Nonlinearity of Microwave Electric Field Coupled Rydberg Electromagnetically Induced Transparency and Autler-Townes Splitting

An electromagnetically induced transparency ( EIT) of a cascade- three- level atom involving Rydberg level in a room- temperature cell, formed with a cesium 6S1/ 2- 6P3/ 2- 66S1/ 2 scheme, is employed to detect the Autler- Townes ( AT) splitting resulted with a 15.21- GHz microwave field coupling the 66S1/ 2 ! 65P1/ 2 transition. Microwave field induced AT splitting, fAT, is characterized by the distance of peak- to- peak of an EIT- AT spectrum. The fAT dependence on the microwave Rabi frequency, W MW, demonstrates two regions, the strong- coupling linear region, fAT similar to W MW and the weak- coupling nonlinear region, fAT. W MW. The fAT dependencies on the probe and coupling Rabi frequency are also investigated. Using small probe- and coupling- laser, the Rabi frequency is found to enlarge the linear regime and decrease the uncertainty of the microwave field measurements. The measurements agree with the calculations based on a four- level atomic model.