Fast, robust and simple all optical far-field nano-positioning measurements

We present a novel method for the nano-positioning measurements of a sample. Based on symmetry considerations, the method transforms the problem of position sensing into identification of topologically robust features obtained from the scattered field. In particular, we exploit the cylindrical symmetry of a control defect appended to the sample. Experimentally, we demonstrate a precision of 10 nm (λ/60) which is limited by the stability of our mechanical set-up, rather than the fundamental resolution of the technique. This far field method provides a deeply sub-wavelength precision and the possibility of rapidly retrieving the initial zero position. In addition, we envision that the principles of the technique could be adapted to other measurements and variety sample geometries.