Electromagnetically induced focussing effects in four level media

Summary form only given. Electromagnetically induced focussing (EIF) results in the control of lensing action on one light field by the application of another. This has been shown to occur in three level cascade systems. We consider an extension to four level cascade schemes where the upper transition is coupled by a RF field. Previous work on EIT in a four level cascade manifold has shown that the EIT can be destroyed by a third field, hence providing a method of sub-Doppler spectroscopy. The idea is based around a three photon interference route which interferes with the one and two photon routes normally associated with EIT and destroys the coherence generated by the EIT. A limitation is that in a Doppler broadened system of atoms if the third field is an optical one, the effects of this extra field are washed out in the broadening process. However we show that by employing a RF field as the third field we can overcome these problems.