Using four-, six-, and eight-wave-mixing geometries, we observe subharmonic resonances in collision-enhanced spectra in a sodium-seeded hydrogen–air flame. Appearing at high intensity, these resonances occur at ±1/2, ±1/3, ±1/4, and ±1/5 times the frequency of the ground-state hyperfine splitting ωhfs. We argue that these resonances result from higher-order processes, specifically χ(n), where n ≥ 5. In particular, to describe the resonances at ±ωhfs/5, at least χ(13) is required. These high-order collision-enhanced effects are true perturbative nonlinearities and not sequential four-wave mixing processes.
© 1987 Optical Society of America
Rick Trebino and Larry A. Rahn, "Subharmonic resonances in higher-order collision-enhanced wave mixing in a sodium-seeded flame," Opt. Lett. 12, 912-914 (1987)