Autler-Townes splitting of three-photon excitation of cesium cold Rydberg gases.

We demonstrate the three-photon Autler-Townes (AT) spectroscopy in a cold cesium Rydberg four-level atom by detecting the field ionized Rydberg population. The ground state |6S〉, two intermediate states |6P〉 and |7S〉 and Rydberg state |60P〉 form a cascade four-level atomic system. The three-photon AT spectra and AT splittings are characterized by the Rabi frequency Ω and Ω and detuning δ of the coupling lasers. Due to the interaction of two coupling lasers with the atoms, the AT spectrum has three peaks denoted with the letters A, B and C. Positions of the peaks and relative AT splittings, γ and γ, strongly depend on two coupling lasers. The dependence of the AT splitting, γ and γ, on the coupling laser detuning, δ, and Rabi frequency, Ω and Ω are investigated. It is found that the AT splitting γ mainly comes from the first photon coupling, whereas the γ mainly comes from the second photon coupling with the atom. The three-photon AT spectra and relevant AT splittings are simulated with the four-level density matrix equation and show good agreement with the theoretical simulations considering the spectral line broadening. Our work is of great significance both for further understanding the interaction between the laser and the atom, and for the application of the Rydberg atom based field measurement.