Atoms can be trapped at the foci of intense laser beams, which can enable the study of interactions and dynamics of ultracold gases. In this thesis, we will describe our\nnew trap designs. A large volume pancake-shaped optical dipole trap is initially used for loading large numbers of atoms from a Magneto-Optical Trap. A BEC with a large number of 84 Sr atoms has been achieved after evaporation in this trap. To form degenerate gas of 88 Sr we compress the mixture of 88 Sr and 87 Sr from the loading trap\ninto a superimposed dimple trap. This combination improves the reproducibility of the experiment and shortens the time required to create quantum degenerate samples,\nwhile we are able to create 88 Sr BEC with high density. In order to generate BEC of 86 Sr, an isotope with large scattering length and extremely high three-body loss\nrate, we implement an optical sheet trap which has an aspect ratio of 1:10. The tight axis in the vertical direction provides strong potential to hold against gravity while the large horizontal dimension brings up the trap volume and keeps down the atomic\ndensity.
Atoms can be trapped at the foci of intense laser beams, which can enable the study of interactions and dynamics of ultracold gases. In this thesis, we will describe our\nnew trap designs. A large volume pancake-shaped optical dipole trap is initially used for loading large numbers of atoms from a Magneto-Optical Trap. A BEC with a large number of 84 Sr atoms has been achieved after evaporation in this trap. To form degenerate gas of 88 Sr we compress the mixture of 88 Sr and 87 Sr from the loading trap\ninto a superimposed dimple trap. This combination improves the reproducibility of the experiment and shortens the time required to create quantum degenerate samples,\nwhile we are able to create 88 Sr BEC with high density. In order to generate BEC of 86 Sr, an isotope with large scattering length and extremely high three-body loss\nrate, we implement an optical sheet trap which has an aspect ratio of 1:10. The tight axis in the vertical direction provides strong potential to hold against gravity while the large horizontal dimension brings up the trap volume and keeps down the atomic\ndensity.
This thesis describes trapping and evaporative cooling of ultracold 87Sr and 88Sr mixtures in an optical dipole trap to produce the first Bose-Einstein condensate (BEC) of 88Sr. Furthermore, this work presents thermalization studies that characterize the scattering properties of these ultracold strontium samples. Such ultracold atomic gases have become an important area of research because of their potential for improving optical frequency standards and for realizing quantum computation using neutral atoms. A BEC of 88Sr is particularly interesting because the small value of its background s-wave scattering length may enable the use of optical Feshbach resonances to create two-dimensional solitons. However, the small scattering length for 88Sr also hinders efficient evaporative cooling in the optical dipole trap, a necessary step to producing a BEC. Experiments with other ultracold gases have successfully overcome this hurdle by mixing in a second atomic species which, by introducing stronger interactions with the weakly interacting species, enables evaporation to colder temperatures via sympathetic cooling. For this work, we use 87Sr to sympathetically cool 88Sr during forced evaporation to quantum degeneracy.\nPrevious experiments in the Killian Lab characterized 88Sr in detail. Here, I emphasize the new or improved aspects that have allowed trapping and cooling of the mixtures of 87Sr and 88 Sr: trapping of 87Sr by itself, spectroscopic measurements of all the stable strontium isotopes to guide the trapping of isotopic mixtures, imaging of both 87Sr and 88Sr, and the various trade-offs necessary to simultaneously trap 87Sr and 88Sr. Finally, I discuss how the thermalization studies of the scattering properties of the isotopes guide the forced evaporation of mixed isotope samples. These efforts result in the production of the BEC of 88Sr, but they also point the way to future studies of fermionic quantum degeneracy in 87Sr and to the rich physics of mixed species ultracold atomic systems.
Ultracold quantum gases have become a frequently used tool for the investigation of a variety of physical issues within the last decade, ranging from general questions in quantum physics up to special topics, e.g. quantum simulations of the physical origin of high temperature superconductivity in condensed matter physics. In this thesis, experiments are presented on the creation of an ultracold quantum gas of the fermionic potassium isotope, 40K, by all-optical means. For this purpose, a new experimental setup was designed, which was started from the scratch, with the long term goal of creating a supersolid in an optical lattice. One focus of this work lies in identifying light-assisted losses as a main restriction for achieving higher 40K atom numbers in a magneto-optical trap and in identifying light-assisted losses as a main reason for a low atomic transfer efficiency from a magneto-optical trap into an optical, quasi-electrostatic dipole trap (further called optical dipole trap) for 40K atoms. Naturally, another focus of this work lies in the application of different techniques to reduce these from light-assisted losses dependent restrictions. A two-dimensional magneto-optical trap, located in the prechamber of a vacuum system, creates a continuous beam of 40K atoms. The atoms of this beam are recaptured in the main chamber of the vacuum system by a spatial dark spot magneto-optical trap. Here, a dark magneto-optical trap is implemented instead of a standard magneto-optical trap in order to reduce light-assisted losses. Up to 1x109 atoms at temperatures around 240 μK are trapped in the dark magneto-optical trap. This is a factor of ten higher compared to atom numbers of a 40K standard magneto-optical trap. The light-assisted losses in the 40K magneto-optical trap were measured as a function of the total trapping intensity and as a function of the repumping intensity. In both cases, the smallest observed loss coefficient was β ≈ 2.5x10-10 cm3/s. Compared to the smallest values of β of all other alkali elements (apart from the bosonic potassium isotopes), the light-assisted losses with 40K atoms are approximately two orders of magnitude larger. Up to 6x105 atoms were transferred from the spatial dark magneto-optical trap into the quasi-electrostatic dipole trap, which was created by using the trapping potential induced by radiation emitted by a CO2 laser operating at a wavelength near 10.6 μm. In order to reach the here reported atom numbers in the optical dipole trap, its trapping volume had to be increased in comparison to a simple dipole trap arrangement as induced with a single focused dipole trapping beam. Simultaneously, the strong confinement, as present in a tigthly focused single beam trap and as required for a subsequent evaporative cooling, had to be maintained. For this purpose, the trapping laser beam of the optical dipole trap was retroreflected in such a way that an overall optical dipole trap with a nearly doubled waist and a slightly deeper potential depth was created. By this trapping arrangement, the number of atoms confined in the optical dipole trap could be nearly doubled compared to a simple dipole trapping arrangement consisting of a single tightly focused trapping beam. The atomic transfer process itself, from the magneto-optical trap into the optical dipole trap, was organized in two steps. In a first step, the dark magneto-optical trap was transformed into a compressed magneto-optical trap by increasing the field gradient and bringing the cooling laser closer to resonance as compared to the detuning of the usual magneto-optical trap. In contrast to other experiments, the repumping laser of the magneto-optical trap was detuned to the blue of the used repumping transition. As a consequence, atoms were optically pumped into the upper hyperfine ground state |S1/2, F=7/2〉, realizing a temporal dark, compressed magneto-optical trap in analogy to the operation of a temporal dark magneto-optical trap. By this approach, light-assisted losses were reduced. In the second step of the transfer process, the light fields of the magneto-optical trap and the optical dipole trap were alternately switched on and off. In this way, a spatially inhomogeneous ac Stark shift of the cooling transitions due to the spatially inhomogeneous intensity distribution of the dipole trapping beam was avoided and again light-assisted losses were reduced during the loading phase of atoms from the compressed magneto-optical trap into the optical dipole trap. The loading rate was measured to be approximately 2.8x108 atoms/s. The light-assisted losses during loading of the optical dipole trap were determined to be β ≈ 4.8x10-10 cm3/s. In the final step of the experiment, the temperature of the atomic ensemble confined in the optical dipole trap was further reduced by evaporative cooling. After an evaporation time of about 30 s, temperatures of 140 nK were reached at typical atom numbers of 12000. The corresponding Fermi temperature is TF = 110 nK yielding a ratio T/TF ≈ 1.3 which indicates that the fermionic potassium atoms are cooled close to the onset of quantum degeneracy. In the future, an additional trapping and evaporative cooling step within a magnetic quadrupole trap should allow for an improved loading efficiency into the far detuned optical dipole trap and finally allow for a subsequent evaporative cooling to a quantum degenerate Fermi gas.
We describe the operation and performance of an ultracold strontium apparatus that is capable of generating quantum degenerate gases. The experiment has produced Bose-Einstein condensates (BECs) of 84Sr and 86Sr as well as degenerate Fermi gases (DFGs) of 87Sr with a reduced temperature of T/TF = 0.2 at a Fermi temperature of TF = 55 nK. Straightforward modifications could be made to allow for isotopic mixtures and BECs of the fourth stable isotope, 88Sr. We also report on a technique to improve the continuous loading of a magnetic trap by adding a laser tuned to the 3P1 - 3S1 transition. The method increases atom number in the magnetic trap and subsequent cooling stages by up to 65% for the bosonic isotopes and up to 30% for the fermionic isotope of strontium. We optimize this trap loading strategy with respect to laser detuning, intensity, and beam size. To understand the results, we develop a one-dimensional rate equation model of the system, which is in good agreement with the data. We discuss the use of other transitions in strontium for accelerated trap loading and the application of the technique to other alkaline-earth-like atoms. Finally, we also report on an updated investigation of photoassociation resonances relative to the 1S0 + 3P1 dissassociation limit in bosonic strontium. Multiple new resonances for 84Sr and 86Sr were measured out to binding energies of -5 GHz and several discrepancies in earlier measurements were resolved. These measurements will allow for the development of a more accurate mass-scaled model and a better theoretical understanding of the molecular potentials near the 3P1 state. We also measure the strength of the 84Sr 0u transitions in order to characterize their use as optical Feshbach resonances.
Summary form only given. Bose-Einstein condensates (BEC) are generally produced by evaporative cooling of laser cooled atomic samples in magnetic traps. Though magnetic traps are indispensable for the production of a BEC, they pose restrictions on the possible studies with a BEC. Previous experiments of our group have demonstrated that it is possible to trap a BEC in the circular focus of a far-detuned infrared laser beam. This first realization of an optical dipole trap (ODT) for BEC was used to prepare and investigate spinor condensates of Na atoms in the F=1 hyperfine manifold. However, the initial peak density in this ODT after loading from the magnetic trap reached 10/sup 15/ atoms/cm/sup 3/, leading to rapid losses due to three-body collisions. To realize an ODT capable of storing large condensates for a long time, we have used a laser beam with an elliptical beam profile. The optical trap is loaded by superimposing the light field onto the magnetic trap and then slowly turning off the magnetic fields. The transfer efficiency is close to 100%, and thus more than 10/sup 7/ Bose-condensed atoms can be loaded into the ODT.
Summary form only given. Recently, we have created a Bose condensate of /sup 87/Rb atoms directly in an optical trap formed by tightly focused laser beams. Following initial loading from a laser cooled gas, evaporative cooling through the BEC transition is achieved by simply lowering the depth of the optical trap. Our success is due in part to a high initial phase space density realized in the loading of our optical dipole trap and in part to the tight confinement of the atoms that permits rapid evaporative cooling to the BEC transition in 2 s.
This thesis describes a new apparatus designed to study ultracold gases of rubidium. The apparatus comprises a six-beam MOT chamber and a dierential pumping stage leading into a 'science chamber'. This science chamber is constructed from a rectangular glass cell. Atomic gases of rubidium are collected in a MOT and then transferred into a magnetic quadrupole trap. This quadrupole trap is mounted on a motorised translation stage. This setup transports the atoms into the science chamber, where they are transferred into a static quadrupole trap which is built around the glass cell. During the transport the atoms are deected over a glass prism, which shields the science chamber from stray rubidium from the MOT chamber. The magnetic transport is studied in detail and the deection over the glass prism is fully described simulating the displacement of the quadrupole trap. Using the magnetic quadrupole trap in the science chamber to store one rubidium isotope, we are able to load the other rubidium isotope in the MOT chamber and transfer it also into the science chamber. There, the two magnetic traps are merged and variable ratios of isotopic mixtures can be created. The merging of the two quadrupole traps could be employed in future experiments to cool 85Rb sympathetically with 87Rb. In the science chamber forced radio-frequency evaporation is performed and the loading of a far-detuned dipole trap is studied. Initially the dipole trap is realised as a hybrid trap, a single beam dipole trap in combination with the quadrupole trap. Further studies include the loading of a crossed beam dipole trap. We demonstrate that the apparatus is capable of producing 87Rb condensates. Preliminary studies of 85Rb in the dipole trap are included which hopefully in future will lead to a quantum degenerate gas of 85Rb.