SPR Effect Controlled by an Electric Field in a Tapered Optical Fiber Surrounded by a Low Refractive Index Nematic Liquid Crystal

Abstract

This paper presents the influence of a thin metal layer deposition on the surface of a tapered optical fiber surrounded by a low liquid crystal, on light propagation inside the taper structure. In this research, three types of liquid crystal cells were under investigation: orthogonal, parallel, and twist. They differed by the rubbing direction of the electrodes in relation to the fiber axis determining the initial molecule arrangement inside the cell. Gold films with thickness d = 30 nm were deposited on the tapered fiber surface in the tapered waist area. Cells including a tapered optical fiber with no metallic layer were also examined and presented as a reference. All measurements were performed at room temperature for a different steering voltage U from 0 to 200 V, with and without any amplitude modulation with a frequency f = 5 Hz, and the wavelength λ range from 550 to 1200 nm. As a result, the resonant peaks were obtained, which depends on a liquid crystal cell type and steering voltage, as well. This paper shows the possibility of sensing the change of applied voltage by the constructed system. During measurements, additional effects as signal overlapping and intermodal interference were observed reducing measured voltage value. In the future, the improved, similar systems that will have a better response could be used as a sensor of factors to which liquid crystal (LC) will be sensitive, especially temperature and electric field.

SPR Sensor Based on a Tapered Optical Fiber with a Low Refractive Index Liquid Crystal Cladding and Bimetallic Ag–Au Layers

“…Because of high intermodal interference, the resonant resolutions have been approximated. For the orthogonal cell, the δλ res = 2.0 nm was obtained for U in the range from 0 to 20 V, and this value is similar to the results obtained for pure gold and pure silver films: [ 14 , 39 ]. In the case of the parallel cell, the resonant resolution was calculated for U: 0–40 V and amounts to δλ res = 0.5 nm.…”

“…In the case of the orthogonal cell, the absorption is the lowest and is approximately 60%. High power fluctuations are visible on the presented relations, but they are slightly lower than in the case of the single metal cover [ 14 , 39 ]. This is probably due to the deposition of a thinner metallic film, in particular the silver one ( h Ag ~ 5 nm).…”

“…If the LCC is thin enough, it is possible to steer the arrangement of LC molecules by using an electric field. Consequently, sensing of changes in the external environment is obtained by changes in the effective RI inside the medium, as was proved in the previous papers [ 14 , 33 ]. The resonant dips occur when no electric field is applied.…”

“…Regarding the modification site on the optical fiber, probes can be divided into side surface modified and forehead modified [ 8 ]. Among the side adjustments, there are few manufacturing methods: etching [ 9 ] or polishing [ 10 ] D-shape fiber formation or heterocore [ 11 ], bending (u-bent) [ 12 ], cavity [ 13 ], and tapering [ 14 ]. The forehead modification can be distinguished by tips, especially mirrors [ 15 ], angle polishing [ 15 ], tip tapering [ 16 ], and LRPR tip [ 17 ].…”

Detection of Organosulfur and Organophosphorus Compounds Using a Hexafluorobutyl Acrylate-Coated Tapered Optical Fibers

“…The aforementioned parameters enable sensors based on optical fibers to detect and measure such factors as, e.g., surrounding refractive index, temperature, pressure, mechanical strain, stress or vibrations and even chemical reagents [ 3 , 4 , 5 ]. To enhance the sensing possibilities of optical fibers, it is possible to combine them with additional functional materials, such as liquid crystals [ 6 ], alkanes [ 7 , 8 ], polymers [ 9 ], metals [ 10 , 11 ] or graphene compounds [ 12 ]. The most common methods of applying these functional materials to optical fibers is by forming an external coating after polishing the fiber [ 13 ] or fabricating a taper [ 14 , 15 , 16 ], or by filling the internal air holes in a photonic crystal fiber [ 17 ].…”