Modulation transfer spectroscopy mediated by spontaneous emission.

We report the polarization dependence of the spectrum in modulation transfer spectroscopy for the transitions from the lower ground state (F(g) = 1) of (87)Rb atoms. We measured the spectra for the two polarization configurations where the carrier and probe beams were linearly polarized in parallel or perpendicular directions. The measured spectra were in excellent agreement with calculated results. The spectra were strongly dependent on the polarization configurations. In particular, the signal for parallel polarization configuration was generated via an incoherent process mediated by spontaneous emission.