Doubly resonant four-photon interactions in Cesium vapor

Four-photon interactions involving both a two-photon resonance with the
9d^{2}D_{3/2}
level and a one-photon resonance with an np
²
P level (
n = 7-10
) are investigated in detail. Gain, gain saturation, gain threshold, and forward-to-backward generation ratios are measured. Comparison with numerical estimates indicates that
9d^{2}D_{3/2}
to
np^{2}P
lasing is the dominant gain mechanism but that a doubly resonant four-photon parametric oscillation makes a significant contribution. Gain saturation is dominated by population of np
²
P levels. Ionization is important, especially at higher pump powers.