This thesis discusses an experiment, which has demonstrated transverse laser cooling of a pulsed supersonic beam of strontium monofluoride (SrF) molecules. Producing ultracold molecules is important because they could advance many fields including many-body physics, quantum chemistry and precision measurements to explore fundamental forces in nature. Direct laser cooling of molecules is a new and promising way to produce molecules with temperatures in the sub-millikelvin range. In the experiment, SrF molecules produced from a pulsed supersonic source were cooled in the transverse direction using light from just two lasers. The molecular beam brightness was increased by about 20%. I discuss the detailed experimental setup, laser system and data analysis. I also present several theoretical models, which give insight into the cooling experiment. Finally, I discuss improvements to this experiment, which should enable higher yields of ultracold molecules to be produced.
This thesis discusses an experiment, which has demonstrated transverse laser cooling of a pulsed supersonic beam of strontium monofluoride (SrF) molecules. Producing ultracold molecules is important because they could advance many fields including many-body physics, quantum chemistry and precision measurements to explore fundamental forces in nature. Direct laser cooling of molecules is a new and promising way to produce molecules with temperatures in the sub-millikelvin range. In the experiment, SrF molecules produced from a pulsed supersonic source were cooled in the transverse direction using light from just two lasers. The molecular beam brightness was increased by about 20%. I discuss the detailed experimental setup, laser system and data analysis. I also present several theoretical models, which give insight into the cooling experiment. Finally, I discuss improvements to this experiment, which should enable higher yields of ultracold molecules to be produced.
Laser cooling and trapping are central to modern atomic physics. It has been roughly three decades since laser cooling techniques produced ultracold atoms, leading to rapid advances in a vast array of fields and a number of Nobel prizes. Prior to the work presented in this thesis, laser cooling had not yet been extended to molecules because of their complex internal structure. However, this complexity makes molecules potentially useful for a wide range of applications. The first direct laser cooling of a molecule and further results we present here provide a new route to ultracold temperatures for molecules. In particular, these methods bridge the gap between ultracold temperatures and the approximately 1 kelvin temperatures attainable with directly cooled molecules (e.g. with cryogenic buffer gas cooling or decelerated supersonic beams). Using the carefully chosen molecule strontium monofluoride (SrF), decays to unwanted vibrational states are suppressed. Driving a transition with rotational quantum number R=1 to an excited state with R'=0 eliminates decays to unwanted rotational states. The dark ground-state Zeeman sublevels present in this specific scheme are remixed via a static magnetic field. Using three lasers for this scheme, a given molecule should undergo an average of approximately 100,000 photon absorption/emission cycles before being lost via unwanted decays. This number of cycles should be sufficient to load a magneto-optical trap (MOT) of molecules. In this thesis, we demonstrate transverse cooling of an SrF beam, in both Doppler and a Sisyphus-type cooling regimes. We also realize longitudinal slowing of an SrF beam. Finally, we detail current progress towards trapping SrF in a MOT. Ultimately, this technique should enable the production of large samples of molecules at ultracold temperatures for molecules chemically distinct from competing methods.
Ultracold polar molecules have emerged as promising platforms for quantum simulation, quantum information science, precision measurements of fundamental symmetry violations, and quantum chemistry. Over the past decade, rapid progress in direct laser cooling of molecules has enabled new capabilities such as magneto-optical traps (MOTs), sub-Doppler cooling, and optical dipole traps (ODTs), expanding the quantum toolbox with ultracold molecules of complex internal structures and strong dipolar interactions. This thesis presents the design, construction, and operation of an experiment on ultracold strontium monofluoride (88Sr19F) molecules, demonstrating state-of-the-art techniques for molecular laser cooling and trapping. The apparatus integrates a broad range of technical sub-systems, including cryogenics, ultra-high vacuum, optics, electronics, and automated laboratory control software. SrF molecules produced in a slowed cryogenic buffer gas beam are captured into a dual-frequency MOT. Subsequent Λ-enhanced gray molasses cooling reduces the molecule temperature to as low as 5 μK. A novel blue-detuned MOT is also implemented to further compress the molecular cloud, increasing the density by a factor of ∼ 400. At this high density, SrF molecules are efficiently loaded into an ODT, a tight, conservative trap with long lifetimes. Population transfer between different rotational levels is achieved via optical pumping and coherent microwave control, yielding ∼ 0.8 ms coherence time in Rabi oscillation and ∼ 90 % transfer efficiency. This thesis also describes the development of an ultracold rubidium (87Rb) atom experiment, in which a loading rate of ∼ 10^10 atoms/s into a radio-frequency (RF) MOT is demonstrated. A modular two-dimensional MOT serves as the Rb source and can be seamlessly integrated into the SrF apparatus, opening the possibility for future studies of co-trapping and sympathetic cooling of SrF molecules with atoms. The results presented in this thesis establish an experimental foundation for exploring new frontiers in ultracold molecular physics.
Quantum gases of diatomic molecules are many body systems that offer unprecedented control over their intrinsic long and short range interactions. This is due to the unique nature of the interactions between molecules, which can be much stronger than those in magnetic atoms and which can be tuned independently through external electromagnetic fields. As a result, diatomic molecules enable both the exploration of novel supersolid states and the creation of dipole dominated and self assembled quantum solids. The molecular properties and internal structure of calcium monofluoride (CaF) make it a promising candidate molecule for the realization of several kinds of strongly correlated quantum systems, such as strongly dipolar Bose Einstein condensates, quantum crystals and arrays of molecules in optical tweezers. This dissertation summarizes the first steps towards the realization of such systems using directly laser cooled CaF molecules. The two major experimental steps which are summarized in this thesis are cryogenic buffer gas cooling and transversal laser cooling of CaF molecules. For the first achievement, cryogenic buffer gas cooling, it is important to observe that CaF molecules are radicals and therefore do not exist in a stable gas phase. As a result, the production of a beam of molecules requires a cryogenic buffer gas cell setup, in which the molecules are produced via laser ablation of a solid precursor target. Two different kinds of ablation target are used to produce a molecular beam. The first is a pellet of a mixture of CaF2/BaF2. The second is a calcium metal target in addition to sulfurhexafluoride gas, which acts as a fluorine donor. The metal target is found to be the much more stable molecule production method, and the buffer gas cooling setup produces a bright and slow beam of molecules with a flux of around 1011 - 1013sr-1 molecules in the ground state of the laser cooling cycle. The second achievement is a significant improvement of the techniques for laser cooling of the resulting molecular beam. Notably, trapped cold gases of directly laser cooled molecules have not reached the quantum degenerate regime yet, but this milestone is within reach. In our experiment we aim to improve the loading of the magneto optical trap from the laser slowed beam, as this has been the most inefficient step in previous experiments with CaF. Our approach is to collimate the molecular beam through transversal laser cooling on the 𝑋 - 𝐴 transition. Due to the angular momentum structure of the type-II cooling transition, Doppler cooling forces in molecules are typically accompanied by sub Doppler Sisyphus heating forces and vice versa. This has made the collimation of molecular beams difficult in previous experiments, because efficient Doppler cooling over a large velocity range is perturbed by the appearance of sub Doppler heating forces at low transversal velocities. Here, we present a new way to address the hyperfine structure in the molecules using carefully chosen laser sidebands, which overcomes this limitation. The sideband spectrum of the cooling laser consists of one strong sideband with ≈ 80% of the laser power addressing a single hyperfine state and several weaker sidebands which repump population from the other hyperfine states. Depending on the detuning of the cooling laser, the strong sideband generates sub Doppler cooling forces on the 𝑋2Σ+(𝐹 = 2) - 𝐴2Π1/2 (𝐹 = 1) or the 𝑋2Σ+(𝐹 = 1+) - 𝐴2Π1/2 (𝐹 = 0) transition. The measured transversal density profiles of the molecular beam suggest that, for certain detunings of the cooling laser, both mechanisms lead to forces that counteract the molecular motion, leading to the simultaneous appearance of Doppler and sub Doppler cooling. This result is contrary to the conventional tendency of type II systems, where sub Doppler cooling forces are usually opposed by Doppler heating forces. The observed behavior is supported by numerical simulations of the laser cooling forces and molecular trajectories based on the optical Bloch equations. Overall, the results highlight the importance of a well tailored cooling laser sideband structure for molecular laser cooling to control the interplay between both cooling mechanisms. Through the collimation of the molecular motion towards the capture region of a magneto-optical trap, the transversal cooling of the molecular beam with simultaneous Doppler and sub-Doppler cooling forces should enable a more efficient trap loading. The experimental results presented here therefore form the basis for future experiments on laser slowing, magneto optical trapping and evaporative cooling towards Bose Einstein condensation or the loading of large optical tweezer arrays of cold CaF molecules.
Polyatomic molecules, while ubiquitous in nature, were generally perceived as too complicated and "unruly" for the majority of quantum physics experiments. However, recent theoretical analyses have pinpointed the multiple advantages of using complex molecules for diverse applications such as the creation of a universal quantum computer, quantum simulation of non-trivial many-body states, and tests of fundamental physical laws. Throughout my dissertation research I focused on extending experimental techniques from atomic physics to complex molecules. The goal of my work was to not only demonstrate new ways to control molecules, but also develop a novel experimental platform for quantum science - complex polyatomic molecules - and provide tools for physicists to conquer new frontiers in physics and chemistry.\nThe main theme of this thesis is the use of laser radiation to control internal and external degrees of freedom for the triatomic radical strontium monohydroxide (SrOH). The research achievements presented here can be primarily divided into the following areas:\ni) out-of-equilibrium studies of vibrationally inelastic collisions at cryogenic temperatures,\nii) demonstration of the radiation pressure force and the direct laser cooling of polyatomic molecules, and\niii) the development of concepts for laser cooling of molecules with six and more atoms.\nFor the collisional studies, off-diagonal optical pumping was used to populate excited Sr-O stretching state and the ratio of elastic to vibrational quenching collisions was determined to be ~700. In the laser cooling work, using only laser light, the transverse temperature of the cryogenic buffer-gas beam of SrOH was reduced from 50 mK to below 1 mK, leading to an order of magnitude increase in phase space density of the molecular beam. Additionally, the prospects of using a coherent bichromatic force for optical deceleration of polyatomics were analyzed and experimentally characterized with the transverse deflection of the SrOH beam. Our experimental results with SrOH and theoretical analysis of more complex strontium alkoxides not only open a wide range of future directions for laser manipulation and trapping of polyatomic molecules but also help overturn long held assumptions about the difficulties of performing "traditional" atomic physics experiments with complex molecules.
Cold and ultracold molecules are highly desirable for a diverse range of applications in physics and chemistry such as precision measurements, tests of fundamental physics, quantum simulation and information processing, quantum chemistry, and the physics of strongly correlated quantum matter. Laser cooling is usually infeasible in molecules because their rotational and vibrational transitions make is difficult to come up with a closed scattering cycle. Recently, a narrow range of diatomic molecules, one of which is CaF, has been shown to possess a convenient electronic structure and a highly-diagonal Franck-Condon matrix and thus be amenable to laser cooling. This thesis describes experiments on laser cooling of CaF radicals produced in a supersonic source. We first investigate the increased fluorescence when multi-frequency resonant light excites the molecules from the four hyperfine levels of the ground X²∑+(N = 1, v = 0) state to the first excited A²π½ (J’ = 1=2; v’ = 0) state. The number of photons scattered per molecule increases significantly from one or two in the single frequency case to more than 50 before the molecules get pumped into the X²∑+(N = 1; v = 1) state. We demonstrate laser cooling and slowing of CaF using counter-propagating laser light which causes the molecules to scatter more than a thousand photons on the X (N = 1, v = 0, 1) <->A (J’ = 1=2; v’ = 0) transition. The effect of the laser cooling is to slow a group of molecules from 600 ms-1 to about 580 ms-1 and to narrow their velocity distribution from an initial temperature of 3 K down to 300 mK. In addition, chirping the frequency of the cooling light to keep it on resonance with the decelerating molecules doubles the deceleration and further compresses the velocity distribution. The effect of the laser cooling is limited by the optical pumping of molecules in the X (N = 1, v = 2) state.