Microwave-dressed spectroscopy of cesium Rydberg atoms under dual-frequency microwave fields

We present a spectroscopic study of cesium Rydberg atoms confined in a vapor cell and driven by dual microwave fields. The primary microwave field drives the 63D 5/2 -64P 3/2 transition at 2.758 GHz, generating an Autler-Townes-modulated heterodyne spectrum. A secondary microwave field, tuned near the 63D 5/2 -61F 7/2 transition at approximately 12.7 GHz, is applied to systematically investigate the effect of an additional resonant coupling channel on the Rydberg spectral response. By recording the optical spectra as functions of the strength and detuning of the secondary microwave field, we observe a field-dependent spectral shift, an optical background offset, and a redistribution of the microwave-to-optical conversion gain. Notably, we quantitatively characterize the conversion gain itself, which has not been the focus of previous Rydberg heterodyne studies. The dynamic spectral response associated with the 2.758 GHz transition remains discernible across the explored perturbation range, albeit with a gradual reduction in conversion gain as the perturbing field strength increases. The main experimental spectral features are well reproduced by wave-packet evolution calculations that incorporate both microwave couplings. These findings elucidate the role of resonant microwave perturbations in Rydberg EIT/AT spectroscopy and provide practical guidance for interpreting microwave-dressed atomic spectra in complex electromagnetic environments.