We demonstrate a scheme for magneto-optically trapping strontium monofluoride (SrF) molecules at temperatures one order of magnitude lower and phase space densities 3 orders of magnitude higher than obtained previously with laser-cooled molecules. In our trap, optical dark states are destabilized by rapidly and synchronously reversing the trapping laser polarizations and the applied magnetic field gradient. The number of molecules and trap lifetime are also significantly improved from previous work by loading the trap with high laser power and then reducing the power for long-term trapping. With this procedure, temperatures as low as 400 μK are achieved.
We demonstrate a scheme for magneto-optically trapping strontium monofluoride (SrF) molecules at temperatures one order of magnitude lower and phase space densities 3 orders of magnitude higher than obtained previously with laser-cooled molecules. In our trap, optical dark states are destabilized by rapidly and synchronously reversing the trapping laser polarizations and the applied magnetic field gradient. The number of molecules and trap lifetime are also significantly improved from previous work by loading the trap with high laser power and then reducing the power for long-term trapping. With this procedure, temperatures as low as 400 μK are achieved.
Abstract We report the production of ultracold, trapped strontium monofluoride (SrF) molecules with number density and phase‐space density significantly higher than previously achieved. These improvements are enabled by three distinct changes to our recently‐demonstrated scheme for radio‐frequency magneto‐optical trapping of SrF: modification of the slowing laser beam geometry, addition of an optical pumping laser, and incorporation of a compression stage to the magneto‐optical trap. With these improvements, we observe a trapped sample of SrF molecules at density 2.5×105 cm−3 and phase‐space density 6×10−14, each a factor of 4 greater than in previous work. Under different experimental conditions, we observe trapping of up to 104 molecules, a factor of 5 greater than in previous work. Finally, by reducing the intensity of the applied trapping light, we observe molecular temperatures as low as 250 μK.
We present experimental results from a new scheme for magneto–optically trapping strontium monofluoride (SrF) molecules, which provides increased confinement compared to our original work. The improved trap employs a new approach to magneto–optical trapping presented by Tarbutt (2015 New J. Phys. 17 015007 ), which provided insight for the first time into the source of the restoring force in magneto–optical traps (MOTs) where the cycling transition includes dark Zeeman sublevels (known as type-II MOTs). We measure a radial spring constant greater than in our original work with SrF, comparable to the spring constants reported in atomic type-II MOTs. We achieve a trap lifetime ms, over longer than originally reported for SrF. Finally, we demonstrate further cooling of the trapped molecules by briefly increasing the trapping lasers' detunings. Our trapping scheme remains a straightforward extension of atomic techniques and marks a step towards the direct production of large, dense, ultracold molecular gases via laser cooling.
We demonstrate significantly improved magneto-optical trapping of molecules using a very slow cryogenic beam source and either rf modulated or dc magnetic fields. The rf magneto-optical trap (MOT) confines 1.0(3)×10^{5} CaF molecules at a density of 7(3)×10^{6} cm^{-3}, which is an order of magnitude greater than previous molecular MOTs. Near Doppler-limited temperatures of 340(20) μK are attained. The achieved density enables future work to directly load optical tweezers and create optical arrays for quantum simulation.
We demonstrate loading of SrF molecules into an optical dipole trap (ODT) via in-trap $\mathrm{\ensuremath{\Lambda}}$-enhanced gray molasses cooling. We find that this cooling can be optimized by a proper choice of relative ODT and cooling beam polarizations. In this optimized configuration, we observe molecules with temperatures as low as $14(1)\text{ }\text{ }\ensuremath{\mu}\mathrm{K}$ in traps with depths up to $570\text{ }\text{ }\ensuremath{\mu}\mathrm{K}$. With optimized parameters, we transfer $\ensuremath{\sim}5%$ of molecules from our radio-frequency magneto-optical trap into the ODT, at a density of $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{9}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$, a phase space density of $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$, and with a trap lifetime of $\ensuremath{\sim}1\text{ }\text{ }\mathrm{s}$.
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.
Laser cooling and magneto-optical trapping of molecules typically involves multiple transitions driven by several laser frequencies. We analyze how magneto-optical trapping forces depend on the angular momenta, F l and F u , and the g -factors, g l and g u , of the lower and upper states. When the polarizations must be reversed relative to cases where The correct choice of circular polarization depends on the sign of g u but not on the sign of g l . If g u is zero there is no trapping force, and the trapping force is very weak whenever g u is small compared to g l , which it usually is when the cooling transition is the to transition of a molecule. For some molecules, mixing of the excited state with a nearby excited state can greatly increase g u , leading to stronger trapping forces. A strong trapping force can also be produced by rapidly and synchronously reversing both the magnetic field and the laser polarizations. We simulate a recent experiment on magneto-optical trapping of SrF molecules, and suggest that an alternative choice of laser beam polarizations will strengthen the trapping force.