Abstract
This paper is a continuation of previous work and shows the enhancement of the surface plasmon resonance effect in a tapered optical fiber device. The study investigated liquid crystal cells containing a tapered optical fiber covered with a silver nanolayer, surrounded by a low refractive index liquid crystal in terms of the properties of light propagation in the taper structure. Silver films with a thickness of d = 10 nm were deposited on the tapered waist area. Measurements were performed at room temperature; liquid crystal steering voltage U from 0 to 200 V, with and without any amplitude modulation with a frequency of f = 5 Hz, and the wavelength λ ranged from 550 to 1200 nm. A significant influence of the initial arrangement of liquid crystals molecules on light propagation was observed. Three types of liquid crystal cells—orthogonal, parallel, and twist—were considered. During the measurements, resonant peaks were obtained—the position of which can also be controlled by the type of liquid crystal cells and the steering voltage. Based on the obtained results, the best parameters, such as highest peak’s width reduction, and the highest SNR value were received for twisted cells. In addition, the present work was compared with the previous work and showed the possibility of improving properties of the manufactured probes, and consequently, the surface plasmon resonance effect. In the presented paper, the novelty is mainly focused on the used materials as well as suitable changes in applied technological parameters. In contrast to gold, silver is characterized by different optic and dielectric properties, e.g., refractive index, extension coefficient, and permittivity, which results in changes in the light propagation and the SPR wavelengths.
Highlights
Surface plasmon resonance (SPR) sensors are widely used to analyze human fluids such as blood, urine, as well as to detect vitamins, e.g., B3, K1, A [1,2,3,4], proteins [5], for chemical analyses [6,7], and environmental screening especially for contamination from biological toxins and pathogens [8,9]
This paper presents the effect of a thin silver layer deposition on the tapered waist area
To achieve the spectral characteristics, a measuring system setup was built and contained the supercontinuum—broadband light source SuperK EXTREME (NKT Photonics, Birkerød, Denmark) which operates on the wavelength range of VIS-NIR (400–2400 nm), optical spectrum analyzer (OSA) AQ6373B (Yokogawa, Tokyo, Japan) with a detection range of 350–1200 nm
Summary
Surface plasmon resonance (SPR) sensors are widely used to analyze human fluids such as blood, urine, as well as to detect vitamins, e.g., B3, K1, A [1,2,3,4], proteins [5], for chemical analyses [6,7], and environmental screening especially for contamination from biological toxins and pathogens [8,9]. Fiber optic plasmonic sensors (FOPS) are probes based on the generation of plasmons that use thin metal films and nanostructures [11]. Several types of probes can be mentioned among the FOPS due to their geometry. The first one is based on modifications placed on the optical fiber face and includes angle-polishing, mirrors, and tips [12,13,14]. The second one is the modification of a side surface including side polishing, etching, bending, tapering, and making hetero-cores [15,16,17,18,19]. The tapered optical fiber (TOF) was investigated and only this modification will be taken into consideration
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