Virtual single-photon transition interrupted: time-gated optical gain   and loss
  The response of matter to an optical excitation consists essentially of absorption and emission. Traditional spectroscopy accesses the frequency-resolved and time-integrated response, while the temporal evolution stays concealed. However, we will demonstrate here that the temporal evolution of a virtual single-photon transition can be mapped out by a second pulsed electromagnetic field. The resulting optical signal shows previously unexpected optical gain and loss, which can be gated and controlled via the relative delay of the electromagnetic fields. The model presented here can be applied to any system that assumes a two-level character through near-resonant optical dipole excitation, whether they are of atomic, molecular or even solid-state nature. These theoretical observations are in excellent qualitative agreement with our transient absorption spectroscopy study in helium. The presented results can act as starting point for a new scheme for creating optical gain, which is a prerequisite for the operation of lasers. It may be possible to open the doors to spectral regions, which were difficult to access until now, e.g. in the extreme ultraviolet. 
Jens Herrmann, Matthias Weger, Reto Locher, Mazyar Sabbar, Paula   Rivi\`ere, Ulf Saalmann, Jan-Michael Rost, Lukas Gallmann...
physics.atom-ph