In this thesis we present novel techniques for the study of ultracold gases of lithium atoms. In the first part of this thesis, we present the development of a narrow-linewidth laser source emitting 840mW of output power in the vicinity of the lithium D-line resonances at 671 nm. The source is based on a diode-end-pumped unidirectional ring laser operating on the 1342-nm transition in Nd:YVO4, capable of producing 1.3W of single-mode light delivered in a diffraction-limited beam. The output beam is subsequently frequency-doubled using periodically-poled potassium titanyl phosphate (ppKTP) in an external buildup cavity. We obtain doubling efficiencies of up to 86%. Tunability of the output frequency over more than 400GHz and frequency-locking of the cavity ensemble with respect to the lithium D-line transitions are accomplished. We measure the linewidth to be 200+400-200kHz. In the second part of this thesis, we employ the source in an experimental setup to produce to cool and trap lithium atoms. We realize samples of finite-temperature unitary Bose gases around the center of a Fano-Feshbach resonance, where interactions between the atoms are maximized. We present temperature-dependent measurements of the unitarity-limited three-body loss rate. The measured losses attain the limiting value imposed by quantum mechanics without adjustable parameters. This measurement allows for the introduction of a criterion for quasi-equilibrium. In this regime, by using technique based on in-situ imaging developed in our group, we provide a first measurement of the equation of state of the unitary Bose gas at low fugacities.
This thesis presents novel techniques for the experimental study of ultracold quantum gases of fermionic lithium and potassium atoms.In the first part of this thesis, we describe the design and characterization of the new components of our experimental apparatus capable of trapping and cooling simultaneously
6Li and
40K atoms to ultracold temperatures.We report on a novel sub-Doppler cooling mechanism, operating on the D
1 line transition of alkali atoms, for laser cooling of lithium and potassium. The measured phase space densities after this molasses phase are on the order of
10−4 for both
6Li and
40K. We present the forced evaporative cooling of
40K atoms, starting in an optically plugged magnetic quadrupole trap and continuing in an optical dipole trap. In this context, we report on the production of a quantum degenerate Fermi gas of
1.5×105 atoms
40K in a crossed dipole trap with
T/TF=0.17, paving the way for the study of strongly interacting superfluids of
40K.In the second part of this thesis, we present a narrow-linewidth, all-solid-state laser source, emitting 5.2 W in the vicinity of the lithium D-line transitions at 671 nm. The source is based on a diode-end-pumped unidirectional ring laser operating on the 1342 nm transition of Nd:YVO
4, capable of producing 6.5 W of single-mode light delivered in a diffraction-limited beam. We report on three different approaches for second-haromonic generation of its output beam, namely by employing an enhancement cavity containing a ppKTP crystal, intracavity frequency doubling and a ppZnO:LN waveguide structure.
We have constructed a solid-state light source for experiments with laser cooled lithium atoms based on a Nd:YVO4 ring laser with second-harmonic generation. Unidirectional lasing, an improved mode selection, and a high output power of the ring laser were achieved by weak coupling to an external cavity which contained the lossy elements required for single frequency operation. Continuous frequency tuning is accomplished by controlling two piezoelectric transducers (PZTs) in the internal and the external cavities simultaneously. The light source has been utilized to trap and cool fermionic lithium atoms into the quantum degenerate regime.
We have constructed a solid-state light source for experiments with laser cooled lithium atoms based on a Nd:YVO4 ring laser with second-harmonic generation. Unidirectional lasing, an improved mode selection, and a high output power of the ring laser were achieved by weak coupling to an external cavity which contained the lossy elements required for single frequency operation. Continuous frequency tuning is accomplished by controlling two piezoelectric transducers (PZTs) in the internal and the external cavities simultaneously. The light source has been utilized to trap and cool fermionic lithium atoms into the quantum degenerate regime.
Ultracold mixture experiments hold the promise of providing new insights into many-body quantum systems as well as ultracold chemistry and few-body phenomena. The work presented in this thesis dealt with the construction of a new apparatus for the production and study of ultracold gases of fermionic lithium-6 and bosonic caesium-133. These isotopes offer a wide tunability in their interaction strength, both in inter-species and intra-species collisions, through magnetic Feshbach resonances. Additionally, the widely different resonance frequencies of lithium and caesium enables independent control of each of the species. With this apparatus, Bose-Einstein condensates (BEC) containing 10^4 lithium Feshbach molecules are routinely produced. The cooling system for caesium has been developed in parallel and important steps towards producing ultracold caesium gases have been made. An optical dipole trap has been loaded with 2x10^6 caesium atoms and evaporative cooling towards quantum degeneracy can now be pursued. Laser, vacuum, magnetic and control systems have been developed for the implementation of this experiment. Light produced with this laser system was used to laser cool atoms, create conservative dipole traps as well as to provide means of imaging atomic clouds. Additionally, a system to produce strong magnetic fields of up to 1400 G has been established in order to exploit the wide tunability in the atomic interactions. Software that was developed for the computerised control system facilitated the coordination of all the components involved in the experimental sequence. Measurements and calculations that showcase the functionality of relevant parts of the setup are presented in this thesis. In this experiment, lithium and caesium atoms are obtained from a novel type of Zeeman slower and are loaded into a magneto-optical trap (MOT). The system is capable of doing this independently for each of the atomic species as well as sequentially. After the MOT has been loaded with atoms, they are transferred into a conservative far-off-resonance optical dipole trap. By adjusting the interactions between atoms and lowering the depth of the dipole trap, efficient evaporative cooling of lithium was carried out from which a molecular BEC was obtained. Time-of-flight measurements were used to characterise the condensate and study its expansion dynamics.
The diode laser system for laser cooling on the 1S 0 → 3P1 intercombination line of strontium discussed in this thesis allows us to cool and trap an atomic strontium sample to 15 muK. Samples in this temperature range are useful for the development of the next generation of atomic frequency standards, cold collision studies, and as a step towards quantum degeneracy. This laser system consists of a Littrow configuration external-cavity diode laser, which is frequency locked to a high finesse cavity. The cavity is subsequently locked to an absorption feature to produce a few mW of tunable, stabilized, single mode power at the target wavelength. We present the design and characterization of this system, including a linewidth measurement of 70 kHz, as well as preliminary results from cooling and trapping.
The low-temperature unitary Bose gas is a fundamental paradigm in few-body and many-body physics, attracting wide theoretical and experimental interest. Here, we present experiments performed with unitary 133Cs and 7Li atoms in two different setups, which enable quantitative comparison of the three-body recombination rate in the low-temperature domain. We develop a theoretical model that describes the dynamic competition between two-body evaporation and three-body recombination in a harmonically trapped unitary atomic gas above the condensation temperature. We identify a universal "magic" trap depth where, within some parameter range, evaporative cooling is balanced by recombination heating and the gas temperature stays constant. Our model is developed for the usual three-dimensional evaporation regime as well as the two-dimensional evaporation case, and it fully supports our experimental findings. Combined 133Cs and 7Li experimental data allow investigations of loss dynamics over 2 orders of magnitude in temperature and 4 orders of magnitude in three-body loss rate. We confirm the 1/T2 temperature universality law. In particular, we measure, for the first time, the Efimov inelasticity parameter η*=0.098(7) for the 47.8-G d-wave Feshbach resonance in 133Cs. Our result supports the universal loss dynamics of trapped unitary Bose gases up to a single parameter η*.