We have theoretically and experimentally studied the dispersive signal of the Rydberg atomic electromagnetically induced transparency (EIT) - Autler-Townes (AT) splitting spectra obtained using amplitude modulation of the microwave (MW) field. In addition to the two zero-crossing points, the dispersion signal has two positive maxima with an interval defined as the shoulder interval of the dispersion signal $\Delta f_{ \text{ sho}}$. The relationship of MW field strength $E_{ \text{ MW}}$ and $\Delta f_{ \text{ sho}}$ are studied at the MW frequencies of 31.6 GHz, 22.1 GHz, and 9.2 GHz respectively. The results show that $\Delta f_{ \text{ sho}}$ can be used to character the much weaker $E_{ \text{ MW}}$ than the interval of two zero-crossing points $\Delta f_{ \text{ zeros}}$ and the traditional EIT-AT splitting interval $\Delta f_{ \text{ m}}$, the minimum $E_{ \text{ MW}}$ measured by $\Delta f_{ \text{ sho}}$ is about 30 times smaller than that by $\Delta f_{ \text{ m}}$. As an example, the minimum $E_{ \text{ MW}}$ at 9.2 GHz that can be characterized by $\Delta f_{ \text{ sho}}$ is 0.056 mV/cm, which is the minimum value characterized by frequency interval using vapour cell without adding any auxiliary fields. The proposed method can improve the weak limit and sensitivity of $E_{ \text{ MW}}$ measured by spectral frequency interval, which is important in the direct measurement of weak $E_{ \text{ MW}}$.
Categories: physics.atom-ph quant-ph