Abstract Laser cooling and trapping of lanthanides has opened the possibility to carry out new experiments with ultracold dipolar gases, for example for quantum simulation of solid state physics. To identify new suitable candidates for laser-cooling, it is important to have a precise spectroscopic knowledge of the atom under consideration. Along this direction, we present here a detailed modeling of the energy levels of neutral neodymium (Nd), an element belonging to the left part of the lanthanide row, which has not yet been considered for laser-cooling. Using the semi-empirical method implemented in the Cowan suite of codes, we are in particular able to interpret more than 200 experimental levels of the NIST database belonging to both parities. The optimal set of atomic parameters obtained after the least-square fitting step can serve to calculate radiative transition probabilities in the future.
Abstract Laser cooling and trapping of lanthanides has opened the possibility to carry out new experiments with ultracold dipolar gases, for example for quantum simulation of solid state physics. To identify new suitable candidates for laser-cooling, it is important to have a precise spectroscopic knowledge of the atom under consideration. Along this direction, we present here a detailed modeling of the energy levels of neutral neodymium (Nd), an element belonging to the left part of the lanthanide row, which has not yet been considered for laser-cooling. Using the semi-empirical method implemented in the Cowan suite of codes, we are in particular able to interpret more than 200 experimental levels of the NIST database belonging to both parities. The optimal set of atomic parameters obtained after the least-square fitting step can serve to calculate radiative transition probabilities in the future.
Within this work a preliminary feasibility study of the laser cooling possibilities for atomic cobalt was performed, motivated by its potential as a platform for quantum-degenerate dipolar matter. Because of the relatively high electronic shells' magnetic moment of the cobalt atom, the approach of cooling and magneto-optical trapping, developed for highly magnetic lanthanide elements, was considered also for this element. Optical leaks from the proposed upper levels were identified and semi-empirical calculations of the transition probabilities for all the relevant transitions were carried out; in all the cases decays to a particular metastable state proved relatively strong - thus Λ-type cooling systems (with one repumper) were further discussed. In order to improve the precision of determination of the hyperfine structure for the proposed upper levels, selected spectral lines directly involving these levels were investigated with the method of laser induced fluorescence in a hollow cathode discharge lamp and evaluated. Additionally, also some spectral lines from Fourier transform spectra available online were analyzed. These experimental data enabled a more accurate determination of the hyperfine structure constants A for 4 odd-parity levels with J=11/2; for 3 of those levels the accuracy for the constants B was also improved, and for one level the constant B was evaluated for the first time. The studies performed in this work allowed to indicate possible candidate cooling schemes, and discuss their basic merits and drawbacks.
We discuss current progress in laser cooling of lanthanides (Er, Yb, Dy, Tm etc.) focusing on applications. We describe some important peculiarities taking Thulium atom as an example: Two stage laser cooling, trapping in an optical lattice, anisotropic interactions and spectroscopy of narrow transitions. Specific level structure and presence of magic wavelengths make ultracold Thulium a favorable candidate for optical clock applications. On the other hand, abundance of Feshbach resonances allow to tune interactions in ultracold gases and thus reach quantum degeneracy. It opens intriguing perspectives for novel quantum simulators employing dipole-dipole interactions in an optical lattice.
Étude théorique des spectres atomiques de lanthanides neutres et trivalents : Application aux gaz ultra-froids et aux solides dopés Le refroidissement laser et le piégeage des lanthanides ont ouvert la possibilité de réaliser de nouvelles expériences avec des gaz dipolaires ultra-froids, qui peuvent être utilisés, par exemple, pour la simulation quantique de la physique du solide. Pour identifier de nouveaux candidats au refroidissement laser, il est crucial de disposer d’une connaissance spectroscopique précise de l’atome considéré. La première grande orientation de ma thèse est la modélisation du spectre des lanthanides neutres, d'une part de l'erbium (Er), un élément de la partie droite de la rangée des lanthanides, particulièrement attractif en raison d'une transition très fine, reliant l'état fondamental et un état excité métastable, et d'autre part du néodyme (Nd), un élément de la partie gauche de la rangée, qui n'a pas encore été explorée pour le refroidissement laser. En utilisant la méthode semi-empirique implémentée dans les codes de Cowan, nous avons amélioré la description des niveaux de configurations impaires de Er.Nous avons aussi calculé des quantités telles que le facteur de Landé, la durée de vie et la polarisabilité de l'état excité cité plus haut, pour les comparer aux résultats expérimentaux obtenus par un groupe d'Innsbruck en Autriche.Ensuite, pour Nd, nous avons pu interpréter plus de 200 niveaux expérimentaux de la base de données du NIST appartenant aux deux parités, puis nous avons proposé de nouvelles transitions de refroidissement laser. La suite logique de ces travaux est le calcul des coefficients d'Einstein, nécessaires pour caractériser l'efficacité du refroidissement laser et du piégeage des atomes.La plupart des lanthanides existent généralement sous forme de cations trivalents. En raison de la présence d’électrons non appariés, les lanthanides présentent d'intéressantes propriétés électromagnétiques et lumineuses. Sous leur forme trivalente, ils sont largement utilisés à des fins industrielles, dans les lasers et comme (co-)dopants dans les amplificateurs optiques à fibres, par exemple dopée à l'erbium.La deuxième grande orientation de ma thèse est consacrée aux lanthanides trivalents, utilisés comme dopants dans un matériau hôte. Toutes les transitions intéressantes pour ces applications sont induites par le champ cristallin. La théorie de Judd-Ofelt est utilisée depuis des décennies pour décrire ces transitions, mais il existe certaines restrictions liées à la manière dont elle est formulée, en particulier pour l'europium (Eu3+). Dans cette thèse, nous avons effectué des calculs spectroscopiques, en utilisant la même méthodologie que pour les lanthanides neutres, pour trois éléments : Eu3+, Nd3+ et Er3+.Nous avons proposé une extension de la théorie de Judd-Ofelt dans laquelle nous incluons une description précise du spectre de l'ion dopant, et nous tenons compte de la variation de l'indice de réfraction du matériau hôte avec la longueur d'onde. Les résultats de notre modèle sont satisfaisants, car il nous donne un aperçu physique de toutes les transitions observées, y compris celles qui sont interdites par la version standard de la théorie. Par ailleurs, notre modèle reproduit quantitativement les forces d'oscillateurs d'absorption expérimentales. Le code utilisé pour effectuer les calculs est disponible sur GitLab.Comme perspective, nous prévoyons de traiter les transitions en lumière polarisée ou entre des sous-niveaux individuels des ions. Cela sera possible dans notre modèle, car les seuls paramètres ajustés sont ceux du champ cristallin. Cela ouvre la possibilité de modéliser les propriétés spectroscopiques de matériaux hôtes à l'échelle nanométrique.
The direct laser cooling of neutral diatomic molecules in molecular beams suggests that trapped molecular ions can also be laser cooled.The long storage time and spatial localization of trapped molecular ions provides an opportunity for multi-step cooling strategies, but also requires careful consideration of rare molecular transitions.We briefly summarize the requirements that a diatomic molecule must meet for laser cooling, and we identify a few potential molecular ion candidates.We then carry out a detailed computational study of the candidates BH + and AlH + , including improved ab initio calculations of the electronic state potential energy surfaces and transition rates for rare dissociation events.On the basis of an analysis of the population dynamics, we determine which transitions must be addressed for laser cooling, and compare experimental schemes using continuous-wave and pulsed lasers.
Abstract The helium dimer in its metastable triplet state is a promising candidate to be the first laser-cooled homonuclear molecule. An ultracold gas of He 2 * would enable a new generation of precision measurements to test quantum electrodynamics for three- and four-electron molecules through Rydberg spectroscopy. Nearly diagonal Franck–Condon factors are obtained because the electron employed for optical cycling occupies a Rydberg orbital that does not take part in the chemical bond. Three possible laser cooling transitions are identified and the spin–rovibronic energy-level structure of the relevant states as well as electronic transition moments, linestrengths, and lifetimes are determined. The production of He 2 * molecules in a supersonic beam is discussed, and a laser slowing scheme to load a magneto-optical trap under such conditions is simulated using a rate equation approach. Various repumping schemes involving one or two upper electronic states are compared to maximize the radiative force. Loss mechanisms such as spin-forbidden transitions, predissociation, and ionization processes are studied and found to not introduce significant challenges for laser cooling and trapping He 2 * . The sensitivity of the vibrational levels of He 2 * with respect to the static polarizability of atomic helium is determined and its implications for a new quantum pressure standard are discussed.
The helium dimer in its metastable triplet state is a promising candidate to be the first laser-cooled homonuclear molecule. An ultracold gas of He2∗ would enable a new generation of precision measurements to test quantum electrodynamics for three- and four-electron molecules through Rydberg spectroscopy. Nearly diagonal Franck-Condon factors are obtained because the electron employed for optical cycling occupies a Rydberg orbital that does not take part in the chemical bond. Three possible laser cooling transitions are identified and the spin-rovibronic energy-level structure of the relevant states as well as electronic transition moments, linestrengths, and lifetimes are determined. The production of He2∗ molecules in a supersonic beam is discussed, and a laser slowing scheme to load a magneto-optical trap under such conditions is simulated using a rate equation approach. Various repumping schemes involving one or two upper electronic states are compared to maximize the radiative force. Loss mechanisms such as spin-forbidden transitions, predissociation, and ionization processes are studied and found to not introduce significant challenges for laser cooling and trapping He2∗. The sensitivity of the vibrational levels of He2+ with respect to the static polarizability of atomic helium is determined and its implications for a new quantum pressure standard are discussed.