Trapping fermionic potassium atoms in a quasi-electrostatic optical dipole potential

Abstract

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.<br /> 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. <br /> 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. <br /> 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. <br /> 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. <br /> 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 |S_1/2, 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. <br /> 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 T_F = 110 nK yielding a ratio T/T_F ≈ 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.