We demonstrate the mixing of rotational states in the ground electronic state using microwave radiation to enhance optical cycling in the molecule yttrium (II) monoxide (YO). This mixing technique is used in conjunction with a frequency modulated and chirped continuous wave laser to slow longitudinally a cryogenic buffer-gas beam of YO. We generate a flux of YO below 10 m/s, directly loadable into a three-dimensional magneto-optical trap. This technique opens the door for laser cooling of diatomic molecules with more complex loss channels due to intermediate states.
We demonstrate the mixing of rotational states in the ground electronic state using microwave radiation to enhance optical cycling in the molecule yttrium (II) monoxide (YO). This mixing technique is used in conjunction with a frequency modulated and chirped continuous wave laser to slow longitudinally a cryogenic buffer-gas beam of YO. We generate a flux of YO below 10 m/s, directly loadable into a three-dimensional magneto-optical trap. This technique opens the door for laser cooling of diatomic molecules with more complex loss channels due to intermediate states.
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.
Complex molecular structure demands customized solutions to laser cooling by extending its general set of principles and practices.Compared with other laser-cooled molecules, yttrium monoxide (YO) exhibits a large electron-nucleus interaction, resulting in a dominant hyperfine interaction over the electron spinrotation coupling.The YO ground state is thus comprised of two manifolds of closely spaced states, with one of them possessing a negligible Landé g factor.This unique energy level structure favors dual-frequency dc magneto-optical trapping (MOT) and gray molasses cooling (GMC).We report exceptionally robust cooling of YO at 4 μK over a wide range of laser intensity, detunings (one-and two-photon), and magnetic field.The magnetic insensitivity enables the spatial compression of the molecular cloud by alternating GMC and MOT under the continuous operation of the quadrupole magnetic field.A combination of these techniques produces a laser-cooled molecular sample with the highest phase space density in free space.
We demonstrate one- and two-dimensional transverse laser cooling and magneto-optical trapping of the polar molecule yttrium (II) oxide (YO). In a 1D magneto-optical trap (MOT), we characterize the magneto-optical trapping force and decrease the transverse temperature by an order of magnitude, from 25 to 2 mK, limited by interaction time. In a 2D MOT, we enhance the intensity of the YO beam and reduce the transverse temperature in both transverse directions. The approach demonstrated here can be applied to many molecular species and can also be extended to 3D.
Laser cooling and magneto-optical trapping of neutral atoms has been a driving force in physics over the last few decades by providing an efficient method to produce cold and dense samples of cold atoms. Polar molecules which can have strong, long-range dipolar interactions and possess more rich internal structure promise a wide range of new physics and chemistry studies if they can be controlled efficiently. Until recently, the only way to achieve sub-milliKelvin level temperatures in molecules was to associate two precooled atomic species, which while very effective, has a firm bound on the chemical diversity accessible. While laser cooling of molecules relies on fortuitous molecular properties, it will significantly increase the choices of species available for molecular cooling.In this thesis I present the improved optical deceleration of the molecule yttrium monoxide (YO) and the loading of molecules into a 3-D magneto-optical trap (MOT). By making an improved choice of repumping scheme from our groups’ previous work, our photon scattering rate has been increased. By also improving our buffer gas cell source, we produce a much larger flux (2000 times higher) of slowed molecules at 5 m/s to reach the trapping region. I have designed and implemented 3-D MOT field coils operating at 5 MHz, and demonstrated a successful operation of a 3-D MOT. This is the first 3-D laser trapping of an oxide and the first of a molecule with an electronic state intermediate to the main cycling transition states.I have also performed a study of the transition from the ground state to the intermediate electronic state, which has a narrower linewidth of 150 kHz, in contrast to the main cycling transition’s linewidth of 5 MHz. This transition is well suited to second-stage narrow line cooling after loading molecules with the main MOT transition, which will allow direct laser cooling of a molecule to the 10 uK regime.
We report three-dimensional trapping of an oxide molecule (YO), using a radio-frequency magneto-optical trap (MOT). The total number of molecules trapped is ∼1.5×10^{4}, with a temperature of 4.1(5) mK. This diversifies the frontier of molecules that are laser coolable and paves the way for the second-stage narrow-line cooling in this molecule to the microkelvin regime. Futhermore, the new challenges of creating a 3D MOT of YO resolved here indicate that MOTs of more complex nonlinear molecules should be feasible as well.
Achieving laser cooling of molecules necessitates the establishment of a closed optical cycling transition. In contrast to atoms, molecules exhibit additional vibrational and rotational structures, which must be carefully addressed-alongside hyperfine splittings-when designing the laser frequency scheme for efficient optical cycling. In this work, we investigated optical cycling in MgF molecules on the laser cooling transition, namely [Formula: see text] transition between the electronic ground state ([Formula: see text]) and the electronic excited state ([Formula: see text]). The ground state comprises four hyperfine levels, of which two are unresolved within the excited-state linewidth; the two hyperfine levels in the excited state are also unresolved. Consequently, three laser frequencies suffice to address all transitions. This was realized by generating three independently tunable frequency components using acousto-optic modulators (AOMs). With optimized detunings, power ratios, and total beam power, simultaneous application of all three frequency components increased the total number of scattered photons by up to [Formula: see text] relative to the sum of the individual single-frequency signals. Applying a tilted magnetic field to remix the dark Zeeman states increased the photon yield by an additional factor of [Formula: see text], leading to an overall enhancement of about sevenfold. These results provide quantitative benchmarks for MgF optical cycling and practical guidance for future molecular slowing and magneto-optical trapping experiments.