Subnanosecond Optical Free-Induction Decay

A novel form of laser frequency switching is devised that extends coherent optical transient studies to a 100-picosecond time scale, the measurements being performed in real time. Free-induction decay (FID) on a subnanosecond time scale reveals new features, such as a first-order FID that dephases with the inhomogeneous dephasing time T
2
* and interferes with the well-known nonlinear FID. A complete analytical expression for optical FID is derived and supports our FID observations for the sodium D
1
transition.