Modulation transfer spectroscopy of $^7$Li D1 line

Abstract: We present the first implementation of modulation transfer spectroscopy (MTS) on the D$1$ line of $^7$Li, carried out in a compact heat-pipe vapor cell with cold windows. By varying the pump and probe intensities and polarization configurations, we systematically map the MTS error-signal amplitude, effective linewidth, and slope for the ground-state crossover resonance $F=1\times2\rightarrow F'=2$. We observe Rabi-induced power broadening and identify optimal conditions for laser frequency stabilization via tightly focused beams with total power below 1 mW. The lin$\perp$lin polarization configuration yields a sharp, symmetric, high-contrast error signal with a maximal slope. Our findings establish the MTS spectrum of the $^7$Li D1 line transitions as a reliable frequency reference for quantum technology and ultracold atom experiments, particularly in scenarios where frequency stabilization must be achieved with low laser optical power.