In femtosecond degenerate four-wave mixing experiments in sodium vapor, quantum beats are observed at the wavelength of the third harmonic when the fundamental beams are three-photon resonant. The period of beating, which corresponds to fine structure energy-level splitting, does not change with the atomic number density, buffer gas pressure, or laser intensity. Surprisingly, the third-harmonic signal is observed only at negative time delays for the transient grating pulse sequence. We relate this to the well-known interference effect between excitation pathways involving the fundamental and the third-harmonic fields, which leaves atoms virtually unexcited and prevents formation of population grating.
© 2013 Optical Society of America
Original Manuscript: July 8, 2013
Revised Manuscript: August 28, 2013
Manuscript Accepted: August 30, 2013
Published: September 23, 2013
Hrvoje Skenderović and Silvije Vdović, "Atomic wave-packet dynamics and third-harmonic generation in sodium," Opt. Lett. 38, 3811-3814 (2013)