REVERSAL OF RADIATION REACTION FORCE AND INSTABILITY OF THE GROUND-STATE OF AN ATOM LOCATED ABOVE THE SURFACE OF AN ACTIVE MEDIUM

It is shown that spontaneous emission of an excited atom is suppressed , and electromagnetic instability of the ground state is possible, if an atom is located in the vicinity of an active medium which amplifies the propagating radiation. The effect is due to a modified and phase- shifted near held of an oscillating atomic dipole, which prevails over the usual radiation reaction field related to the wave zone. The nece ssary conditions for the reversal of the total electromagnetic back re action are found, and possible experimental realizations are discussed .