Effect of NaCl on the modulation of LPFG double peak by PDDA/PSS nanomembrane

In this paper, poly(diallyldimethylammonium chloride)(PDDA)/poly(sodium styrene sulfonate)(PSS) nanomembranes were deposited on the surface of long-period fiber gratings (LPFG) using the electrostatic layer-by-layer (LBL) assembly method, and the effect of NaCl on the modulation of LPFG double peaks by PDDA/PSS nanomembranes was investigated. The principle behind the emergence of double peaks was first explored using coupled mode theory, revealing that changes in the mode effective refractive index(RI) occur as the number of nanomembrane layers increases. The experimental results showed that under the conditions of PDDA with NaCl/PSS without NaCl and PDDA without NaCl/PSS with NaCl, double peaks do not appear in the spectra of LPFG as the number of thin film layers increases. In contrast, highly sensitive double peaks appear in the spectra of LPFG when both PDDA and PSS contain NaCl. Additionally, the range of these double peaks was explored. It was shown that the sensitivity of LPFG to the nanomembrane reached a maximum at 79-81 bilayers, with a sensitivity of 37.8 nm/bilayer, and the appearance of double peaks was consistent with the theoretical simulation. The findings of this study will be applied to the modification of LPFG surface biofunctional membranes and the highly sensitive detection of biochemical substances.