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The cavity was designed to operate as an asymmet- ric cavity with flat input mirror and curved back mirror with a 25-cm radius of curvature and 22-cm separation between the mirrors. The atomic beam intersects the cavity mode in the middle of the cavity, where the 1 /e 2 radius of the mode in intensity is 172 µ m. The asymmet- ric configuration has the advantage of larger mode radius at the interaction position compared to a symmetric cav- ity. A larger mode allows us to reduce the ac-Stark shifts, consequently reducing the width of the 408 nm transition. Alternatively, the cavity can be modified to operate in the symmetric confocal condition where multiple transverse modes can be excited, thereby increasing the effective “mode” size. However, we were unable to obtain high power and stable lock for the confocal configuration. 

The cavity mirrors were purchased from Research Elec- tro Optics, Inc. For the flat input mirror the transmission is 350 ppm with the absorption and scattering losses of 150 ppm total at 408 nm. The curved back mirror is de- signed to have a lower transmission of 50 ppm in order to additionally suppress the net light wave vector and, therefore, the M1 transition amplitude. The absorption and scattering losses in the curved mirror are 120 ppm. The finesse and the circulating power of the PBC are up to F = 9000 and P = 8 W. These parameters were 

routinely monitored during the PV measurements. De- tails of the characterization of the PBC are addressed in Appendix B. 

We found that the use of the 408-nm-PBC in vacuum is accompanied by substantial degradation of the mirrors. Typically after 6 hours of operation, the finesse drops by a factor of two. This is a limiting factor for the duration of the measurement run. The degradation of the finesse is due to the increased absorption and scattering losses. This effect is reversible: the mirror parameters can be restored by operating the PBC for several minutes in air, which makes performing a number of runs possible without replacing the mirrors. However, it takes several hours with the present apparatus to reach the desired vacuum after exposing the PBC to air. Presently, this effect is under investigation aiming for longer-duration experiments and shorter breaks in between. 

A schematic of the PBC setup is presented in Fig. 4. The mirrors are mounted on precision optical mounts (Lees mounts) with micrometer adjustments for the hor- izontal and vertical angles and the pivot point of the mirror face. The mirror mounts are attached to an Invar rod supported by adjustable table resting on lead blocks. The input mirror is mounted on a piezo-ceramic trans- ducer allowing cavity scanning. 

