Multi-frequency Optical Forces
The astounding general features of optical forces from multi-frequency light arise because the limits imposed on them are independent of the atomic properties, and instead can be set at the discretion of the experimentor. The most important of these features are force magnitude and velocity dependence. In single-frequency light, both of these are limited by the lifetime of the excited atomic state. But with more than one frequency, the additional experimental parameters of frequency separation or sweep rate set these limits, and these are selectable. Both force magnitude and velocity capture range can be orders of magnitude greater than the single-frequency light values. Moreover, all transitions can be stimulated, enabling coherent control
In the case of atoms with hfs or molecules with rovibrational ground state sublevels, several states can be populated by spontaneous decay, and thus a repump laser(s) may be needed, but that is not the case considered here. Instead, more than one frequency is tuned near the single resonance of a two-level atom, so that the atomic response depends on all the available frequencies. There are many forms of polychromatic light, but the types that we have studied so far are the simplest case, bichromatic, and the case of swept frequency light applied to adiabatic rapid passage. Each of these is described on other pages of this website.
The article located at RMP provides a broad overview of these topics.