UHV Compatible Power Build-up Cavity for Francium Spectroscopy

    Studying the highly forbidden 7s→8s magnetic dipole (M1) transition is key for future atomic parity-violating (APV) experiments in francium. Relativistic effects and hyperfine interactions give rise to an extremely weak M1 transition, with oscillator strength f ~ 10^-13(100× larger than in cesium). To increase the M1 transition rate, one can use a power build-up cavity (PBC) to increase the overall intensity of the spectroscopy laser within the interaction region. At the Francium Trapping Facility located at TRIUMF, Canada's Particle Accelerator Center, we collect neutral francium atoms using magneto-optical traps. These laser-cooled and trapped francium atoms are then used in 7s→8s (506 nm) spectroscopy experiments. We have designed a high finesse PBC that will increase the power of the spectroscopy laser by a gain factor of 4034 ± 50 (preliminary) within the interaction region. This cavity uses the Pound-Drever-Hall locking technique and is UHV compatible. Here we will discuss the design and construction of the PBC, as well as the required stability needed for locking. Further discussion will review measurements of the gain factor and losses of the PBC and compare it to theoretical calculations.