Laser cooling of molecules for precision measurements of parity violation

Abstract

Understanding the fundamental laws of physics has been an intrinsic motivation for the scientific community for many generations. The Standard Model of particle physics (SM), while extraordinarily successful in describing a wide range of phenomena, does not answer all fundamental questions. The weak interaction, especially its parity-violating nature, remains a promising area to search for new physics. Precision measurements using laser-cooled heavy molecules offer a tabletop-scale approach to probe poorly characterized properties of nuclear-spin-dependent parity violation (NSD-PV) connected to this force, and could complement findings from high-energy particle collider facilities. However, the complex hyperfine structure of suitable molecular species substantially complicates the creation of cold samples since conventional laser cooling techniques, which have emerged over the last decade, remain constrained to the most abundant isotopologues of simpler and mostly lighter molecules. This thesis presents a novel strategy for designing optimized optical spectra and implementing them via serrodyne waveforms to selectively laser cool heavy, low abundant barium monofluoride (BaF) molecules, whose additional nuclear spin leads to a level structure significantly exceeding the complexity of other laser-cooled species. A general prerequisite to any type of laser interaction is the thorough characterization of the molecular level structure. For this purpose, high-resolution spectroscopy is performed and transition spectra are modeled to disentangle the hyperfine and rovibrational spectra of the five most abundant isotopologues, from 138BaF to 134BaF, enabling a King plot analysis of the isotope shifts. Laser cooling in complex multi-level systems is modeled numerically using a dedicated simulation software package. Combined with spectroscopic input, this approach enables the identification of optimized laser sideband configurations for various BaF isotopologues. To realize the simulated laser forces, an experimental setup is designed that includes the generation of these sideband configurations via serrodynes. This enables the observation of strong and efficient Sisyphus cooling forces for the most abundant isotopologue 138BaF. High-fidelity detection and manipulation of select low-abundance isotopologues in the same molecular beam is first shown for 136BaF as a test system. Finally, these principles are combined to demonstrate optical cycling and one-dimensional transverse Sisyphus laser cooling for 137BaF which features an exceptionally complex hyperfine structure. These results enable efficient state preparation, detection, and intense collimated molecular beams of 137BaF molecules, which will significantly enhance the sensitivity to NSD-PV effects in the future. Moreover, the demonstrated techniques are also broadly applicable to other previously inaccessible molecules with complex hyperfine structure, thereby paving the way for their use in other sensitive tests of physics beyond the SM.