Radiative decay-induced narrow resonances in four-wave mixing due to nonconservation of population, alignment, and orientation.

It is demonstrated how spectroscopic line shape analysis of backward nearly degenerate four-wave mixing (NDFWM) can be used to determine all the relaxation parameters characterizing a two-level atom. This problem is investigated in the context of a simple gas-phase collisionless system but has important implications for interpretation of nonlinear spectroscopy data in more complicated systems, such as solids or liquids. The authors also report the observation of linewidths smaller than the radiative linewidth associated with the two-level transition. The narrow resonances that are observed can serve as the basis for constructing a narrowband tunable optical filter or for locking two laser frequencies together.