  Excess "micromotion" of trapped ions due to the residual radio frequency (rf)
trapping field at their location is often undesirable and is usually carefully
minimized. Here, we induce precise amounts of excess micromotion on individual
ions by adjusting the local static electric field they experience. Micromotion
modulates the coupling of an ion to laser fields, ideally tuning it from its
maximum value to zero as the ion is moved away from the trap's rf null. We use
tunable micromotion to vary the Rabi frequency of stimulated Raman transitions
over two orders of magnitude, and to individually control the rates of resonant
fluorescence from three ions under global laser illumination without any
changes to the driving light fields. The technique is amenable to situations
where addressing individual ions with focused laser beams is challenging, such
as tightly packed linear ion strings or two-dimensional ion arrays illuminated
from the side.
