Scanning evanescent fields using a pointlike light source and a nanomechanical DNA gear.

The characterization of three-dimensional inhomogeneous illumination fields is a challenge in modern microscopy. Here we use a four-arm DNA junction as a nanomechanical translation stage to move a single fluorescent quantum dot through an exponentially decaying evanescent field. Recording the emission of the quantum dot within the evanescent field as well as under homogeneous illumination allows one to directly obtain the intensity distribution of the excitation field without additional deconvolution. Our method will allow the characterization of a broad range of illumination fields and to study near-field effects between small optical probes.