2, University of California, Riverside, Riverside, California, United States
In recent years, there has been an increasing demand for fibers and textile materials with integrated sensing and monitoring capabilities. One possible solution for producing multi-functional fibers is near-field electrospinning (NFE). This direct-write fabrication approach enables fast, yet precise positioning of micron-sized fibers for low-cost, scalable manufacturing. In addition, NFE is able to merge the strength and durability of polymers with the additional functionality of emitters for optically active sensing and/or receptors for enhanced selectivity. Here, fluorescent dye-doped polymer fiber sensors that support whispering gallery mode (WGM) resonance within the fiber cross-sections were fabricated and their sensing ability was demonstrated. Dye-doped polymer solutions were mixed, and were rheologically and optically characterized to determine the solution that would produce fibers with high quality (Q) resonance. Fibers were fabricated using NFE to draw fibers onto a substrate from a 25 wt% poly(vinyl) alcohol polymer solution doped with 1.74 mM rhodamine 6G. The substrates were patterned with deep trenches to prevent unwanted optical coupling, thus allowing WGM resonance to occur. The effect of different NFE parameters such as stage speed and applied voltage on fiber diameter was studied. The resulting fibers ranged from 2 to 22 µm in diameter and displayed circular cross sections. Using microphotoluminescence, resonant peaks with high Q factors (Q > 14,000) were measured in the wavelength range of 590 – 640 nm. Using size-dependent mode spacing predicted by finite-difference time domain simulations, the resonances were identified as first order WGMs. Each centimeter of fiber containing several resonators and the likelihood of finding a resonator in a suspended region of fiber was determined to be above 90%. Furthermore, isopropanol vapor sensing experiments confirmed the ability of the electrospun WGM fibers to detect small changes in the surrounding environment. The fibers fabricated here have demonstrated the potential of near field electrospinning to manufacture fibers with incorporated functionality for sensing and monitoring textile applications.