Unbounded cooling force in traveling waves excitation of a four-level system

It is generally held that the process known as stimulated cooling requires the presence of intensity gradients. However, this study has found a system where the stimulated redistribution of photons can occur even without the presence of any intensity gradients. Specifically, this scheme employs a four-level system with two ground and two excited levels (the double Λ system). It is found that the cooling force obeys very simple resonance conditions and does not change sign over a large range of velocities. Moreover, this force scales linearly with rabi frequency, without bound (in the limit of rotating wave and electric dipole approximations). Therefore this process may be used to provide very strong cooling over a short period of time and over a broad range of velocities. These properties make this scheme a very attractive candidate for cooling open systems such as molecules.