Negative Photochromism in Hybrid Organic–Inorganic PES Optical Fibers Functionalized with Active WO₃ Nanoparticles

Incorporation of nanostructured photochromic WO3–x (NP-WO) in polymer optical fibers (POFs) made of polyethersulfone using a postdrawing coating method is reported. Upon ultraviolet (UV) irradiation, a higher coloring intensity of the cladding of nanostructured fibers is linked with a remarkably clear amplification (negative photochromism) of the transmission signal through the fiber. The postcoating method was thoroughly optimized by using poly(vinylpyrrolidone) (PVP) both as a dispersant agent and a cladding matrix for NP-WO. Modeling shows that the decrease of the real refractive index n in the transparency range of the POF (600–1200 nm) associated with the increase of the absorption coefficient k versus coloring under UV light allows the controllable "opening" of the fiber transmission. The fiber core negative photochromism implying the coloration under UV irradiation of the NP-WO-based fiber cladding represents a brand new optical effect, which could be used in various applications such as sensing, camouflage, or smart envelops.