| We have demonstrated several sub-Doppler laser cooling schemes using rubidium atoms in a one-dimensional optical molasses. We have shown the discovery of a new cooling process, magnetically induced laser cooling, using the combination of Larmor precession and optical pumping in the presence of a weak magnetic field. This is an example showing that sub-Doppler laser cooling works without the presence of optical polarization gradient. By increasing the magnetic field, we have shown that the atoms are cooled to the non-zero velocity with a spread below the Doppler limit. This phenomenon can be explained by coherent two-photon velocity selective resonances between the ground state sublevels. We have re-examined schemes using polarization gradients in the presence of the magnetic field. We show that velocity selective resonances exist in all well known sub-Doppler laser cooling schemes, thus enabling us to give a new interpretation for sub-Doppler laser cooling and to predict new effects. An extended theory has been developed to describe various sub-Doppler laser cooling schemes. We provide both detailed calculations and comparison with our experimental results. Extensions of the sub-Doppler laser cooling to more complicated atomic systems and the applications of cold atoms have been pointed out. |
