Applied sciences  
Electrical Special Issue on Optical Sensors and Gauges Based on Plasmonic Resonance  
[PERSON]  
Centre for Applied Sciences and Engineering, University of Granada, 18071 Granada, Spain; fperez@ugres  
A surface plasmon is a plasmon that propagates through a surface; i.e., it is an electromagnetic wave from the interaction of an electromagnetic wave with the oscillations of free electrons on the surface of a metal, which propagates at the separation interface of a metal (Drude model) [1]. There are two types of plasmons, the polariton surface plasmon that has the form of an electromagnetic wave, which propagates through the interface of a metal-dielectric surface [2], and the localized polariton surface plasmon, which is comprised of localized electron oscillations in metallic nanoparticles that are found inside a dielectric [3]. From the Lycurgus Cup in 272-337 AD [4], through the wonderful stained glass of Romanesque and Gothic churches and cathedrals [5] to the most modern sensors, plasmons have always been present in the life of human beings, although we did not know about them. The first to obtain the mathematical expression of a plasmon was [PERSON] in 1908 [6]. Humanity had to wait almost 70 years until work with particles began in the 1980s, resulting in one of the first articles on plasmonic sensors to be published in the world [7]. It has been a hard task; physicists and engineers have had to work together, first to formalize the equations of plasmons and evanescent waves at dielectric-metal interfaces and second to build structures that would make it possible to obtain these plasmons. Later, these scientists, working together with biologists, medical doctors, chemists, etc. paved the way for the creation of plasmonic sensors gradually designed to measure all kinds of variables, not just physical ones. The last step has been the design and manufacture of new materials, metal alloys, or the doping of materials to replace metals and dielectrics and achieve better resolutions and sensitivities [8]. Considering the structure of the sensor, the only configurations are Otto (prism-metal-dielectric) [9] and Kretschmann (prism-dielectric-metal) [10]. In terms of sensor design, there are intensity, wavelength, and angle interrogation sensors. Although they may all have the same sensitivity and resolution, sensors based on intensity interrogation have the advantage of not having moving parts, so there is no mechanical wear and tear, and they will have a long useful life [11]. Do plasmonic sensors really measure the amount of substance in a certain medium, for example, do they measure salinity, sucrose concentration, the presence of CO, etc.? The answer is no. These sensors measure differences in the refractive index of the medium, and from here, we can relate, for example, the concentration of sucrose in a medium to the refractive index. Measuring the refractive index (or changes in refractive indices) is what makes them better, more general and universal, since they do not analyze a specific substance but rather a property. They are nanophotonic sensors, so their tiny size allows them to be integrated with more sensors to build an array and be able to detect different substances, materials, etc. without more than multiplexing in time or space the input and output optical power [12] or integrating them into more complex sensors capable of measuring more than one parameter at the same time [13]. The applications of these sensors are not only in the field of physics; they are also being used very successfully in chemistry, biology, medicine, etc. In the latter case, as they are made of biocompatible materials, they can be introduced into the patient's body (if necessary) without putting humans (or animals) at risk.