Bichromatic Optical Force
In the early days of laser cooling, the view of two-level atoms moving in a monochromatic laser beam provided a sufficient picture. The topics that could be described this way included atomic beam slowing and cooling, optical molasses, optical dipole traps, lattices and band structure effects, and a host of others. Within a few years, however, it became clear that this simple two-level atom view was inadequate, and that the multiple level structure of atoms was necessary to explain some of the experiments. This opened the way for a new era in laser cooling by providing descriptions of sub-Doppler cooling, magneto-optical trapping, velocity selective coherent population trapping, and many others. One might expect a similar plethora of new phenomena to emerge from the use of multiple beams of light of different frequencies or changing frequencies, (i.e., non-monochromatic) light, but this topic has been largely ignored.
The bichromatic force derives from improved control of the momentum exchange between the atoms and the light field. The forces arise by implementing a carefully orchestrated, rapid, coherent sequence of absorptions followed by stimulated emissions using non-monochromatic light, while comparatively infrequent spontaneous emission provide the irreversible processes required for the phase space compression that constitutes cooling. This force is both very much stronger and spans a very much larger velocity range than the ordinary radiative force that has been used for laser cooling since the early 1980's. Since it covers such a large range of velocities, Doppler compensation is not necessary for slowing a thermal beam. It also has a strong velocity dependence at its range boundaries, so that it can cool.