  We present topological derivative and energy based procedures for the imaging
of micro and nanostructures using one beam of visible light of a single
wavelength. Objects with diameters as small as 10 nm can be located, and their
position tracked with nanometer precision. Multiple objects distributed either
on planes perpendicular to the incidence direction or along axial lines in the
incidence direction are distinguishable. More precisely, the shape and size of
plane sections perpendicular to the incidence direction can be clearly
determined, even for asymmetric and non-convex scatterers. Axial resolution
improves as the size of the objects decreases. Initial reconstructions may
proceed by glueing together 2D horizontal slices between axial peaks or by
locating objects at 3D peaks of topological energies, depending on the
effective wavenumber. Below a threshold size, topological derivative based
iterative schemes improve initial predictions of the location, size and shape
of objects by postprocessing fixed measured data. For larger sizes, tracking
the peaks of topological energy fields that average information from additional
incident light beams seems to be more effective.
