Doppler cooling of ion cyclotron motion in counter-propagating Laguerre-Gaussian beams

We demonstrate theoretically that ions moving in the field of two axial counter-propagating Laguerre-Gaussian (LG) light modes and a constant axial magnetic field, exhibit novel cooling effects. When the beams have orbital angular momentum I of the same sign we find that in addition to an axial force similar to the conventional plane wave Doppler cooling force, there is a velocity-independent quantised torque about the common beam axis which significantly influences the subsequent motion of the ion. The new effects are illustrated for the cooling of Mg-24(+) ions in the presence of an axial magnetic field.