Transverse shift of helical beams and subdiffraction imaging.

An imaging technique is here proposed to overcome the classical "diffraction limit" by using helical beams. This technique and the analysis presented are valid for all kinds of waves (either optical or acoustical) as long as the field can be considered as scalar. We show that the stable structure of such phase singularities turns out to be appropriate to measure both the position and the diameter of subdiffraction circular apertures. The property used is a shift of the scattered vortex. Its location is obtained with a very high resolution thanks to a nonclassical correlation method exploiting the superoscillating property of a vortex near its axis. This theoretical analysis is supported by acoustic experiments performed underwater evidencing subdiffraction imaging.