OPTICAL FORCE ON THE RAMAN DARK STATE IN 2 STANDING WAVES

We use the normal modes of atom-field coupling to explain the force on a three-level (LAMBDA) atom excited by two standing wave optical fields. An expression for the force in the weak field, small detuning limit is derived by inspection, and shown to agree with the exact solution to the optical Bloch equations. We describe the physical origin of the completely rectified force components as well as components which vary on scales much narrower than the optical wavelength. We emphasize the importance of the force component associated with the "dark state" and discuss the influence of the phase difference between the fields.