Four-wave parametric interaction of ultrashort optical pulses in a two-level system of barium atoms

An experimental investigation was made of a four-wave parametric interaction (FWPI) in a field of ultrashort optical pulses (λ_p ≈0.55 μ) acting on a ¹S–¹P⁰ two-level system of barium atomic vapor. It was observed that a pair of new frequencies ν₃ and ν₄ appeared as a result of FWPI, ν₃ being close to ν'_tps = 2ν_p – ν'_sp and ν₄ being close to ν'_sp. This process was observed on approaching the resonance from the low-frequency side. When the resonance was approached from the high-frequency side, "conical" emission was recorded. It was found that a FWPI in a two-level system subjected to a strong pump field is governed by the third- and fifth-order nonlinear susceptibilities χ⁽³⁾ and χ⁽⁵⁾.