## 2 Design of the Plasmonic Sensor  
The theoretical basis of surface plasmons and SPR sensors can be consulted in [14; 15]. The sensor is composed of the following parts:P-polarized incident laser at 589 nm (sodium D line) and with S polarization, surface plasmon polaritons (SPPs) are not excited, for this reason, it is used the P polarization [16]. - Hemispherical prism SF10 with refractive index \(\mathrm{n_{p}}\) = 1.72803 [17] has a radius of curvature of approximately 0.56 cm with a spherical surface of approximately 4 cm\({}^{2}\) and a flat surface, corresponding to the contact surfaces of the layers with the medium of 2 cm\({}^{2}\). We propose two different materials for the metal medium, either aluminum \(\mathrm{n_{m}}\) = 0.94483 + 6.2571i [18], a thickness of 10 nm, a dielectric layer of magnesium fluoride (MgF\({}_{2}\)), refractive index \(\mathrm{n_{d}}\) = 1.3776 [19] and thickness of 110 nm or silver (Ag) \(\mathrm{n_{m}}\) = 0.049787 + 3.9063i [20], a thickness of 30.2 nm and a dielectric layer of MgF\({}_{2}\), refractive index \(\mathrm{n_{d}}\) = 1.3776 [19] and thickness of 81 nm. Figure 1 shows the diagram of the two sensors. The medium is a sucrose solution the refractive index of which is measured. Data of the refractive index and the concentration are in the chemistry and physics handbooks [19]. All the data are obtained at 20 \({}^{\circ}\)C, normal pressure (1 atm), and are measured at a wavelength of 589 nm (sodium D line). * Detector: this is used to collect the signal transported by the optical fiber from the hemispherical prism. The angle of half sensitivity is not critical as the radiation will hit the perpendicular detector. The detector must be a high-speed and highly sensitive PIN photodiode chip with, at least, \(\pi\) mm\({}^{2}\) sensitive circular area to cover the straight section of the output optical fiber. It has to be sensitive to visible light. It should have peak sensitivity at 589 nm or near to it. The minimum accuracy of the detector must be 0.2%. * Optical fiber: this is used only to transport the light from the laser to the hemispherical prism and from the hemispherical prism to the detector. The input and output optical fibers must be the same so that the losses are the same, and there is no asymmetry, and the reflectance can be calculated with minimum error. Taking into account the size of the hemispherical prism, the dimensions of the fibers can be: core diameter, cladding, and buffer 1 mm, 1.05 mm, and 1.25 mm, respectively. The core is pure fused silica, and the cladding is a polymer. The optical fiber is a multimode step-index and has an attenuation of 10 dB/km and a transmittance close to 100% at the working wavelength. To reduce costs, low working temperatures are not needed but it may  
Figure 1: Diagram of the two plasmonic sensors. At the top, on the left, the optical fiber transports the incident radiation to the hemispherical prism, and at the top, on the right, another optical fiber collects the reflected radiation. The SPPs are also shown propagating through the MgF\({}_{2}\)-silver/aluminum and/or silver/aluminum-medium interfaces. be that high temperatures are needed, around 100 \({}^{\circ}\)C when working with aqueous solutions close to boiling point. Electronic device: this processes the signal collected from the detector. It is connected to a PC to have the measurements in real-time (the refractive indexes and the concentration of sucrose). The system is connected with the sucrose filling machine. If the concentration of the sucrose is not high enough, the electronic device orders more sucrose to be added to the solution. If it is the correct amount, the electronic device remains on standby. If the concentration of sucrose is higher than the correct amount, the electronic device orders more water to be added until the correct concentration is achieved. The system is equipped with programmable alarms if is required. As can be seen in Figure 1, the light which travels through an optical fiber is injected into the hemispherical prism.