(color online) Plasmon rulers associated with symmetry breaking. (a) Displacement sensors based on a radiative sensing scheme.^[61] (b) Three-dimensional displacement sensing based on Fano resonance.^[130] (c) Displacement sensing in vertical and horizontal directions measured via the spectral shift and splitting of the cavity plasmons.^[28] Reprinted with permission from Copyright 2013 American Chemical Society, Copyright 2011 American Association for the Advancement of Science, Copyright 2016 Royal Society of Chemistry.