UC Riverside

Because of their ability to serve in chemical and biological applications, there is a growing need for sensitive, compact, readily fabricated, and inexpensive chemical sensors. In recent years, devices that utilize whispering gallery modes (WGMs) have proven versatile in many such applications, however, these reports have typically used fabrication techniques that are either complex, do not produce sensitive devices, or are costly. One possible solution is to use polymer materials in conjunction with near-field electrospinning (NFES). This direct-write fabrication approach enables fast, yet precise positioning of micron-sized fibers for low-cost, scalable sensor manufacturing. Moreover, NFES can be used to incorporate additional functionality such as emitters for optically active sensing or receptors for enhanced selectivity. Here, fluorescent dye-doped polymer fiber refractive index sensors that support WGMs within the fiber cross-section were demonstrated. Poly(vinyl) alcohol resonators were first simulated with finite-difference time-domain simulations to show that the devices needed to be several microns in size and a few hundred nanometers from the substrate to support resonance. Polymer solutions of 25 wt% PVA and doped at with varied amounts of Rhodamine 6G were mixed and shown to have consistent physicochemical properties. Resonators were fabricated using NFES by drawing fibers from the polymer solution onto substrates patterned with deep trenches. The electrospun fibers ranged from 2 to 33 µm in diameter, displayed smooth surfaces, and had circular cross sections. Using microphotoluminscence, resonant groups of peaks with Q factors as high as 14,191 were observed in the emission range of the dye, from 590-700 nm. Using size-dependent mode spacing predicted by finite-difference time domain simulations, as well as a mode-fitting technique, the resonances were identified as WGMs. The response of the resonance peaks to exposure to isopropanol and methanol vapors was evaluated using a range of fiber diameters. The fibers were also crosslinked to make them water insoluble and their in-air and in-water resonance capabilities were analyzed. The crosslinked fibers displayed Q factors as high as 19,700 when measured in air and values as high as 7,500 when measured in water. Furthermore, aqueous sensing of ethanol was performed. The WGM resonators fabricated here have demonstrated the potential of near-field electrospun polymer-based fibers for sensing applications.