Three-photon frequency shift on non-collinear excitation

The authors calculate the three-photon resonance absorption shift obtained with two non-collinear excitation beams of arbitrary orientation incident on an ensemble of collisionally broadened two-level atoms excited close to one-third of the resonance frequency. For counterpropagating beams the absorption resonance is narrow and shifted by the order of the linewidth to the violet. As the beams are angled away from the counterpropagating condition the shift increases, and if the angle approaches copropagation the shift becomes very large. When the shift exceeds the single-photon absorption linewidth by more than L/ lambda , the shifted line weakens and disappears, but a separate unshifted peak persists at copropagation. The third-harmonic generation response is more complicated. Except in the region where the beams are almost copropagating the shift in third-harmonic generation response follows the shift in the absorption response. In contrast to the absorption response the third-harmonic generation response increases as the excitation beams approach copropagation, then peaks, then diminishes, broadens and forms a complicated structure whose character is determined by phase matching.