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

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 polarisation 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 polarisation configuration.