<p>This study presents a dual-frequency modulation scheme in which both optical and microwave fields are simultaneously modulated in a microwave–optical double resonance system within a hot rubidium vapor cell. The combined modulation produces harmonic spectral features at the sum and difference of the individual modulation frequencies, indicating nonlinear coupling between the fields. Experimental results reveal that this approach enables modulation of atomic population dynamics and facilitates efficient population transfer between hyperfine levels. The observed frequency mixing highlights the system’s sensitivity to dual modulation and suggests its potential for enhancing the spectral control of atomic transitions. These findings may support future developments in atomic clocks, high-precision magnetometers, and quantum information processing systems.</p>