Decay induced narrow resonances in backward nearly degenerate four-wave mixing spectroscopy

We present results showing narrow resonant behavior in a collisionless four-wave mixing interaction in atomic sodium. In a two-level system the excited state radiatively decays to the ground state at rate ${\gamma }_{\mathrm{sp}}$, and the two states decay to the reservoir at ${\gamma }_1$ and ${\gamma }_2$. If ${\gamma }_1$ and ${\gamma }_2$ equal the transit time, the resonance width is determined by the total decay rate out of state 2. In the presence of additional excited-state decay to another state, a narrow resonance with width ${\gamma }_1$ is oberved as a result of the effect of the additional excited-state decay on the magnitude of the ground-state probability amplitude.