that by contrast to the standard perturbative series in the binding potential obtained within the strong field approximation, the first terms of the new series converge rapidly towards the results we get by solving the corresponding time-dependent Schrödinger equation.

Sir Peter Knight is a pioneer in quantum optics which has now grown to an important branch of modern physics to study the foundations and applications of quantum physics. He is leading an effort to develop new technologies from quantum mechanics. In this collection of essays, we recall the time we were working with him as a postdoc or a PhD student and look at how the time with him has influenced our research.

Photovoltaics, which convert sunlight into electricity, is one of the sustainable solutions to meet the global energy demand. In this framework, Cu2ZnSnS4 (CZTS) has attracted considerable attention as a potential earth-abundant and non-toxic absorber material for thin film solar cells due to its favorable electronic and optical properties. Despite the considerable efforts, its efficiency has remained low. One possible explanation is the cation disorder in the system. In this thesis, we investigate this issue using two different approaches. Firstly, since the arrangement of cations over several possible sites makes first-principles density functional theory (DFT) calculations impractical, we rely on a combination of DFT with cluster expansion and Monte Carlo methods to evaluate the thermodynamical properties. We identify the phase transitions and also analyze the nature of bandgap change with temperature. Secondly, since Raman and infrared spectroscopies are considered to be robust techniques for analyzing disorder in CZTS, we compute the vibrational properties of well-known ordered structures of CZTS (kesterite, stannite and PMCA) and compare them with experiments. The effect of different exchange-correlation functionals is also addressed both in a qualitative and quantitative manner. Further, newer features arising from the spectra of many other structures show a better matching with experiments. Both approaches highlight the importance of cation disorder on the properties of CZTS., (FSA - Sciences de l'ingénieur) -- UCL, 2017

The work presented in this thesis is dedicated to three aspects of the detachment and ionization of anions and atoms by photoabsorption and electron impact. The photodetachment of the hydrogen and oxygen anions is first studied. We report on experiments aiming at measuring, in an absolute manner, the cross sections governing the process. The development of the animated-crossed-beam technique for laser-ion interactions is described along with the design and operation of a velocity map imaging photoelectron spectrometer. The results obtained provide a complete picture of the photodetachment of O-, for which long-standing disagreements between theory and experiment are resolved, and provide benchmark values for H-. The double detachment of He- by intense, short light pulses is then investigated. The He+ spectrum associated with this 4-photon process exhibits series of peaks which are attributed to resonances between the initial state of the transient atom and high-lying Rydberg states. The combination of experiment, ab initio calculations and accurate modeling allows to understand the intricate dynamics underlying double detachment and to study their complex dependence on the light polarization and atom orientation. Finally, the electron impact ionization of metastable helium and He- is studied. A novel source of a fast, intense and pure beam of metastable helium is devised based on the photodetachment of the He- ion. The measured, absolute, total ionization cross section of metastable helium resolves a long-standing discrepancy between theory and experiment. The double ionization cross sections of metastable helium and the helium anion exhibit surprising behavior., (SC - Sciences) -- UCL, 2017

During recent years, new types of materials have been discovered with unique properties. One family of such materials is two-dimensional (2D) materials with unusual electronic, mechanical and optical properties, which led to their extensive study for diverse applications. Transition metal carbides, carbonitrides and nitrides (MXenes) are among the latest additions to the 2D world, which are derived from their layered precursors, MAX phases, layered early transition metal ternary carbides and nitrides, composed of M, an early transition metal, A, a group A element and X is C and/or N. In this thesis, as its self-telling title indicated, we present our work on the synthesis, structural characterization and the electron transport in the MAX phases and 2D MXenes. For MAX phases: using high temperature solution growth and slow cooling technique, several MAX phases single crystals have been successfully grown, including Cr2AlC, V2AlC, Ti3SiC2, etc. Structural characterization confirms the single crystalline character of the samples. Experimentally, a set of transport data was obtained from single crystals of V2AlC and Cr2AlC as a function of temperature and magnetic field. Theoretically, a general 2D nearly free electron model was proposed for describing the weak field magneto-transport properties of MAX phases. For MXenes: large-scale V2CTx (T stands for the surface terminations, hydroxyl, oxygen or fluorine)) MXene flakes were successfully synthesized. Mechanical delamination of multilayered V2CTx flakes into few layer flakes and transfer on Si/SiO2 substrate was also achieved. We then detailed the electrical device fabrication and proceeded with electrical measurements down to low temperature, with the aim to extract information on charge carrier behavior. First-hand transport data were obtained on V2CTx MXenes, which brings new understandings to this novel type of 2D material., (FSA - Sciences de l'ingénieur) -- UCL, 2017

We consider the interaction of a molecule with an ultrashort laser pulse within the mean field approximation.a model has been generalized to treat the interaction of atomic hydrogen with an electromagnetic pulse. The kernel of the Coulomb potential in the momentum space is replaced by a sum of products of separable potentials supporting a bound state of the atom and built from the corresponding bound state wave function. The molecular orbitals are expressed in terms of Gaussians and generated by means of a quantum chemistry software. We analyze the ionization yield as a function of the size of the Gaussian basis for H2 and H2O molecules and the results are compared to those already existing in the literature.

We analyze the interaction of a one-active electron system with a strong laser field. In order to extract information about the interaction mechanisms, it is important to consider perturbative expansions of the ionization probability amplitude in terms of the Coulomb potential. Based on some of Faddeev ideas, we introduce a new perturbative series that involves both the Coulomb and the Volkov propagators. We show that for photon energies higher than the ionization potential, the series converges rapidly towards the exact result as calculated by solving numerically the time-dependent Schrödinger equation. In the same conditions, the well known strong field approximation series is shown to diverge.

In the fields of condensed matter physics and electronics, the advent of new types of materials with unconventional charge carrier dynamics first gives possibilities to explore peculiar physical phenomena, which can then open novel routes for the design of electronic devices. It is the case of graphene, a two-dimensional carbon crystal where charge carriers behave like massless relativistic particles. To benefit from the outstanding properties potentially offered by such a material, the first step is to understand how charge carriers propagate through graphene devices in order to control their motion inside the considered structures. In this thesis, we present experimental results on the electronic transport inside two types of graphene-based devices : (I) nitrogen-incorporated graphene Hall bars and (II) graphene quantum ring (QR) interferometers. A particular attention was brought to the investigation of coherent effects, involving charge carrier interferences. In addition, a scanning probe technique, called scanning gate microscopy (SGM), was used to image and control charge carrier dynamics inside mesoscopic graphene QRs at the local nm-scale. In nitrogen-incorporated graphene, our results evidence the substantial influence of the disorder induced by the presence of nitrogen atoms in the graphene lattice. We then directly observe in real space the effect of disorder in different transport regimes in SGM images of QRs. Beside the influence of disorder, SGM mapping of the charge carrier wave functions inside graphene QRs reveals the formation of scarlike features reminiscent of semi-classical periodic orbits scarring the local density of states along the QRs’ arms. Patterns imaged in SGM are found to be recurrent when varying the charge carrier energy, consistent with theoretical predictions for relativistic quantum scars., (FSA - Sciences de l'ingénieur) -- UCL, 2016

We give a detailed account of an ab initio spectral approach for the calculation of energy spectra of two active electron atoms in a system of hyperspherical coordinates. In this system of coordinates, the Hamiltonian has the same structure as the one of atomic hydrogen with the Coulomb potential expressed in terms of a hyperradius and the nuclear charge replaced by an angle dependent effective charge. The simplest spectral approach consists in expanding the hyperangular wave function in a basis of hyperspherical harmonics. This expansion however, is known to be very slowly converging. Instead, we introduce new hyperangular Sturmian functions. These functions do not have an analytical expression but they treat the first term of the multipole expansion of the electron-electron interaction potential, namely the radial electron correlation, exactly. The properties of these new functions are discussed in detail. For the basis functions of the hyperradius, several choices are possible. In the present case, we use Coulomb-Sturmian functions of half integer angular momentum. We show that, in the case of H-, the accuracy of the energy and the width of the resonance states obtained through a single diagonalization of the Hamiltonian, is comparable to the values given by state-of-the-art methods while using a much smaller basis set. In addition, we show that precise values of the electric-dipole oscillator strengths for transitions in helium are obtained thereby confirming the accuracy of the bound state wave functions generated with the present method.

We present a detailed analysis of the time-scaled coordinate approach and its implementation for solving the time-dependent Schr¨odinger equation describing the interaction of atoms or molecules with radiation pulses.We investigate and discuss the performance of multiresolution schemes for the treatment of the squeezing around the origin of the bound part of the scaled wave packet. When the wave packet is expressed in terms of B splines, we consider two different types of breakpoint sequences: an exponential sequence with a constant density and an initially uniform sequence with a density of points around the origin that increases with time. These two multiresolution schemes are tested in the case of a one-dimensional Gaussian potential and for atomic hydrogen. In the latter case, we also use Sturmian functions to describe the scaled wave packet and discuss a multiresolution scheme which consists of working in a Sturmian basis characterized by a set of nonlinear parameters. Regarding the continuum part of the scaled wave packet, we show explicitly that, for large times, the group velocity of each ionized wave packet goes to zero while its dispersion is suppressed, thereby explaining why, eventually,the scaled wave packet associated with the ejected electrons becomes stationary. Finally, we show that only the lowest scaled bound states can be removed from the total scaled wave packet once the interaction with the pulse has ceased.

The research conducted during this PhD thesis aims at studying the microwave emission and the synchronization properties of nanoscale oscillators driven by the spin-transfer torque (STT) phenomenon. Moreover, the spin-transfer nano-oscillators (STNOs) considered in this work take advantage of the so-called non-uniform magnetic vortex states for creating new microwave emission technologies. The experimental device employed for the investigation of the vortex based STNOs, also called spin-transfer vortex oscillator (STVO), is a network of multilayered ferromagnetic nanowires produced electrochemically inside nanoporous anodic alumina oxide (AAO) templates. In addition to the device fabrication and characterization steps, a theoretical study of the magnetization dynamics in the constituent magnetic layers is conducted in the framework of nanomagnetism (micromagnetic simulations and analytical modelization). In our approach, each nanowire is composed of one or several Co/Cu/Co trilayers corresponding to the so-called spin valve (SV) structures that appear as a set of potential STNOs connected in series and/or parallel. Each SV is separated from the others by a relatively thick non-magnetic layer (50 to 100 nm). An interesting perspective of the nanowire devices is that such in series connected STNOs are expected to lead to synchronization phenomena. Synchronization between STNOs is at the center of the current concern since it represents a promising way for increasing the emitted power of such nano devices and for reaching the power required for practical applications., (FSA - Sciences de l) -- UCL, 2015

Accurate theoretical data on many time-dependent processes in atomic and molecular physics and in chemistry require the direct numerical ab initio solution of the time-dependent Schrödinger equation, thereby motivating the development of very efficient time propagators. These usually involve the solution of very large systems of first-order differential equations that are characterized by a high degree of stiffness. In this contribution, we analyze and compare the performance of the explicit one-step algorithms of Fatunla and Arnoldi. Both algorithms have exactly the same stability function, therefore sharing the same stability properties that turn out to be optimum. Their respective accuracy, however, differs significantly and depends on the physical situation involved. In order to test this accuracy, we use a predictor-corrector scheme in which the predictor is either Fatunla's or Arnoldi's algorithm and the corrector, a fully implicit four-stage Radau IIA method of order 7. In this contribution, we consider two physical processes. The first one is the ionization of an atomic system by a short and intense electromagnetic pulse; the atomic systems include a one-dimensional Gaussian model potential as well as atomic hydrogen and helium, both in full dimensionality. The second process is the decoherence of two-electron quantum states when a time-independent perturbation is applied to a planar two-electron quantum dot where both electrons are confined in an anharmonic potential. Even though the Hamiltonian of this system is time independent the corresponding differential equation shows a striking stiffness which makes the time integration extremely difficult. In the case of the one-dimensional Gaussian potential we discuss in detail the possibility of monitoring the time step for both explicit algorithms. In the other physical situations that are much more demanding in term of computations, we show that the accuracy of both algorithms depends strongly on the degree of

In this thesis we present novel methods to improve the limitations in Optical Coherence Tomography (OCT). They are divided into two parts. The first part deals with the axial resolution limitation in OCT systems. We give a description of the Fourier Domain OCT (FDOCT), and then we show theoretically how to enhance the axial resolution in a particular case of a two-layer sample where one of the two layers is moving continuously, while these two layers are separated by a distance lower than the physical axial resolution. The second part of the thesis demonstrates a novel full theoretical model that uses the second order autocorrelation interferometry technique in nonlinear optics to eliminate the strong background DC, and therefore enhance the imaging contrast. A femtosecond laser emits the light at 1300nm wavelength, it travels through a polarized beam splitter which divides the light in two orthogonal polarizations, one will probe the reference and the other will probe the sample. By realizing the second harmonic generation, in type II configuration, it is possible to eliminate the DC. The setup is adapted, in an original manner, so as to realize interferometric measurements on the sample arm using three waves coupling. A full description of the method with solid theoretical models justified by simulation and experimental results is presented in this part to validate the proof of concept. This part of the thesis shows the possibility to use non-linear interferometry in order to provide interferometric measurements on a dark field even if strong parasitic light is reflected by the sample., (SC - Sciences) -- UCL, 2014

Ce travail porte sur le développement de nouvelles méthodes dépendantes du temps pour l'étude de l'interaction laser-atome; il vise deux objectifs. Le premier fait suite au désaccord significatif existant entre les résultats de différents calculs de section efficace de double ionisation par absorption de deux photons de l'atome d'hélium. Il s'agit de déterminer si ce désaccord est lié à la prise en compte ou non de la corrélation électronique dans la voie de sortie. Pour répondre à cette question, nous développons une méthode hybride combinant la résolution numérique de l'équation de Schrödinger (ES) avec une méthode de type R-matrice. L’analyse des résultats pour l'ionisation de l'hélium par un et deux photons montre que le désaccord n’est pas lié à la corrélation dans la voie de sortie. Le second objectif, lié à l'inexistence de fonctions d'onde analytiques décrivant les continua multiples, est de développer une méthode permettant d'éviter la projection des paquets d'ondes (PO) sur ces fonctions pour en extraire l'information pertinente. Cet objectif est atteint en introduisant dans l’ES un facteur d’échelle dépendant du temps qui conduit à un confinement du PO. Cette méthode est analysée en l'appliquant à l'étude de l’interaction d’atomes à un électron actif avec une impulsion laser. Sa mise en oeuvre nécessite par ailleurs le développement d'algorithmes performants de propagation temporelle des PO. Différents algorithmes sont étudiés en détail. Pour tester ces derniers, nous étudions en profondeur l'ionisation de l'hydrogène atomique en régime basse fréquence pour expliquer l'inattendue présence des structures observées à basse énergie dans les spectres de photo-ionisation d’atomes et molécules, mesurés dans la direction de polarisation du champ., (SC - Sciences) -- UCL, 2014

Les études développées dans cette thèse de doctorat reposent sur l’axe de recherche principal : la production d’effets visuels particuliers dus à une structure multi-échelle. Notre objectif est la réalisation en laboratoire des structures à deux échelles qui tirent leur origine d’observations biologiques : (1) Une structuration de surface de forme multicouche, à l’échelle nanométrique, inspirée d’une étude menée au sein de notre laboratoire Optics laboratory (NAPS) en collaboration avec l’Université de Namur qui a déjà permis la synthèse artificielle des multicouches planes, bio-inspirées reproduisant les propriétés optiques de la cuticule d’un coléoptère, à savoir le Chrysochroa vittata. La compréhension du mécanisme de coloration sur cet insecte a révélé un empilement des multicouches minces de chitine et d’air intercalés. La structure artificielle bio-inspirée a pu donc être mise au point grâce à la maitrise du modèle théorique (cristaux photoniques) et de la technique de dépôt sous vide. (2) Des structurations à l’échelle microscopique de formes différentes : pyramidale, hémisphérique et cylindrique inspirées des espèces Calidea panaethiopica et Pamphobeteus antinous. (3) À cela s’ajoute l’insertion de la fluorescence dans une structure multicouche pour reproduire les propriétés optiques des papillons Troïdes magellanus. De nombreuses mesures ont été consacrées à la compréhension des propriétés optiques de telles structures. La caractérisation optique a été approfondie afin de bien cerner le comportement de ces structures. Et Grâce aux outils de calcul basant sur le formalisme des matrices de transfert et la théorie de Mie, les simulations ont été détaillées. Par le biais de ces recherches, il apparait que la nature dispose d’une large diversité de structures produisant des effets visuels inattendues. Cette créativité se devait de compenser le manque de matériaux à disposition. En effet, nous avons distingué la présence d’un seul bipolymère « la chitine » dans t, (SC - Sciences) -- UCL, 2014

In the recent work of Herath et al. [T. Herath, L. Yan, S. K. Lee, and W. Li, Phys. Rev. Lett. 109, 043004 (2012)PRLTAO0031-900710.1103/PhysRevLett.109.043004] the first experimental observation of a dependence of strong-field ionization rate on the sign of the magnetic quantum number m [of the initial bound state (n,l,m)] was reported. The experiment with nearly circularly polarized light could not distinguish which sign of m favors faster ionization. We perform ab initio calculations for the hydrogen atom initially in one of the four bound substates with the principal quantum number n=2, and irradiated by a short circularly polarized laser pulse of 800nm. In the intensity range of 1012-1013W/cm2 excited bound states play a very important role, but also up to some 1015W/cm2 they cannot be neglected in a full description of the laser-atom interaction. We explore the region that with increasing intensity switches from multiphoton to over-the-barrier ionization and we find, unlike in tunneling-type theories, that the ratio of ionization rates for electrons initially counter-rotating and corotating (with respect to the laser field) may be higher or lower than 1.

We consider the interaction of hydrogen atom with a very intense low frequency laser pulse. The Henneberger-Kramers representation of the time-dependent Schrödinger equation is the most appropriate one for this purpose. It is shown that in the case of very low frequencies, the quantum dispersion of the electron wave packet plays a dominant role in the dynamics of the atom.