Modulation transfer spectroscopy of the D1 transition of potassium: theory and experiment

Abstract

We report on a study of modulation transfer spectroscopy of the 4S1/2 → 4P1/2 (D1) transition
of naturally abundant potassium in a room-temperature vapour cell. This transition is critical for
laser cooling and optical pumping of potassium and our study is therefore motivated by the need for
robust laser frequency stabilisation. Despite the absence of a closed transition, the small ground-
state hyperfine splitting in potassium results in strong crossover features in the D1 modulation
transfer spectrum. To emphasise this we compare the D1 and D2 spectra of potassium with those
of rubidium. Further, we compare our experimental results with a detailed theoretical simulation,
examining different pump-probe polarization configurations to identify the optimal signals for laser
frequency stabilisation. We find good agreement between the experiment and the theory, especially
for the lin ∥ lin polarization configuration.