Litografia con atomi di Cesio raffreddati

Candidato: Matteo Zaccanti Titolo Tesi: Litografia con atomi di Cesio raffreddati Relatore: Prof. Ennio Arimondo Questo lavoro di tesi descrive il montaggio ex novo, da noi eseguito, di un apparato sperimentale- evoluzione di un precedente esperimento -destinato alla nanodeposizione ed alla litografia atomica, che utilizza un fascio di atomi di Cesio freddi, nonché la caratterizzazione delle varie parti dell’ esperimento, e l’ esposizione dei primi risultati da noi ottenuti. Nonostante oggigiorno sia stata già dimostrata la possibilità di fare litografia atomica con svariati elementi (quali Cesio, Cromo, Alluminio, ed altri) - opportunamente raffreddati sfruttando tecniche di laser cooling (che saranno esposte nella prima parte della tesi)- con la realizzazione di nanostrutture di varie forme (linee, reticoli, reticoli esagonali), l’ esperimento da noi messo a punto ha la originalità assoluta di utilizzare atomi ”lenti”, oltreché ”freddi”: mentre in tutti gli esperimenti di litografia atomica si utilizza, infatti, un forno per generare gli atomi che andranno a formare il fascio, i.e. si scalda il metallo di interesse fino a temperature dell’ ordine di 500 K, e si manipolano gli atomi così liberati, fino ad ottenere un fascio freddo (i.e. con basse velocità longitudinali), il fascio atomico da noi messo a punto è costituito, invece, da atomi dotati di velocità basse sia nelle direzioni trasversali a quella di propagazione del fascio (pochi cm/s), sia in quella longitudinale(10 m/s); ciò permette una maggiore sensibilità sperimentale ed una migliore risoluzione del processo litografico. L’ apparato messo a punto nel corso di questa tesi è costituito da una sorgente di atomi di Cs freddi (MOT piramidale), dalla quale essi vengono estratti con continuità (imbuto atomico), e collimati in un unico step (fase di collimazione) che genera il fascio atomico; questo viene infine fatto incidere su un substrato su cui è creata una nanostruttura 1D, attraverso l’ interazione del fascio con una maschera ottica (fase di deposizione). L’ elaborato è articolato secondo il seguente schema: • Viene fornita una descrizione teorica delle tecniche di rareddamento e di manipolazione di atomi neutri che si sono utilizzate per realizzare questo esperimento, e sono messi in evidenza i parametri fondamentali che regolano tali processi, e che sono stati pertanto esaminati e studiati nel corso della caratterizzazione delle varie parti costituenti l’ esperimento. • Viene descritto l’ apparato sperimentale nelle sue varie parti. • Viene analizzata la sorgente di atomi dell’ apparato, e caratterizzata al variare dei principali parametri sperimentali. • Viene riportata la caratterizzazione del fascio atomico in uscita dall’ imbuto atomico e sono evidenziate le sue caratteristiche di divergenza, velocità, profilo, etc..Vengono descritte quindi le principali peculiarità del fascio dopo la fase di collimazione. • Viene descritto come, a partire dal fascio collimato, siamo stati in grado di produrre una nanostruttura su un substrato opportunamente trattato, attraverso l’ interazione del fascio con una maschera ottica (formata da un’ onda stazionaria), e dopo un processo chimico di sviluppo del campione impressionato (etching)

Polarons, Dressed Molecules, and Itinerant Ferromagnetism in ultracold Fermi gases

In this review, we discuss the properties of a few impurity atoms immersed in a gas of ultracold fermions, the so-called Fermi polaron problem. On one side, this many-body system is appealing because it can be described almost exactly with simple diagrammatic and/or variational theoretical approaches. On the other, it provides quantitatively reliable insight into the phase diagram of strongly interacting population imbalanced quantum mixtures. In particular, we show that the polaron problem can be applied to study itinerant ferromagnetism, a long standing problem in quantum mechanics.Comment: Review paper; published version, 48 pages and 23 figure

Realization of a high power optical trapping setup free from thermal lensing effects

Transmission of high power laser beams through partially absorbing materials modifies the light propagation via a thermally-induced effect known as thermal lensing. This may cause changes in the beam waist position and degrade the beam quality. Here we characterize the effect of thermal lensing associated with the different elements typically employed in an optical trapping setup for cold atoms experiments. We find that the only relevant thermal lens is represented by the $TeO_2$ crystal of the acousto-optic modulator exploited to adjust the laser power on the atomic sample. We then devise a simple and totally passive scheme that enables to realize an inexpensive optical trapping apparatus essentially free from thermal lensing effects

Accurate near-threshold model for ultracold KRb dimers from interisotope Feshbach spectroscopy

We investigate magnetic Feshbach resonances in two different ultracold K-Rb mixtures. Information on the K(39)-Rb(87) isotopic pair is combined with novel and pre-existing observations of resonance patterns for K(40)-Rb(87). Interisotope resonance spectroscopy improves significantly our near-threshold model for scattering and bound-state calculations. Our analysis determines the number of bound states in singlet/triplet potentials and establishes precisely near threshold parameters for all K-Rb pairs of interest for experiments with both atoms and molecules. In addition, the model verifies the validity of the Born-Oppenheimer approximation at the present level of accuracy.Comment: 9 pages, 7 figure

Observation of a Strong Atom-Dimer Attraction in a Mass-Imbalanced Fermi-Fermi Mixture

We investigate a mixture of ultracold fermionic $^{40}$K atoms and weakly bound $^{6}$Li$^{40}$K dimers on the repulsive side of a heteronuclear atomic Feshbach resonance. By radio-frequency spectroscopy we demonstrate that the normally repulsive atom-dimer interaction is turned into a strong attraction. The phenomenon can be understood as a three-body effect in which two heavy $^{40}$K fermions exchange the light $^{6}$Li atom, leading to attraction in odd partial-wave channels (mainly p-wave). Our observations show that mass imbalance in a fermionic system can profoundly change the character of interactions as compared to the well-established mass-balanced case

Repulsive Fermi and Bose Polarons in Quantum Gases

Polaron quasiparticles are formed when a mobile impurity is coupled to the elementary excitations of a many-particle background. In the field of ultracold atoms, the study of the associated impurity problem has attracted a growing interest over the last fifteen years. Polaron quasiparticle properties are essential to our understanding of a variety of paradigmatic quantum many-body systems realized in ultracold atomic gases and in the solid state, from imbalanced Bose-Fermi and Fermi-Fermi mixtures to fermionic Hubbard models. In this topical review, we focus on the so-called repulsive polaron branch, which emerges as an excited many-body state in systems with underlying attractive interactions such as ultracold atomic mixtures, and is characterized by an effective repulsion between the impurity and the surrounding medium. We give a brief account of the current theoretical and experimental understanding of repulsive polaron properties, for impurities embedded in both fermionic and bosonic media, and we highlight open issues deserving future investigations

Feshbach resonances in ultracold K(39)

We discover several magnetic Feshbach resonances in collisions of ultracold K(39) atoms, by studying atom losses and molecule formation. Accurate determination of the magnetic-field resonance locations allows us to optimize a quantum collision model for potassium isotopes. We employ the model to predict the magnetic-field dependence of scattering lengths and of near-threshold molecular levels. Our findings will be useful to plan future experiments on ultracold potassium atoms and molecules.Comment: 7 pages, 6 figure

Realization of a Cold Mixture of Fermionic Chromium and Lithium Atoms

We report on the production of a novel cold mixture of fermionic $^{53}$Cr and $^{6}$Li atoms delivered by two Zeeman-slowed atomic beams and collected within a magneto-optical trap (MOT). For lithium, we obtain clouds of up to $4 \,10^8$ atoms at temperatures of about $500\,\mu$K. A gray optical molasses stage allows us to decrease the gas temperature down to $45(5)\,\mu$K. For chromium, we obtain MOTs comprising up to $1.5\, 10^6$ atoms. The availability of magnetically trappable metastable $D$-states, from which $P$-state atoms can radiatively decay onto, enables to accumulate into the MOT quadrupole samples of up to $10^7$ $^{53}$Cr atoms. After repumping $D$-state atoms back into the cooling cycle, a final cooling stage decreases the chromium temperature down to $145(5)\,\mu$K. While the presence of a lithium MOT decreases the lifetime of magnetically trapped $^{53}$Cr atoms, we obtain, within a 5 seconds duty cycle, samples of about $4\, 10^6$ chromium and $1.5\,10^8$ lithium atoms. Our work provides a crucial step towards the production of degenerate Cr-Li Fermi mixtures.Comment: 14 pages, 8 figure

Gains and losses of coral skeletal porosity changes with ocean acidification acclimation

Ocean acidi\ufb01cation is predicted to impact ecosystems reliant on calcifying organisms, potentially reducing the socioeconomic bene\ufb01ts these habitats provide. Here we investigate the acclimation potential of stony corals living along a pH gradient caused by a Mediterranean CO2 vent that serves as a natural long-term experimental setting. We show that in response to reduced skeletal mineralization at lower pH, corals increase their skeletal macroporosity (features >10 micrometers) in order to maintain constant linear extension rate, an important criterion for reproductive output. At the nanoscale, the coral skeleton\u2019s structural features are not altered. However, higher skeletal porosity, and reduced bulk density and stiffness may contribute to reduce population density and increase damage susceptibility under low pH conditions. Based on these observations, the almost universally employed measure of coral biomineralization, the rate of linear extension, might not be a reliable metric for assessing coral health and resilience in a warming and acidifying ocean

All-optical production of 6Li quantum gases

We report efficient production of quantum gases of 6Li using a sub-Doppler cooling scheme based on the D1 transition. After loading in a standard magneto-optical trap, an atomic sample of 109 atoms is cooled at a temperature of 40 μK by a bichromatic D1 gray-molasses. More than 2×107 atoms are then transferred into a high-intensity optical dipole trap, where a two-spin state mixture is evaporatively cooled down to quantum degeneracy. We observe that D1 cooling remains effective in the deep trapping potential, allowing an effective increase of the atomic phase-space density before starting the evaporation. In a total experimental cycle of 11 s, we produce weakly-interacting degenerate Fermi gases of 7×105 atoms at T/TF < 0.1 and molecular Bose-Einstein condensates of up 5×105 molecules. We further describe a simple and compact optical system both for high-resolution imaging and for imprinting a thin optical barrier on the atomic cloud; this represents a first step towards the study of quantum tunneling in strongly interacting superfluid Fermi gases.ISSN:1742-6588ISSN:1742-659