An experimentally feasible strategy for direct laser cooling of polyatomic molecules with six or more atoms is presented. Our approach relies on the attachment of a metal atom to a complex molecule, where it acts as an active photon cycling site. We describe a laser cooling scheme for alkaline earth monoalkoxide free radicals taking advantage of the phase space compression of a cryogenic buffer-gas beam. Possible applications are presented including laser cooling of chiral molecules and slowing of molecular beams using coherent photon processes.
An experimentally feasible strategy for direct laser cooling of polyatomic molecules with six or more atoms is presented. Our approach relies on the attachment of a metal atom to a complex molecule, where it acts as an active photon cycling site. We describe a laser cooling scheme for alkaline earth monoalkoxide free radicals taking advantage of the phase space compression of a cryogenic buffer-gas beam. Possible applications are presented including laser cooling of chiral molecules and slowing of molecular beams using coherent photon processes.
The authors discuss prospects for laser cooling of polyatomic molecules of increasing complexity from expanding alkaline chains to exohedral and endohedral fullerenes with one or two optical cycling centers attached. They demonstrate the degree to which these cycling centers retain their beneficial photon scattering properties for a diverse list of molecules. Their computational results indicate that a second optical cycling center potentially doubles the photon scattering rate and, therefore, improves the efficiency of the laser-cooling process.
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.
Atoms can be cooled and trapped efficiently with the help of lasers. So-called Doppler cooling takes advantage of momentum transfer upon absorption and emission of photons and of Doppler shifts to facilitate effectively closed optical absorption-emission loops, by which atoms are slowed down and cooled. Due to the wealth of internal degrees of freedom accessible in molecules, it was assumed for a long time that similarly closed optical loops cannot be realised for molecules. After an early theoretical proposal by Di Rosa for diatomic molecules, such cooling has been achieved in this decade for SrF, YO, CaF and YbF. It has been outlined recently that also polyatomic molecules should be coolable with lasers and classes of molecules expected to be amenable to this have been proposed by the present authors. Experimental success in laser cooling of SrOH has been reported. The status of cooling polyatomic molecules with lasers and the prospects for obtaining ultracold chiral molecules is reviewed herein.
Molecules controlled at the single-quantum-state level offer wide-ranging applications spanning many of the frontiers of modern physics. Within the past decade, simple diatomic molecules have been brought under full quantum control both via assembly from pre-cooled atoms and by direct laser cooling. Polyatomic molecules\footnote{Molecules that contain three or more atoms.} have attracted focus as novel quantum resources providing distinct advantages (and challenges) compared to both atoms and diatomic molecules. For example, nearly all polyatomic molecules have long-lived states that can be fully polarized at low applied electric fields. These and other features generic to polyatomic molecules can be applied to quantum simulation, quantum chemistry, and tests of fundamental physics beyond the Standard Model. While these structural features of polyatomic molecules are highly promising for applications, they complicate the process of bringing molecules under full quantum control. In this thesis, we describe a series of experimental and theoretical advances toward the production of ultracold complex polyatomic molecules. We demonstrate the first laser cooling of the polyatomic radical ytterbium monohydroxide (YbOH), a promising candidate to search for particles and interactions beyond the Standard Model of particle physics. We develop a novel deceleration technique (``Zeeman-Sisyphus deceleration") capable of slowing molecular beams to trappable velocities while scattering fewer than 10 optical photons. This deceleration method is demonstrated using CaOH molecules, a lighter analog of YbOH. In order to determine a viable pathway to full three-dimensional cooling and trapping of YbOH, we develop a spectroscopic technique to measure vibrational branching ratios with relative intensity sensitivity around 1 part in 105, approximately two orders of magnitude beyond the previous state of the art. We also perform the first spectroscopic observations and characterizations of the related molecule ytterbium monomethoxide, YbOCH3, a promising species to probe beyond-the-Standard-Model physics. In addition, we show theoretically that an entire class of asymmetric top molecules can be laser cooled using essentially the same techniques used for much simpler species. Measurements of the vibrational branching ratios for a few prototypical asymmetric to molecules (CaSH and CaNH2) are presented to confirm these predictions.
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.
Abstract The methods of laser cooling of atoms have long been applied to obtain cold and ultracold atomic gases, including degenerate states and the atomic Bose–Einstein condensate in particular. Until recently the application of laser cooling methods to molecules was assumed to be impossible because of the complex structure of molecular levels and the absence of closed cooling cycles for transitions between the electron levels of molecules in the general case. However, it has recently become clear that laser cooling can be performed for a large class of molecules, including not only the simplest diatomic molecules but also polyatomic molecules. We here present the general principles for identifying suitable molecules and discuss current studies on and further developments in the laser cooling of molecules.