Phase control of electromagnetically induced transparency in a four-level system

We present a theoretical study on the frequency position and the linetype of Electromagnetically Induced Transparency (EIT) in a four-level atomic system with a coupling field, two microwave fields and a probe field. The absorption spectrum which is characterized by two EIT windows is obtained by a weak probe field scanning corresponding transition. It can be found that the frequency position and the linetype of EIT change with the relative phase of the two microwave fields. The frequnecy interval reaches the minimum when the relative phase is reverse. If two microwave fields have the same strength and reverse phase, the absorption spectrum will exhibit an EIT. This proposes a way to controlling frequency position of EIT by modulating the relative phase between two fields.