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Laser Slowing of CaF Molecules and Progress Towards a Dual-MOT for Li and CaF

Eunmi Chae

Published Jan 27, 2016• 1 citation
OpenAlex

Abstract

Diatomic molecules are considered good candidates for the study of strongly correlated systems and precision measurement searches due to their combination of complex internal states and strong long-range interactions. Cooling molecules down to ultracold temperatures is often a necessary step for fully utilizing the power of the molecule. This requires a trap for molecules and the ability to cool molecules to the mK regime and below. A magneto-optical trap (MOT) is a good tool for achieving mK temperatures. However, extra care is needed for molecules to form the necessary quasi-closed cycling transitions due to molecule's complicated energy structure. In our work with CaF, we use two repump lasers to block vibrational leakage and selection rules for the rotational degree of freedom to achieve about 10^{5} photon cycles.\n\n\n\nThe two-stage buffer gas beam source is a general method to generate a cold and slow beam of molecules with a forward velocity of about 50 m/s. The compatibility of the buffer-gas source with a MOT is studied and we confirm that such beams can be nicely compatible with MOTs using various atomic species. \n\n\n\nIn order to load molecules into a MOT from even such a slow beam, additional slowing is required due to the low capture velocity of a molecular MOT (< 10 m/s). We apply a frequency-broadened “white-light” slowing on CaF from a two-stage source, demonstrating slowing of CaF below 10 m/s. An AC MOT, which provides active remixing of dark substates, is also developed and Li atoms are slowed and trapped. These are crucial ingredients for co-trapping CaF molecules and Li atoms and study their collisional properties, which would lead to sympathetic cooling of molecules down to ultracold temperatures. \n\n\n\nThe achievement of slowing and development of this system allowed for the detailed study of the CaF laser cooling system, as well as physical processes involved with AC MOTs and the proposed MOT for CaF. Crucial knowledge of this archetypal system provides significant progress toward manipulation and control of molecules similar to what has been achieved with atoms and what is necessary for searches for new physics with ultracold molecules.

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Laser slowing of CaF molecules down to the capture velocity of a magneto-optical trap for molecules is achieved. Starting from a two-stage buffer gas beam source, we apply frequency-broadened 'white-light' slowing and observe approximately 6 x 10⁴ CaF molecules in a single pulse with velocities 10 ± 4 m s⁻¹. CaF is a candidate for collisional studies in the mK regime. This work represents a significant step towards magneto-optical trapping of CaF.

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Laser slowing of CaF molecules to near the capture velocity of a molecular MOT

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Abstract Laser slowing of CaF molecules down to the capture velocity of a magneto-optical trap for molecules is achieved. Starting from a two-stage buffer gas beam source, we apply frequency-broadened ‘white-light’ slowing and observe approximately 6 × 10 4 CaF molecules in a single pulse with velocities 10 ± 4 m s −1 . CaF is a candidate for collisional studies in the mK regime. This work represents a significant step towards magneto-optical trapping of CaF.

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Laser cooling and trapping in a magneto-optical trap (MOT) have been essential to the success of cold atom physics in the last decades. Recently, the application of the same techniques to molecules has begun. The complexity of even a simple diatomic molecule makes laser cooling difficult, but promises new applications in many areas of research. In this thesis I describe the development of the first three-dimensional MOT of calcium fluoride (CaF) molecules. First, a cryogenic buffer gas source was set up, producing a pulsed beam of 9.3*10^10 molecules per steradian per pulse with forward velocities around 170 m/s. A similar source for very large molecules was set up during a 5 month internship at the University of Vienna. Next, the molecular pulse was slowed down to the capture velocity of a MOT using chirped laser slowing, resulting in about 7*10^5 CaF molecules passing through the typical MOT volume of 1 cm^3 at velocities of 15+-5 m/s. A new deceleration method, called Zeeman-Sisyphus deceleration, was also investigated. In this method molecules move through a spatially varying magnetic field and are optically pumped between low- and high-field seeking states in such as a way that they are always losing kinetic energy. The method promises to deliver more slow molecules because the molecules are guided transversely as they are decelerated. A small prototype was built and the optical pumping step was tested successfully. Finally, 7.6*10^3 CaF molecules were trapped in a MOT and cooled to a temperature of 8.5 mK. The radial trap frequency is 2 pi*130 Hz and the damping constant is beta=9.5*10^2 s^-1. The lifetime is about 100 ms and depends strongly on the scattering rate. This MOT is an an ideal starting point for a wide range of new experiments with ultracold molecules.

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We present the properties of a magneto-optical trap (MOT) of CaF molecules. We study the process of loading the MOT from a decelerated buffer-gas-cooled beam, and how best to slow this molecular beam in order to capture the most molecules. We determine how the number of molecules, the photon scattering rate, the oscillation frequency, damping constant, temperature, cloud size and lifetime depend on the key parameters of the MOT, especially the intensity and detuning of the main cooling laser. We compare our results to analytical and numerical models, to the properties of standard atomic MOTs, and to MOTs of SrF molecules. We load up to molecules, and measure a maximum scattering rate of s −1 per molecule, a maximum oscillation frequency of 100 Hz, a maximum damping constant of 500 s −1 , and a minimum MOT rms radius of 1.5 mm. A minimum temperature of 730 μ K is obtained by ramping down the laser intensity to low values. The lifetime, typically about 100 ms, is consistent with a leak out of the cooling cycle with a branching ratio of about . The MOT has a capture velocity of about 11 m s −1 .

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Diatomic molecules are considered good candidates for the study of strongly correlated systems and precision measurement searches due to their combination of complex internal states and strong long-range interactions. Cooling molecules down to ultracold temperatures is often a necessary step for fully utilizing the power of the molecule. This requires a trap for molecules and the ability to cool molecules to the mK regime and below. A magneto-optical trap (MOT) is a good tool for achieving mK temperatures. However, extra care is needed for molecules to form the necessary quasi-closed cycling transitions due to molecule's complicated energy structure. In our work with CaF, we use two repump lasers to block vibrational leakage and selection rules for the rotational degree of freedom to achieve about 10^{5} photon cycles.\n\n\n\nThe two-stage buffer gas beam source is a general method to generate a cold and slow beam of molecules with a forward velocity of about 50 m/s. The compatibility of the buffer-gas source with a MOT is studied and we confirm that such beams can be nicely compatible with MOTs using various atomic species. \n\n\n\nIn order to load molecules into a MOT from even such a slow beam, additional slowing is required due to the low capture velocity of a molecular MOT (< 10 m/s). We apply a frequency-broadened “white-light” slowing on CaF from a two-stage source, demonstrating slowing of CaF below 10 m/s. An AC MOT, which provides active remixing of dark substates, is also developed and Li atoms are slowed and trapped. These are crucial ingredients for co-trapping CaF molecules and Li atoms and study their collisional properties, which would lead to sympathetic cooling of molecules down to ultracold temperatures. \n\n\n\nThe achievement of slowing and development of this system allowed for the detailed study of the CaF laser cooling system, as well as physical processes involved with AC MOTs and the proposed MOT for CaF. Crucial knowledge of this archetypal system provides significant progress toward manipulation and control of molecules similar to what has been achieved with atoms and what is necessary for searches for new physics with ultracold molecules.
Laser cooling and trapping in a magneto-optical trap (MOT) have been essential to the success of cold atom physics in the last decades. Recently, the application of the same techniques to molecules has begun. The complexity of even a simple diatomic molecule makes laser cooling difficult, but promises new applications in many areas of research. In this thesis I describe the development of the first three-dimensional MOT of calcium fluoride (CaF) molecules. First, a cryogenic buffer gas source was set up, producing a pulsed beam of 9.3*10^10 molecules per steradian per pulse with forward velocities around 170 m/s. A similar source for very large molecules was set up during a 5 month internship at the University of Vienna. Next, the molecular pulse was slowed down to the capture velocity of a MOT using chirped laser slowing, resulting in about 7*10^5 CaF molecules passing through the typical MOT volume of 1 cm^3 at velocities of 15+-5 m/s. A new deceleration method, called Zeeman-Sisyphus deceleration, was also investigated. In this method molecules move through a spatially varying magnetic field and are optically pumped between low- and high-field seeking states in such as a way that they are always losing kinetic energy. The method promises to deliver more slow molecules because the molecules are guided transversely as they are decelerated. A small prototype was built and the optical pumping step was tested successfully. Finally, 7.6*10^3 CaF molecules were trapped in a MOT and cooled to a temperature of 8.5 mK. The radial trap frequency is 2 pi*130 Hz and the damping constant is beta=9.5*10^2 s^-1. The lifetime is about 100 ms and depends strongly on the scattering rate. This MOT is an an ideal starting point for a wide range of new experiments with ultracold molecules.
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.
Diatomic molecules are promising systems for quantum science applications due to their complex energy structures and strong dipole-dipole interactions. Achieving ultracold temperatures is essential for these applications, but the complexity of molecular energy levels requires precise optimization of experimental parameters for laser slowing and magneto-optical trapping (MOT). Here, we simulate and optimize the complete process of slowing and trapping MgF molecules, from a buffer-gas beam source to MOT capture, using Bayesian optimization. By combining laser slowing and MOT simulations, we identify parameters that maximize the capture velocity and the ratio of trapped molecules. Our results demonstrate a maximum MOT capture velocity of 82.5 m/s, and 28.6% of the molecules that reach the MOT region are trapped under optimal conditions. These findings provide insights into experimental setups for MgF and similar molecules, offering a framework for advancing molecular laser cooling and quantum experiments.
We present the properties of a magneto-optical trap (MOT) of CaF molecules. We study the process of loading the MOT from a decelerated buffer-gas-cooled beam, and how best to slow this molecular beam in order to capture the most molecules. We determine how the number of molecules, the photon scattering rate, the oscillation frequency, damping constant, temperature, cloud size and lifetime depend on the key parameters of the MOT, especially the intensity and detuning of the main cooling laser. We compare our results to analytical and numerical models, to the properties of standard atomic MOTs, and to MOTs of SrF molecules. We load up to molecules, and measure a maximum scattering rate of s −1 per molecule, a maximum oscillation frequency of 100 Hz, a maximum damping constant of 500 s −1 , and a minimum MOT rms radius of 1.5 mm. A minimum temperature of 730 μ K is obtained by ramping down the laser intensity to low values. The lifetime, typically about 100 ms, is consistent with a leak out of the cooling cycle with a branching ratio of about . The MOT has a capture velocity of about 11 m s −1 .