Observation of parametric instabilities in high-power cavities for optical trapping of molecules*

Despite much interest in studying cold molecules, access to cold, trapped molecules has been limited to only a few select species. Here we present progress towards trapping a variety of small, chemically stable molecules, such as N₂, CO, O₂, and HCl [1]. The molecules will be trapped at cryogenic temperatures by buffer-gas loading a deep optical dipole trap. The ~10 K trap depth is produced by a tightly-focused, 1064-nm cavity capable of reaching intensities of hundreds of GW/cm². Molecules will be directly buffer-gas loaded into the trap using a helium buffer gas at 1.5 K. Both buffer-gas cooling and the very far-off-resonant, quasi-electrostatic trapping mechanism are insensitive to a molecule’s energy level structure and dipole moments, allowing for co-trapping of multiple species. Our trap opens new possibilities in molecular spectroscopy, studies of cold chemical reactions, and precision measurement, amongst other fields of physics.

 

To this end, we have set-up a 100-mm-long, high-intensity cavity and achieved intensities of ~75 GW/cm², close to the design target. At higher intensities, the parametric oscillatory instability (POI) was found to be limiting for our initial setup. POI is a nonlinear phenomenon based on the interaction between optical modes in a cavity and mechanical modes, here at frequencies of tens of MHz, in the cavity mirrors [2]. POI is rare and has only been reported before in experiments with microresonators or in-cavity membranes and at LIGO. We characterize the POI both experimentally and theoretically, and present these results along with techniques to mitigate the instability. We believe that these results will be of interest to a variety of future experiments aiming to use high-intensity optical cavities. We will also discuss recent progress on our cold buffer-gas system and its integration with the cavity.

[1] A. Singh et al., Physical Review Research 5, 033008 (2023).

[2] V.B. Braginsky et al., Physics Letters A 287, 331 (2001).