Standing-wave-like radiation force under travelling wave excitation

It is found that a quantum system excited by travelling laser waves can demonstrate inherent features of standing wave excitation, such as spatial inhomogeneous interaction and multiresonant radiation force. This behaviour takes place only if the transitions of the system interacting with resonant fields form a closed loop. Laser cooling of Λ-atoms excited by three travelling electromagnetic waves is investigated. In this case the temperature of cooled atoms can be two orders of magnitude below the Doppler limit.