nS_1/2→(n+1)S_1/2 two-photon excitation EIT-AT spectrum of Rydberg atom

In this work, we present an nS_1/2→(n + 1)S_1/2 two-photon excitation EIT-AT spectrum of Rydberg atom in the vapor cell. A ground state (6S_1/2), a first excited state (6P_3/2) and Rydberg state (69S_1/2) of cesium atoms constitute a three-level system. A weak probe laser locking to the transition of 6S_1/2 (F = 4)→6P_3/2 (F′ = 5) couples the ground-state transition, and the strong coupling laser drives the Rydberg transition of 6P_3/2→69S_1/2 to yield electromagnetically induced transparency (EIT) effect, which realizes the optical detection of Rydberg atoms. Two Rydberg 69S_1/2 and 70S_1/2 levels are coupled with the microwave field at a frequency of f_MW = 11.735 GHz, forming a microwave two-photon spectrum. To observe the influence of microwave electric field power on two-photon spectrum, we investigate the microwave coupled Rydberg EIT-AT spectra at different microwave fields. The measurements show that the EIT-AT splitting interval is proportional to the square of the microwave electric field at strong microwave field, and indicvates a nonlinear dependence at weak microwave electric field. The theoretical calculation accords with the experimental measurement. The work here is of significance in precisely measuring the microwave electric field.