Nonlinear spectroscopy of three-photon excitation of cesium Rydberg atoms in vapor cell*

Abstract

We present nonlinear spectra of four-level ladder cesium atoms employing 6S 1/2 → 6P 3/2→ 7S 1/2 → 30P 3/2 scheme of a room temperature vapor cell. A coupling laser drives Rydberg transition, a dressing laser couples two intermediate levels, and a probe laser optically probes the nonlinear spectra via electromagnetically induced transparency (EIT). Nonlinear spectra are detected as a function of coupling laser frequency. The observed spectra exhibit an enhanced absorption (EA) signal at coupling laser resonance to Rydberg transition and enhanced transmission (ET) signals at detunings to the transition. We define the enhanced absorption (transmission) strength, H EA (H ET), and distance between two ET peaks, γ ET, to describe the spectral feature of the four-level atoms. The enhanced absorption signal H EA is found to have a maximum value when we vary the dressing laser Rabi frequency Ω d, corresponding Rabi frequency is defined as a separatrix point, Ω dSe . The values of Ω dSe and further η = Ω dSe /Ω c are found to depend on the probe and coupling Rabi frequency but not the atomic density. Based on Ω dSe , the spectra can be separated into two regimes, weak and strong dressing ranges, Ω d ≲ Ω dSe and Ω d ≳ Ω dSe , respectively. The spectroscopies display different features at these two regimes. A four-level theoretical model is developed that agrees well with the experimental results in terms of the probe-beam absorption behavior of Rabi frequency-dependent dressed states.