Fig. 1. Experimental setup and energy-level diagram. (a) Diagram of experimental setup; (b) diagram of two-photon transition ladder Rydberg EIT energy levels, where Ω_p and Ω_c are probe and coupling Rabi frequencies, respectively

Fig. 2. Parallel-plate waveguide and its S-parameter test curve. (a) Parallel-plate waveguide physical object and its experimental sketch in off-resonant region. Vapor cell is placed in center between upper and lower plates of parallel-plate waveguide, and lasers are incident into vapor cell through optical apertures; (b) parallel-plate waveguide S-parameter test curve. Insertion loss is 0.3 dB at 300 MHz

Fig. 3. Measurements of radio frequency electric field (hollow dots) by EIT-AT spectra versus square root of signal generator output power, and its linear fit (solid line) with gradient of 3.929 (mV·cm^-1)/mW^1/2

Fig. 4. Beat-note intensity as a function of signal generator output power

Fig. 5. Variation curve of normalized signal amplitude under test with difference frequency