  The dynamic Stark shift of a high-lying atom in a system of Bose Einstein
condensation (BEC) of photons is discussed within the framework of
nonrelativistic quantum electrodynamics (QED) theory. It is found that the
Stark shift of an atom in BEC of photons is modified by a temperature dependent
factor, compared to that in a normal two-dimensional photonic fluid. In
photonic BEC, the value of Stark shift is always greater than that in
two-dimensional free space. Physical origin of the phenomenon is presented and
potential application is also discussed.
