Four-color stimulated optical forces for atomic and molecular slowing

Stimulated optical forces offer a simple and efficient method for providing optical forces far in excess of the saturated radiative force. The bichromatic force, using a counterpropagating pair of two-color beams, has so far been the most effective of these stimulated forces for deflecting and slowing atomic beams. We have numerically studied the evolution of a two-level system under several different bichromatic and polychromatic light fields, while retaining the overall geometry of the bichromatic force. New insights are gained by studying the time-dependent trajectory of the Bloch vector, including a better understanding of the remarkable robustness of bi- and polychromatic forces with imbalanced beam intensities. We show that a four-color polychromatic force exhibits great promise. By adding new frequency components at the third harmonic of the original bichromatic detuning, the force is increased by nearly 50$%$ and its velocity range is extended by a factor of 3, while the required laser power is increased by only 33$%$. The excited-state fraction, crucial to possible application to molecules, is reduced from 41$%$ to 24$%$. We also discuss some important differences between polychromatic forces and pulse trains from a high-repetition-rate laser.