Optical detection of the percolation threshold of nanoscale silver coatings with optical fiber gratings

The metal-to-dielectric transition of silver films deposited on single-mode optical fibers is monitored by measurements of the transmission spectra of tilted fiber Bragg gratings inscribed in the core of the fiber. In situ, real-time measurements of the spectrum at wavelengths near 1550 nm during the wet etching of a 50 nm thick silver coating show a sudden and temporary decrease of more than 90% in the amplitudes of the core-to-cladding mode coupling resonances when the film thickness reaches 18 nm. Confirmation that this observation corresponds to the percolation threshold is obtained from simultaneous measurements of the silver coating conductivity during etching and by simulations of the grating response. The characteristic spectral signature of the percolation threshold is only obtained for cladding modes polarized azimuthally in the fiber cross section, i.e., parallel to the film surface.