Modulation transfer spectroscopy for two-level atoms at high laser intensity

We present a theoretical study of lineshapes in modulation transfer spectroscopy for two-level atoms at high laser intensity. We consider the interaction involving photon numbers up to seven when calculating the density matrix equations and compare the lineshapes for different photon numbers, by changing the laser intensity and its modulation frequency. As the modulation frequency becomes smaller or the laser intensities become stronger, interactions involving higher photon numbers are required to generate accurate modulation transfer spectra.