Rydberg electromagnetically induced transparency and absorption of strontium triplet states in a weak microwave field

We study theoretically laser excitation of Rydberg triplet states of strontium atoms in the presence of weak microwave (MW) fields. Starting from the ground state 5s²0.16em0ex¹S₀, the Rydberg excitation is realized through the metastable, triplet 5s5p³P₁ state, whose decay rate γ₂ is 2π×7.5 kHz, much smaller than the one in the singlet state or alkali-metal atoms. The influences of γ₂ on the transparency and absorption spectrum in the electromagnetically induced transparency (EIT), and electromagnetically induced absorption (EIA) regime are examined. Narrow transparent windows (EIT) or absorption peaks (EIA) are found, whose distance in the spectrum depends on the Rabi frequency of the weak MW field. It is found that the spectrum exhibits higher contrast than using the singlet state or alkali-metal atoms in typical situations. Using the metastable intermediate state, we find that the resonance fluorescence of Sr gases exhibits very narrow peaks, which are modulated by the MW field. When the MW field is weaker than the probe and control light, the spectrum distance of these peaks is linearly proportional to Ωₘ. This allows us to propose an alternative way to sense very weak MW fields through resonance fluorescence. Our study shows that the Sr triplet state could be used to develop the Rydberg MW electrometry that gains unique advantages.