Monitoring layer-by-layer assembly of polyelectrolyte multi-layers using high-order cladding mode in long-period fiber gratings

We have undertaken a combined theoretical and experimental investigation to illustrate and demonstrate the strong correlation between the order of the coupled cladding modes in long-period gratings (LPG) in conventional single mode fiber and their sensitivity to the process of layer-by-layer (LbL) assembly of polyelectrolyte multi-layers. We show that high-order cladding modes such as LP_0,10 in LPG are significantly more sensitive than their lower-order counterparts such as LP_0,2, with LP_0,10 yielding a shift of 1.575 nm in resonance wavelength per polyelectrolyte repeating unit of poly(vinyl pyrrolidone)/poly(methacrylic acid) bi-layer. The integrated LPG/LbL strategy as a robust LbL test-bed has broad ramifications in exploring and exploiting sensors and devices enabled by versatile, stimuli-responsive polyelectrolyte thin films.