A high efficiency label-free photonic biosensor based on vertically stacked ring resonatorsC.E. Campanella, C.M. Campanella, F. De Leonardis and V.M.N. PassaroEur. Phys. J. Special Topics, 223 10 (2014) 2009-2021DOI: https://doi.org/10.1140/epjst/e2014-02244-5