SPR Sensor Based on Tapered Optical Fiber with a Low Refractive Index Liquid Crystal Cladding ^†

Tapered optical fibers, due to their specific optical properties, are great sensing elements. During the elongation of an optical fiber, a tapered waist forms, which allows the electromagnetic wave to propagate in the whole volume of this structure; simultaneously, some part of this wave leaks out of the taper as an evanescent wave. In fiber-based surface plasmon resonance sensors, the evanescent field of propagating modes in a fiber penetrates a thin metal layer and, thus, the surface plasmons are excited at a metal/dielectric interface. Additionally, to control and strengthen the SPR effect, liquid crystal cladding with controllable refractive indices by voltage has been applied. This research focused on an electric field sensor based on a tapered optical fiber probe. The manufactured sensor consisted of gold- and silver-coated thin films in the tapered waist area of an optical fiber with the low-refractive-index liquid crystal named 3092A as cladding. The Au and Ag layers with thicknesses from 10 to 30 nm were deposited using the sputtering method. Because of the significant influence of liquid crystal molecules’ initial arrangement on light propagation, three types of liquid crystal cells, orthogonal, parallel, and twist, were considered. Measurements were performed at room temperature, and the steering voltage ranged U from 0 to 200 V in a wide optical range. The obtained results allowed us to establish resonant peaks, the depths and positions of which depended on the metallic layer used, liquid crystal cell type, and voltage, and could be controlled by the mentioned factors.