Investigation of two- and three-photon atomic resonances in a strong nonmonochromatic field

An experimental investigation was made of the dispersion of the cubic susceptibility of sodium vapor in the region of two- and three-photon resonances with the 3s–4s and 3s–7p transitions. A compensation of the contributions to the nonlinear susceptibility of two- and three-photon resonances was observed. A theory was developed for the generation of the third harmonic in a nonmonochromatic field under resonance conditions and this theory was found to be in good agreement with the experimental results. A reduction in the effective susceptibility occurred in a strong field and this was due to saturation and Stark shift of the levels.