  We report an experimental technique to map and exploit the local density of
optical states of arbitrary planar nano-photonic structures. The method relies
on positioning a spontaneous emitter attached to a scanning probe
deterministically and reversibly with respect to its photonic environment while
measuring its lifetime. We demonstrate the method by imaging the enhancement of
the local density of optical states around metal nanowires. By
nano-positioning, the decay rate of a pointlike source of fluorescence can be
reversibly and repeatedly changed by a factor of two by coupling it to the
guided plasmonic mode of the wire.
