Fig. 3: Simulated output spectra and evolution of involved fields in atomic vapors in probe only (red), co-propagating (backward, blue), and counter-propagating (forward, green) cases.

All intensities are normalized by the input intensity of the probe beam. a Spectra of the probe field. b Spectra of the spontaneous parametric and pumped FWM (inset) signals at Δ₂′=−2π × 8.2 GHz. c Evolutions of the probe field. d Evolutions of the spontaneous parametric signal at Δ₁ = Δ₂′ = −2π × 8.2 GHz. The propagation of pumped FWM signal (black curve) is also provided for the forward case. Other parameters: N = 1.24 × 10¹⁴ cm⁻³ (the forward case) and 1.43 × 10¹⁴ cm⁻³ (the backward case), G₁=2π × 6 GHz, G₂=G₂′= 2π × 30 GHz, and M = 600