Abstract We demonstrate optical cycling and laser cooling of a cryogenic buffer-gas beam of calcium monohydride (CaH) molecules. We measure vibrational branching ratios for laser cooling transitions for both excited electronic states A and B. Furthermore, we measure that repeated photon scattering via the A ← X transition is achievable at a rate of ∼ 1.6 × 1 0 6 photons s−1 and demonstrate interaction-time limited scattering of ∼ 200 photons by repumping the largest vibrational decay channel. We also demonstrate a sub-Doppler cooling technique, namely the magnetically assisted Sisyphus effect, and use it to cool the transverse temperature of a molecular beam of CaH. Using a standing wave of light, we lower the transverse temperature from 12.2(1.2) mK to 5.7(1.1) mK. We compare these results to a model that uses optical Bloch equations and Monte Carlo simulations of the molecular beam trajectories. This work establishes a clear pathway for creating a magneto-optical trap (MOT) of CaH molecules. Such a MOT could serve as a starting point for production of ultracold hydrogen gas via dissociation of a trapped CaH cloud.
Abstract We demonstrate optical cycling and laser cooling of a cryogenic buffer-gas beam of calcium monohydride (CaH) molecules. We measure vibrational branching ratios for laser cooling transitions for both excited electronic states A and B. Furthermore, we measure that repeated photon scattering via the A ← X transition is achievable at a rate of ∼ 1.6 × 1 0 6 photons s−1 and demonstrate interaction-time limited scattering of ∼ 200 photons by repumping the largest vibrational decay channel. We also demonstrate a sub-Doppler cooling technique, namely the magnetically assisted Sisyphus effect, and use it to cool the transverse temperature of a molecular beam of CaH. Using a standing wave of light, we lower the transverse temperature from 12.2(1.2) mK to 5.7(1.1) mK. We compare these results to a model that uses optical Bloch equations and Monte Carlo simulations of the molecular beam trajectories. This work establishes a clear pathway for creating a magneto-optical trap (MOT) of CaH molecules. Such a MOT could serve as a starting point for production of ultracold hydrogen gas via dissociation of a trapped CaH cloud.
We present the first experimental demonstration of radiation pressure force deflection and direct laser cooling for barium monohydride (BaH) molecules resulting from multiple photon scattering. Despite a small recoil velocity (2.7 mm s −1 ) and a long excited state lifetime (137 ns), we use 1060 nm laser light exciting the X → A electronic transition of BaH to deflect a cryogenic buffer-gas beam and reduce its transverse velocity spread. Multiple experimental methods are employed to characterize the optical cycling dynamics and benchmark theoretical estimates based on rate equation models as well as solutions of the Lindblad master equation for the complete multilevel system. Broader implications for laser cooling and magneto-optical trapping of heavy-metal-containing molecules with narrow transition linewidths are presented. Our results pave the way for producing a new class of ultracold molecules—alkaline earth monohydrides—via direct laser cooling and trapping, opening the door to realizing a new method for delivering ultracold hydrogen atoms (Lane 2015 Phys . Rev . A 92 , 022511).
Molecular predissociation, the spontaneous nonradiative bond-breaking process, can limit the ability to scatter a large number of photons required to reach the ultracold regime in laser cooling. Unlike rovibrational branching, predissociation is irreversible since the fragments fly apart with high kinetic energy. Of particular interest is the simple diatomic molecule CaH, for which the two lowest electronically excited states used in laser cooling, $A{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Pi}}}_{1/2}$ and $B{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}$, lie above the dissociation threshold of the ground potential. In this work, we present measurements and calculations that quantify the predissociation probabilities Ppd affecting the cooling cycle. For the lowest vibrational levels, we find Ppd of $\ensuremath{\sim}{10}^{\ensuremath{-}6}$ for $A({v}^{\ensuremath{'}}=0)$ and $\ensuremath{\sim}{10}^{\ensuremath{-}3}$ for $B({v}^{\ensuremath{'}}=0)$. The results allow us to design a laser-cooling scheme that will enable the creation of an ultracold and optically trapped cloud of CaH molecules. In addition, we use the results to propose a two-photon pathway to controlled dissociation of the molecules in order to gain access to their ultracold fragments, including hydrogen.
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.
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.
Only bosonic molecular species have been directly laser cooled to date, primarily due to an abundance of bosonic isotopes in nature and to their simpler hyperfine structure. Fermionic molecules provide new opportunities for ultracold chemistry, quantum simulation, and precision measurements. Here we report direct laser cooling of a fermionic molecular isotopologue, calcium monodeuteride (CaD). With a nuclear spin I = 1 , only 5 hyperfine states need to be addressed for rotational closure in optical cycling. These hyperfine states are unresolved for typical experimental linewidths. We present a method for efficiently producing alkaline-earth metal hydrides and deuterides. We demonstrate rotational closure and show magnetically assisted Sisyphus cooling in one dimension for a beam of CaD molecules. Our results indicate that the experimental complexity for laser cooling CaD is similar to that of calcium monohydride (CaH). Laser cooling of CaD is a promising first step for production of ultracold and trapped atomic deuterium. Published by the American Physical Society 2024