Enhanced four-wave mixing via crossover resonance in cesium vapor
  We report on the observation of enhanced four-wave mixing via crossover resonance in a Doppler broadened cesium vapor. Using a single laser frequency, a resonant parametric process in a double-$\Lambda$ level configuration is directly excited for a specific velocity class. We investigate this process in different saturation regimes and demonstrate the possibility of generating intensity correlation and anti-correlation between the probe and conjugate beams. A simple theoretical model is developed that accounts qualitatively well to the observed results. 
T. Passerat de Silans, C. S. L. Goncalves, D. Felinto, and J. W. R.   Tabosa
quant-ph