Modification of Higher Alkanes by Nanoparticles to Control Light

micromachines

Article Modiﬁcation of Higher Alkanes by Nanoparticles to Control Light Propagation in Tapered Fibers

Karol A. Stasiewicz 1,* , Iwona Jakubowska 1, Joanna Korec 1 and Katarzyna Matras-Postołek 2

1 Institute of Applied Physics, Faculty of Advanced Technologies and Chemistry, Military University of Technology, 2 Kaliskiego St., 00-908 Warsaw, Poland; [email protected] (I.J.); [email protected] (J.K.) 2 Department of Biotechnology and Physical Chemistry, Faculty Chemical Engineering and Technology, Cracow University of Technology, 24 Warszawska St., 31-155 Cracow, Poland; [email protected] * Correspondence: [email protected]

Received: 20 October 2020; Accepted: 13 November 2020; Published: 14 November 2020

Abstract: This study presents the doping of higher alkanes, namely, pentadecane (C15) and hexadecane (C16), with ZnS:Mn nanoparticles to create new types of in-line optical ﬁber sensors with unique optical properties. In this research, the phenomenon of light beam leakage out of the taper and its interaction with the surrounding materials is described. The fabricated new materials are used as cladding in a tapered optical ﬁber to make it possible to control the optical light beam. The manufactured sensor shows high sensitivity and fast response to the change in the applied materials. Results are presented for a wide optical range of 1200–1700 nm with the use of a supercontinuum source and an optical spectrum analyzer, as well as for a single wavelength of 800 nm, corresponding to the highest transmitted power. The results present a change in the optical property dependence on the temperature in the cooling and heating process. For all materials, the measurements in a climatic chamber are provided between 0 and 40 ◦C, corresponding to the phase change of the alkanes from solid to liquid. The addition of nanoparticles to the volume of alkanes is equal to 1 wt%. To avoid a conglomeration of nanoparticles, the anti-agglomeration material, Brij 78 P, is used.

Keywords: nanoparticles; optical fiber taper; fiber technology; sensors; higher alkanes; advancements in fiber forming technologies; functional micro- and nanofibers for miniaturisation technologies

1. Introduction In recent years, a rapid development in optical fiber technology has been observed [1]. Light propagation in a fiber is associated with the difference between the fiber structure, especially the properties of the core and cladding materials, corresponding in most cases with their refractive indices ncore and ncladding, and the core diameter. For all kinds of fibers, a fundamental requirement is to protect the light inside the structure. The electromagnetic field of light waves propagating inside the fiber can be derived from solving Maxwell’s equations with boundary conditions given by the material structure [1–3]. Two methods of light propagation are currently possible. The first method is based on Snell’s law, where the total internal reflection (TIR [3]) phenomenon is used. This method of propagation is mostly used for standard fibers with a solid core and cladding, and with an mTIR modified version for photonic crystal fibers with the structure of air holes representing the cladding and solid core. The second method of light propagation involves the phenomenon of photonic bandgap propagation [4], which is mostly used in a photonic crystal fibers with air hole cores.

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In standard fibers, the difference between the value of the refractive index of the core and cladding determines the number of modes propagating in a fiber, known as the normalized frequency V, which is equal to 2405. This parameter is described by [5]: q 2π 2 2 V = a ncore n (1) λ − cladding Additionally, this parameter is dependent on the wavelength and the core diameter. To change the propagation properties in both cases, it is necessary to change the refractive index or the core diameter. A second method for changing the light propagation parameter is to modify the size of the fiber core. It should be noted that the propagation of a light beam in fibers, especially in a standard one, due to Snell’s law, is also connected with a reflection on the border of two materials from which the core and cladding are formed. In such a propagation, part of the power passes from the core to the cladding for a certain distance. This distance is known as the penetration depth of light from the core to the cladding and can be described by [6–8]:

λ dp = q (2) 2 2 2 2π ncore sin θ n − cladding where λ is the determined wavelength of the light, θ is the angle of the incident plane wave on the core/cladding surface and ncore and ncladding are the core and cladding refractive indices, respectively. This parameter depends on wavelength propagation in fibers, as well as on the angle of an incident plane wave on the core and cladding border [8–10]. It is noteworthy that this parameter enables direct access to the light and increases the sensitivity to external changes in the environment. In the presented case, the applied alkanes with nanoparticles interact in a leaking light as double-clad materials that change their phase state. Theoretical and experimental investigation of changing core and cladding diameters, optical beam parameters, as well as a refractive index profile along with the fiber show that with the reduction of the diameter/dimensions of the core and the clading, the diameter of the propagating beam increases, filling the entire taper structure and extending beyond it [11]. For both kinds of fibers, a technique known as tapering can be applied for changing the core and cladding diameters [6–11]. Tapering technology uses a relatively easy technique to fill the air holes and provides the possibility of the continuous monitoring of changes of the light propagation through the fiber during element manufacturing, i.e., a diameter of taper region control. The above description of light propagation in different fibers shows its wide range of applications. The telecommunication application of fibers is widely described in the literature [12]. Nowadays, significant interest is directed to optical fiber sensors [12,13] due to their small dimensions, low weight, rapid response, and the possibility of operating in different environments. Their construction is divided into several groups. One of the groups is based on an interferometry measurement that enables very precise measurements [13] but requires an advanced system of acquisition and causes an increase in dimensions. Some sensors use fibers as elements that deliver light to the measurement places and their light beam is output from the fiber and interacts with a measurement factor [14]. The final part of sensors uses the change of light amplitude as an interaction of the external parameters of the fiber [14]. The accuracy and sensitivity of such sensors are strictly connected with the influence of the detected factors on light propagation. For these reasons, in many detectors, additional materials are introduced as media that multiply the signal level, which influences the light beam. In all the above mentioned cases, the main advantage is that the light measures the change of the surrounding environment without changing the material and geometrical properties of the investigated material. In addition, in recent years, nanoparticles have become a significant topic of interest. Unique results for light interaction with matter can be observed in semiconducting nanoparticles. They possess interesting optical, electronic, chemical, and electrochemical properties that are useful for a Micromachines 2020, 11, 1006 3 of 13

Micromachines 2020, 11, x 3 of 14 wide variety of applications, including optoelectronics [15,16], catalysis [17,18] and sensing [19]. Among II-VI semiconducting nanoparticles, manganese-doped zinc sulfide (ZnS:Mn) nanocrystals Among II-VI semiconducting nanoparticles, manganese-doped zinc sulfide (ZnS:Mn) nanocrystals have been substantially studied due to their unique properties and their diverse applications, have been substantially studied due to their unique properties and their diverse applications, including photovoltaics [20,21] and transparent nanocomposites [22]. Zinc sulfide is a large band-gap including photovoltaics [20,21] and transparent nanocomposites [22]. Zinc sulfide is a large band-gap energy (3.6 eV) semiconductor that is well known for its photoluminescence, electroluminescence, energy (3.6 eV) semiconductor that is well known for its photoluminescence, electroluminescence, and thermoluminescence. ZnS:Mn and its orange emission have been known since the early days of and thermoluminescence. ZnS:Mn and its orange emission have been known since the early days of luminescence research because of its high quantum efficiency at room temperature. ZnS:Mn is non- luminescence research because of its high quantum efficiency at room temperature. ZnS:Mn is non-toxic, toxic, chemically more stable, and technologically better than many other semiconducting materials chemically more stable, and technologically better than many other semiconducting materials based based on toxic elements, such as CdSe and CdS [23,24]. on toxic elements, such as CdSe and CdS [23,24]. In this study, threshold temperature sensors based on the use of additional materials, namely, In this study, threshold temperature sensors based on the use of additional materials, namely, alkanes with a nanoparticle admixture that influence light propagation in a biconical fiber taper, are alkanes with a nanoparticle admixture that influence light propagation in a biconical fiber taper, presented. are presented.

2.2. Materials Materials and and Methods Methods

2.1.2.1. Materials Materials InIn the the research, research, a a standard standard optical optical fiber ﬁber (SMF28e) (SMF28e) was was used used and and built built as as two two concentrically concentrically arrangedarranged materials materials to to form form the the structure structure of of core core and and cladding cladding (Figure (Figure 11).).

FigureFigure 1. 1. PropagationPropagation of of light light in in an an optical optical fiber ﬁber taper taper [25,26]. [25,26].

TableTable 11 presentspresents thethe main main parameters parameters of of the the used used fiber fiber in thein the future future technological technological process. process. As canAs canbe seen,be seen, the the refractive refractive index index of of the the core core is is slightly slightly higher higher thanthan thethe refractive index of of the the cladding. cladding. SomeSome part part of of the energy leaks toto thethe claddingcladding as as an an evanescence evanescence field field (Figure (Figure1) 1) within within a distance a distance of upof upto severalto several dozen dozen nm [10nm], known[10], know as then penetrationas the penetration depth. Indepth. such aIn structure, such a structure, there are no there possibilities are no possibilitiesto directly interact to directly with interact the light with propagating the light inside propagating the core dueinside to the muchcore due larger to dimensionsthe much larger of the dimensionsthickness of of the the fiber thickness cladding. of the fiber cladding.

TableTable 1. 1. ParametersParameters of of applied applied standard standard telecommunication telecommunication fiber. ﬁber.

ParameterParameter [Units] [Units] Core Core Cladding Cladding Mode ModeField Field DiameterDiameter [µm] [µm] 8.2 8.2 125 125 10.98 10.98 RefractiveRefractive index index [a.u.] [a.u.] 1.4548 1.4548 1.443 1.443 -- –

One of the methods used to obtain a decreasing diameter of the core and the cladding is a process in whichOne part of the of methodsthe fiber usedis heated to obtain to the adecreasing melting point diameter and is of then the corestretche andd the out. cladding This technique is a process is calledin which the parttapering of the technique. fiber is heated Due to to the the wide melting descript pointion and of tapers is then in stretched literature out., hence, This it technique will not be is presentedcalled the in tapering this paper technique. [7,10]. Due to the wide description of tapers in literature, hence, it will not be presentedThe obtained in this paper structure [7,10 ].can be described as a structure in which the light propagates in a dielectricThe structure obtained structure(taper waist) can becorresponding described as to a structure the fiber incore which and thethe lightair surrounding propagates inbecomes a dielectric the cladding.structure The (taper difference waist) corresponding in refractive indices to the fiberbetween core the and core the and air surrounding the cladding becomes in a standard the cladding. fiber is ~1%The (see diff erenceTable 1). in For refractive a structure indices with between surrounding the core air (refractive and the cladding index of in ~1003), a standard the value fiber is is~45% ~1% (assuming the average value of the collapse coverage made the structures 1.45). In such

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Micromachines(see Table1 ).2020 For, 11, a x structure with surrounding air (refractive index of ~1003), the value is ~45%4 of 14 (assuming the average value of the collapse coverage made the structures 1.45). In such arrangements, arrangements,there exists an opportunitythere exists toan replace opportunity air with to various replace materials. air with Suchvarious works materials. have been Such widely works used have to beenbuild widely sensors, used ﬁlters, to build ampliﬁers sensor [27s, –filters,35]. amplifiers [27–35]. As base materials that act as double cladding, higher alkanes, such as n-pentadecane (C15) and n-hexadecane (C16), have been applied. Table 2 2 shows shows thethe mainmain parametersparameters ofof thesethese alkanesalkanes [36[36,37].,37].

Table 2. Chemical parameters of n-pentad n-pentadecaneecane and n-hexadecane [[36,37].36,37].

Refractive Molecular Melting Boiling Flash RefractiveRefractive Alkane Formula Molecular Melting Boiling Flash DensityRefractive Index at Alkane Formula Mass Point Point Point Density Index Index at Mass Point Point Point Index 20.4 C 20.4 °C◦ n- 132 ◦C n- CHCH33(CH(CH2)13 CH3 212.42 9–10 ◦C 269–270 ◦C 132 °C 0.769 1.4320 1.4317 Pentadecane 212.42 9–10 °C 269–270 °C (269 ◦F) 0.769 1.4320 1.4317 Pentadecane CH3 (269 °F) n- 135 ◦C CH3(CH2)14CH3 226.45 18 ◦C 287 ◦C 0.773 1.4345 1.4344 Hexadecanen- CH3(CH2)14 135 (275°C F) 226.45 18 °C 287 °C ◦ 0.773 1.4345 1.4344 Hexadecane CH3 (275 °F) The presented chemical properties of the used materials were checked in our laboratories at The presented chemical properties of the used materials were checked in our laboratories at a a temperature of 20.4 ◦C using a Hanna Instruments HI 968000 (Woonsocket, RI, USA) electronic temperature of 20.4 °C using a Hanna Instruments HI 968000 (Woonsocket, Rhode Island, USA) refractometer in the range of 1.3330–1.5040 nD20, with a control material standard water used for the electronic refractometer in the range of 1.3330–1.5040 nD20, with a control material standard water measurement of the refractive index equal to 1.3329. used for the measurement of the refractive index equal to 1.3329. These materials are very well known and have been described widely [38–40]. As seen from These materials are very well known and have been described widely [38–40]. As seen from Table2, the refractive indices are close to those of the standard cladding of the fiber. Table 2, the refractive indices are close to those of the standard cladding of the fiber. It should be mentioned that higher alkanes change their transparency for light together with It should be mentioned that higher alkanes change their transparency for light together with phase change in a melting point [41,42]. Analyzing the state of RI in a solid stated, the extinction ratio phase change in a melting point [41,42]. Analyzing the state of RI in a solid stated, the extinction ratio is high, which also causes that the real part of RI of alkanes is higher than RI of fibers which influences is high, which also causes that the real part of RI of alkanes is higher than RI of fibers which influences the light attenuation. the light attenuation. The innovation in this study is an admixture of the mentioned alkanes with nanoparticles of The innovation in this study is an admixture of the mentioned alkanes with nanoparticles of ZnS:Mn + DBSA + CYS, which means that nanoparticles consist of the material ZnS: Mn (zinc sulfide: ZnS:Mn + DBSA + CYS, which means that nanoparticles consist of the material ZnS: Mn (zinc sulfide: manganese) and have a combination of dodecylbenzenesulfonic acid (DBSA) and 2-mercaptoethylamine manganese) and have a combination of dodecylbenzenesulfonic acid (DBSA) and 2- hydrochloride (cysteaminium chloride, cys determination) on the surface. Images of the used mercaptoethylamine hydrochloride (cysteaminium chloride, cys determination) on the surface. nanoparticles are presented in Figure2. Figure3 shows the particle size distribution of the Images of the used nanoparticles are presented in Figure 2. Figure 3 shows the particle size ZnS:Mn nanoparticles. distribution of the ZnS:Mn nanoparticles.

Figure 2.2. Picture of applied nanoparticles of ZnS:Mn + DBSA + CYS.CYS.

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Micromachines 2020, 11, x 6 of 14 wave were maintained, as was the response for the influences on optical propagation of light inside of the created structure. Table 4 presents the cumulative refractive indices of the mixtures of alkanes with Brij 78 P (0.5 wt%) and nanoparticles (1 wt%).

Table 4. Refractive indices of investigated mixtures at 20.4 °C.

C15 with Brij and C16 with Brij and C15 C15 with Brij C16 C16 with Brij Nanoparticles Nanoparticles Figure 3.3.Particle Particle size size distribution distribution of ZnS:Mn of ZnS:Mn nanoparticles nanoparticles stabilised bystabilised CYS and DBSAby CYS in tetrahydrofuran.and DBSA in 1.4317 1.4319 1.4319 1.4344 1.4346 1.4346 tetrahydrofuran. The size of the ZnS:Mn nanoparticles dispersed in THF (Tetrahydrofuran) was determined by 2.2. Technology DLS (dynamicThe size of light the scattering).ZnS:Mn nanoparticles Figure4 shows disperse the resultingd in THF distribution, (Tetrahydrofuran) with a hydrodynamic was determined radius by ofDLS ~6 In(dynamic nm. this According section, light the scattering). to technology the Scherrer Figure of equation, threshold 4 shows the se thensor calculated resulting manufacturing crystallite distribution based size, ofwith on the a a particlesbiconical hydrodynamic isoptical three fibertimesradius taper smaller of ~6 covered nm. than According the in higher diameter, to alka the asnes Scherrer determined with nanoparticlesequation, by dynamic the iscalculated lightpresente scattering. crystallited. For the This sizemanufacturing can of be the explained particles of the byis biconicalthethree intensity times optical smaller weighted fiber than tapers, indication the the diameter, author’s of the particle arrangas determements diameterined named by and dynamic theFOTET organic lightII (Fiber shell scattering. Optic around Taper This the Element particle,can be Technology)leadingexplained to by higher werethe intensity valuesused [34], for weighted theas presented particle indication diameters. in Figure of the 4. Additionally, particle diameter it has and to be the considered organic shell that around crystal imperfectionsthe particle, leading contribute to higher to additional values for line the broadening particle diameters. in X-ray diAdditionally,ffraction. it has to be considered that crystal imperfections contribute to additional line broadening in X-ray diffraction. The materials for the synthesis of the ZnS:Mn nanoparticles were used without additional purifications. Manganese (II) acetate tetrahydrate (~99%), zinc (II) acetate dehydrate (~98%), and tetratrahydrofuran (~99%) were purchased from Sigma Aldrich. Cysteamine chloride (~97%) and 4- dodecylbenzenesulphonic acid (~90%) were purchased from Merck and Fluka, respectively. Sodium sulfide hydrate (~60%) was pursed from Riedel-de Haan. The synthesis procedure for the ZnS:Mn nanoparticles is patented [27]. The ZnS:Mn nanoparticles were stabilised by cysteaminium chloride and 4-dodecylbenzylsulphonic acid were used in our previous work as an element of hybrid photovoltaic devices [20]. In a typical synthesis, 10.3 mmol of zinc (II) acetate dehydrate, 1.95 mmol of manganese (II) acetate tetrahydrate, and 7.40 mmol of cysteamine hydrochloride were dissolved in 125 mL of distilled water (solution I) under stirring for ~15 min. Simultaneously, 7.4 mmol 4- Figure 4. Schematic of FOTET system for biconical optical fiber manufacturing [34]. dodecylbenzenesulphonic acid was dissolved in 2 mL of tetrahydrofuran (solution II) and 5.80 mmol Figure 4. Schematic of FOTET system for biconical optical fiber manufacturing [34]. of sodiumThe materials sulfide was for dissolved the synthesis in 50 ofmL the of water ZnS:Mn (solution nanoparticles III), respectively. were used Solution without I was additional placed in a 250 mL three-neck flask. Solution II was very slowly injected under stirring into solution I. To purifications.A detailed description Manganese of (II) the acetate manufacturing tetrahydrate proces (~99%),s are included zinc (II) in the acetate previous dehydrate articles, (~98%), hence, precipitate the ZnS:Mn QDs, the aqueous solution of sodium sulfide (solution III) was slowly added itand will tetratrahydrofuran not be presented in (~99%) this paper were purchased[11,34]. Prepared from Sigmatapers Aldrich.used for Cysteaminepresented threshold chloride sensors (~97%) dropwise under stirring into the reaction mixture inside the round-bottom flask. Finally, the turbid areand characterized 4-dodecylbenzenesulphonic by low losses below acid α (~90%) = 0.2 dB were in a purchased whole investigated from Merck range, and elongation Fluka, respectively. L = 20.20 dispersion was refluxed at 85 °C under stirring for 3.5 h. The precipitated nanoparticles were collected ±Sodium 0.05 mm sulfide and diameter hydrate (~60%)of the taper was waist pursed φ = from 14.50 Riedel-de ± 0.50 µm. Haan. The synthesis procedure for the by centrifugation at 10,000 rpm for 15 min. Finally, the white powder of the ZnS:Mn nanoparticles ZnS:MnIn the nanoparticles next step, due is patentedto the mechanical [27]. The properties ZnS:Mn nanoparticles of the prepared were taper, stabilised the protection by cysteaminium tube was was dried in a vacuum oven at 60 °C for 24 h. usedchloride with and a schematic 4-dodecylbenzylsulphonic and cross-section acidpresented were used in Figures in our 5a,b, previous respectively, work as anand element a sample of hybridimage From our measurements and observations, the most important information was that the photovoltaicin Figure 5c. devices [20]. In a typical synthesis, 10.3 mmol of zinc (II) acetate dehydrate, 1.95 mmol ofagglomeration manganese (II)of nanoparticles acetate tetrahydrate, occurs very and 7.40often. mmol This of problem cysteamine was hydrochlorideresolved through were the dissolved use of a inspecial 125 mLmaterial of distilled known wateras polyethylene (solution I)glycol under monooctadecyl stirring for ~15 ether, min. Brij Simultaneously, 78 P, with its parameters 7.4 mmol 4-dodecylbenzenesulphonicpresented in Table 3 [28]. acid was dissolved in 2 mL of tetrahydrofuran (solution II) and 5.80 mmol of sodium sulfide was dissolved in 50 mL of water (solution III), respectively. Solution I was placed in a 250 mLTable three-neck 3. Chemical flask. parameters Solution of IIapplied was veryanti-aggl slowlyomeration injected material under Brij stirring 78 P [29,30]. into solution I. To precipitateFormula theMolecular ZnS:Mn QDs, Mass the Melting aqueous Point solution of sodium sulfide Solubility (solution III) was slowly added dropwiseC58H118O under24 stirring 1199.57 into the reaction ~56–60 mixture °C inside At 20 the°C in round-bottom methanol, chloroform flask. Finally, and ethanol the turbid dispersion was refluxed at 85 ◦C under stirring for 3.5 h. The precipitated nanoparticles were collected by centrifugationBy applying atthe 10,000 Brij 78 rpm P material, for 15 min. the Finally, optical the and white chemical-physical powder of the parameters, ZnS:Mn nanoparticles especially wasthe driedrefractive in a vacuumindex and oven melting at 60 ◦temperature,C for 24 h. for the used higher alkane materials in the propagated

(a) (b)

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wave were maintained, as was the response for the influences on optical propagation of light inside Micromachinesof the created2020 structure., 11, 1006 6 of 13 Table 4 presents the cumulative refractive indices of the mixtures of alkanes with Brij 78 P (0.5 wt%)From and nanoparticles our measurements (1 wt%). and observations, the most important information was that the agglomeration of nanoparticles occurs very often. This problem was resolved through the use Table 4. Refractive indices of investigated mixtures at 20.4 °C. of a special material known as polyethylene glycol monooctadecyl ether, Brij 78 P, with its parameters presented in Table3[28]. C15 with Brij and C16 with Brij and C15 C15 with Brij C16 C16 with Brij Nanoparticles Nanoparticles 1.4317 Table 3.1.4319Chemical parameters 1.4319 of applied anti-agglomeration 1.4344 1.4346 material Brij 78 P [29 1.4346,30]. Formula Molecular Mass Melting Point Solubility 2.2. Technology C58H118O24 1199.57 ~56–60 ◦C At 20 ◦C in methanol, chloroform and ethanol In this section, the technology of threshold sensor manufacturing based on a biconical optical fiber taper covered in higher alkanes with nanoparticles is presented. For the manufacturing of the By applying the Brij 78 P material, the optical and chemical-physical parameters, especially the biconical optical fiber tapers, the author’s arrangements named FOTET II (Fiber Optic Taper Element refractive index and melting temperature, for the used higher alkane materials in the propagated wave Technology) were used [34], as presented in Figure 4. were maintained, as was the response for the inﬂuences on optical propagation of light inside of the created structure. Table4 presents the cumulative refractive indices of the mixtures of alkanes with Brij 78 P (0.5 wt%) and nanoparticles (1 wt%).

Table 4. Refractive indices of investigated mixtures at 20.4 ◦C.

C15 with Brij and C16 with Brij and C15 C15 with Brij C16 C16 with Brij Nanoparticles Nanoparticles 1.4317 1.4319 1.4319 1.4344 1.4346 1.4346

2.2. Technology

In this section, the technology of threshold sensor manufacturing based on a biconical optical ﬁber taper coveredFigure 4. in Schematic higher alkanes of FOTET with system nanoparticles for biconical is optical presented. fiber manufacturing For the manufacturing [34]. of the biconical optical ﬁber tapers, the author’s arrangements named FOTET II (Fiber Optic Taper Element Technology)A detailed were description used [34], of as the presented manufacturing in Figure proces4. s are included in the previous articles, hence, it willA not detailed be presented description in this of paper the manufacturing [11,34]. Prepared process tapers are used included for presented in the threshold previous articles,sensors hence,are characterized it will not by below presented losses below in α this = 0.2 paper dB in [a11 whole,34]. investigated Prepared tapers range, usedelongation for presented L = 20.20 threshold± 0.05 mm sensorsand diameter are characterized of the taper bywaist low φ losses= 14.50 below ± 0.50α µm.= 0.2 dB in a whole investigated range, elongationIn the Lnext= 20.20 step, due0.05 to mm the mechanical and diameter properties of the taper of the waist preparedϕ = 14.50 taper,0.50 the µprotectionm. tube was ± ± usedIn with the a next schematic step, due and to cross-section the mechanical presented properties in Figures of the prepared 5a,b, respectively, taper, the and protection a sample tube image was usedin Figure with 5c. a schematic and cross-section presented in Figure5a,b, respectively, and a sample image in Figure5c.

(a) (b)

Figure 5. Cont.

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(c) (c) Figure 5. (a,b) Schematic of arrangements for protection taper with possibilities of alkanes and FigurenanoparticleFigure 5. (a(a, b,claddingb) )Schematic Schematic application, of of arrangements arrangements and (c) cross-section for for protection protection of a sample.taper taper with with possibilities possibilities of of alkanes alkanes and and nanoparticlenanoparticle cladding cladding application, application, and ( cc)) cross-section cross-section of a sample. Measurements were performed for 1550 nm. The choice of this value was dictated by a used Measurements were performed for 1550 nm. The choice of this value was dictated by a used opticalMeasurements fiber with the were cut offperformed for 1260 nm—afor 1550 single nm. Themode choice works of in this a third value telecommunication was dictated by a range. used optical fiber with the cut off for 1260 nm—a single mode works in a third telecommunication range. opticalAdditionally, fiber with for thethis cut wavelength, off for 1260 allnm—a the singleavailable mode measurement works in a thirdequipment telecommunication possesses the range. best Additionally, for this wavelength, all the available measurement equipment possesses the best quality Additionally,quality and sensitivity.for this wavelength, Furthermore, all theall avaprovilableided measurementworks are focused equipment on implementationpossesses the best for and sensitivity. Furthermore, all provided works are focused on implementation for industrial use. qualityindustrial and use. sensitivity. Measurements Furthermore, were provided all prov in idedstages. works The first are onefocused was usedon implementationfor spectral analysis for Measurements were provided in stages. The first one was used for spectral analysis and therefore industrialand therefore use. Measurementswas equipped were with provided a wide inrang stages.e diode The firsttype one SLED was usedfrom forExcalos spectral (Schlieren, analysis was equipped with a wide range diode type SLED from Excalos (Schlieren, Switzerland) with a range andSwitzerland) therefore with was a equippedrange of 1550with nm a wide± 100 nmrang ande diode an OSA-AQ6375 type SLED opticalfrom Excalosspectrum (Schlieren, analyzer of 1550 nm 100 nm and an OSA-AQ6375 optical spectrum analyzer (Yokogawa, Tokyo, Japan). Switzerland)(Yokogawa, ±Tokyo, with a Japan). range Theof 1550 second nm measurement± 100 nm and system an OSA-AQ6375 was designed optical to measure spectrum the hysteresis analyzer The second measurement system was designed to measure the hysteresis of the temperature change (Yokogawa,of the temperature Tokyo, changeJapan). Thetime second response. measurement An S3FC1550 system single-mode was designed laser tosource measure for 1550the hysteresis nm from time response. An S3FC1550 single-mode laser source for 1550 nm from Thorlabs (Newton, NJ, USA) ofThorlabs the temperature (Newton, change NJ, USA) time wasresponse. used Anand S3FC a PM100D1550 single-mode detector laserfrom sourceThorlabs for 1550with nma S140C from was used and a PM100D detector from Thorlabs with a S140C photodiode power sensor was applied. Thorlabsphotodiode (Newton, power NJ,sensor USA) was was applied. used andAll temperaturea PM100D detectorchanges fromand Thorlabstheir stabilization with a S140C were All temperature changes and their stabilization were provided by a climatic chamber VCL7010 from photodiodeprovided by powera climatic sensor chamber was VCL7010applied. fromAll temperatureVotche Ltd., (Balingen,changes and Germany). their stabilization were providedVotche Ltd., by (Balingen,a climatic chamber Germany). VCL7010 from Votche Ltd., (Balingen, Germany).

3. Results Results and and Discussion Discussion 3. Results and Discussion The measurements were divided into a few steps, which clearly show the influence influence of materials The measurements were divided into a few steps, which clearly show the influence of materials that create thethe tapertaper claddingcladding with with di differentfferent admixtures. admixtures. The The first first step step provides provides the the manufacturing manufacturing of thatof an create optical the fiber taper adiabatic cladding taper with with different low insertion admixtures. losses The below first 0.2 step dB provides in the whole the manufacturing range. A taper ofan an optical optical fiber fiber adiabatic adiabatic taper taper with with low low insertion insertion losses losses below below 0.2 0.2 dBdB inin thethe wholewhole range.range. A taper taper secured in a glass tube was measured in the climatic chamber for a SLED light source, as well as the securedpolarization in a glassparameters tube was (Figure measured 6). in the climatic chamber for a SLED light source, as well as the polarizationpolarization parameters (Figure 66).).

Figure 6. Setup for measurement of optical fiber devices. Figure 6. Setup for measurement of optical fiber devices. Figure 6. Setup for measurement of optical fiber devices. C15 and C16 were investigated in the temperature range corresponding to the two-phase state—solidC15 and and C16 liquid. were investigated For the solid in phase the temperature in both cases, range optical corresponding transmission to in the the two-phase fiber sensor state— was C15 and C16 were investigated in the temperature range corresponding to the two-phase state— verysolid low.and Thisliquid. is connectedFor the solid with phase the value in both of thecases, refractive optical indextransmission in the solid in the state fiber and, sensor as well was as very with solid and liquid. For the solid phase in both cases, optical transmission in the fiber sensor was very low.the properties This is connected of materials, with theythe value are not of transparent,the refractive which index causesin the solid strong state scattering and, as ofwell light as bywith kinds the low. This is connected with the value of the refractive index in the solid state and, as well as with the propertiesof microstructures of materials, within they the are solid not statetransparent, of all power which from causes the strong optical scatteri fiber taper.ng of Atlight the by transition kinds of properties of materials, they are not transparent, which causes strong scattering of light by kinds of microstructurestemperature, sharp within increases the solid in power state areof all observed power andfrom the the power optical is maintainedfiber taper. inAt the the liquid transition state. microstructures within the solid state of all power from the optical fiber taper. At the transition Thetemperature, additional sharp material increases Brij 78in power P was are applied observed to provide and the non-agglomeration power is maintained of in the the nanoparticles. liquid state. temperature, sharp increases in power are observed and the power is maintained in the liquid state. TheFigure additional7 shows the material hysteresis Brij of78 power P was change applied for to both provide pure materialsnon-agglomeration and for those of with the thenanoparticles. admixture The additional material Brij 78 P was applied to provide non-agglomeration of the nanoparticles. Figureof Brij 787 P.shows the hysteresis of power change for both pure materials and for those with the Figureadmixture 7 shows of Brij the 78 P.hysteresis of power change for both pure materials and for those with the admixture of Brij 78 P.

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Figure 7. Hysteresis of power change for a pure material and with the Brij admixture for the example Figure 7. HysteresisHysteresis of of power power change change for for a a pure pure material material and with the Brij admixture for the example of C15 materials. of C15 materials. As can be observed, there exists a hysteresis in the cooling and heating process. In all cases, it As can bebe observed,observed, therethere exists exists a a hysteresis hysteresis in in the the cooling cooling and and heating heating process. process. In allIn cases,all cases, it can it can be observed that the transmission of light in the heating process is slower than in the cooling one. canbe observed be observed that that the the transmission transmission of of light light in in the the heating heating process process is is slower slower thanthan inin the cooling one. one. Additionally, the difference in temperature in which full power is being observed to the temperature Additionally, thethe didifferencefference in in temperature temperature in in which which full full power power is is being being observed observed to to the the temperature temperature it it starts to see light transmission is changed from 3 to 8 °C. For the cooling process, the change of itstarts starts to to see see light light transmission transmission is changed is changed from from 3 to 83 ◦toC. 8 For °C. the For cooling the cooling process, process, the change the change of power of power exhibits a step change from maximum to zero. The hysteresis relates to the volume of the powerexhibits exhibits a step changea step change from maximum from maximum to zero. to The ze hysteresisro. The hysteresis relates to relates the volume to the of volume the materials of the materials used. The heating process starts from the outer edge and spreads to the optical taper materialsused. The used. heating The process heating starts process from thestarts outer from edge the and outer spreads edge to and the opticalspreads taper to the materials. optical Ataper full materials. A full switching range can be observed over 5 °C (from 10 to 15 °C), which is connected materials.switching A range full canswitching be observed range can over be 5 ◦observedC (from 10ov toer 155 °C◦C), (from which 10 isto connected15 °C), which with is the connected uneven with the uneven process of melting materials. In the change of power during the cooling process, withprocess the of uneven melting process materials. of melting In the change materials. of power In the during change the coolingof power process, during levels the cooling can be described process, levels can be described as a step change. The change from the liquid state to the solid state is 1 °C (8– levelsas a step can change.be described The as change a stepfrom change. the The liquid change state from to the the solid liquid state state is to 1 theC (8–7solid stateC). This is 1 °C phase (8– 7 °C). This phase transition is much faster than in the heating process. ◦ ◦ 7transition °C). This isphase much transition faster than is much in the faster heating than process. in the heating process. In all cases, Brij 78 P does not influence significantly the heating process, as well as the cooling In all cases, Brij 78 P does not influenceinfluence significantlysignificantly the heating process, as well as the cooling one, so it can be accepted that there is no change of the base material properties. The next part of the one, so it can be accepted that there is no change of the base material properties.properties. The next part of the investigation relates to the admixture of nanoparticles from ZnS:Mn to both materials. investigation relates to the admixture of nanoparticles fromfrom ZnS:MnZnS:Mn toto bothboth materials.materials. Figure 8 shows the hysteresis of the power change for a 1550 nm laser (S3FC1550 from Thorlabs) Figure 88 showsshows thethe hysteresishysteresis ofof thethe powerpower changechange forfor aa 15501550 nmnm laserlaser (S3FC1550(S3FC1550 fromfrom Thorlabs)Thorlabs) for C15 materials with Brij and with ZnS:Mn is presented. for C15 materials with Brij an andd with ZnS:Mn is presented.

(a) (b) (a) (b) Figure 8. TemperatureTemperature hysteresis hysteresis of of C15 C15 ( aa)) and and C16 ( b) for pure materials and with nanoparticle Figure 8. Temperature hysteresis of C15 (a) and C16 (b) for pure materials and with nanoparticle changes for a 1550 nm light beam. changes for a 1550 nm light beam. For both C15 andand C16,C16, thethe hysteresishysteresis of of a a phase phase change change from from solid solid to to liquid, liquid, and and vice vice versa, versa, can can be For both C15 and C16, the hysteresis of a phase change from solid to liquid, and vice versa, can beobserved. observed. C15 C15 in in the the heating heating process process has has a a much much longer longer timetime ofof fullfull changechange of the physical state. state. be observed. C15 in the heating process has a much longer time of full change of the physical state. For C16, the time of a physical state complete change change is is similar similar for for the the heating heating and and cooling cooling process. process. For C16, the time of a physical state complete change is similar for the heating and cooling process. An admixtureadmixture ofof nanoparticles nanoparticles for for both both alkanes alkanes changes changes the the temperature temperature of phase of phase transition transition from from solid An admixture of nanoparticles for both alkanes changes the temperature of phase transition from solidto liquid to liquid one: C15 one: from C15 20 from to 25 20 Cto and 25 C16°C and from C16 22 tofrom 24 22C. to This 24 is°C. related This tois therelated heating to the capacity heating of solid to liquid one: C15 from 20◦ to 25 °C and C16 from ◦22 to 24 °C. This is related to the heating capacity of the used nanoparticles. Furthermore, for the C15 material, a reduction in the phase change capacity of the used nanoparticles. Furthermore, for the C15 material, a reduction in the phase change

Micromachines 2020, 11, x 9 of 14

MicromachinesMicromachines 20202020,, 1111,, x 1006 99 of of 14 13 fromMicromachines solid to 2020 liquid, 11, x from 10 to 5 °C can be observed. For the C15 alkane with nanoparticles 9in of a14 fromheating solid process, to liquid the maximumfrom 10 to value 5 °C verycan beslowly observed. reaches For 40 the°C. C15 alkane with nanoparticles in a from solid to liquid from 10 to 5 °C can be observed. For the C15 alkane with nanoparticles in a heatingthe usedThese process, nanoparticles. results the were maximum Furthermore,confirmed value for very for a wide-range the slowly C15 material,reaches diode. 40 a Figures reduction°C. 9 and in the 10 phasefor C15 change and Figures from solid 11 heating process, the maximum value very slowly reaches 40 °C. andto liquid 12These for from C16results 10present to were 5 C theconfirmed can changes be observed. for of powera wide-range For in the a C15wi diode.de alkane range Figures withfor the nanoparticles 9 heatingand 10 forand inC15 cooling a heatingand Figuresprocesses, process, 11 These results were◦ confirmed for a wide-range diode. Figures 9 and 10 for C15 and Figures 11 andrespectively.the maximum12 for C16 valuepresent very the slowly changes reaches of power 40 C.in a wide range for the heating and cooling processes, and 12 for C16 present the changes of power◦ in a wide range for the heating and cooling processes, respectively.These results were conﬁrmed for a wide-range diode. Figures9 and 10 for C15 and Figures 11 and 12 respectively. for C16 present the changes of power in a wide range for the heating and cooling processes, respectively.

(a) (b) (a) (b) Figure 9. Heating process(a) in C15 (a) and with an admixture of NPs (b) for( bSLED) 1400–1700 nm. Figure 9. Heating process in C15 (a) and with an admixture of NPs (b) for SLED 1400–1700 nm. FigureFigure 9.9. Heating process inin C15C15 ((aa)) andand withwith anan admixtureadmixture ofof NPsNPs ((bb)) forfor SLEDSLED 1400–17001400–1700 nm.nm.

(a) (b) (a) (b) Figure 10. Cooling process(a) in C15 (a) and with an admixture of NPs (b) for(b SLED) 1400–1700 nm. FigureFigure 10. 10. CoolingCooling process process in in C15 C15 ( (aa)) and and with with an an admixture admixture of of NPs NPs ( (bb)) for for SLED SLED 1400–1700 1400–1700 nm. nm. Figure 10. Cooling process in C15 (a) and with an admixture of NPs (b) for SLED 1400–1700 nm.

(a) (b) (a) (b) FigureFigure 11. 11. HeatingHeating process process(a) in in C16 C16 ( (aa)) and and with with an an admixture admixture of of NPs NPs ( (bb)) for for(b SLED SLED) 1400–1700 1400–1700 nm. nm. Figure 11. Heating process in C16 (a) and with an admixture of NPs (b) for SLED 1400–1700 nm. Figure 11. Heating process in C16 (a) and with an admixture of NPs (b) for SLED 1400–1700 nm.

Micromachines 2020, 11, 1006 10 of 13 Micromachines 2020, 11, x 10 of 14

Micromachines 2020, 11, x 10 of 14

(a) (b)

Figure 12. Cooling process in C16 (a) and with an admixture of NPs (b) for SLED 1400–1700 nm. Figure 12. Cooling process(a) in C16 (a) and with an admixture of NPs (b(b)) for SLED 1400–1700 nm.

In a wideIn a wideFigure range, range, 12. theCooling the measurement measurement process in C16 for (fora) and a a single single with an wavelength admixture of was NPs was confirmed. (b confirmed.) for SLED The 1400–1700 The presented presented nm. devices devices can work for different wavelengths with the same level of temperature detection. Nanoparticles of can work forIn dia widefferent range, wavelengths the measurement with for the a single same wavelength level of temperature was confirmed. detection. The presented Nanoparticles devices of ZnS:Mn introduce a new kind of nucleating agent, which absorbs some part of the temperature ZnS:Mncan introduce work for different a new kindwavelengths of nucleating with the same agent, level which of temperature absorbs somedetection. part Nanoparticles of the temperature of energy (with a higher heat capacity than alkanes) and allows for shifting of the phase state. Moreover, energy (withZnS:Mn a higherintroduce heat a new capacity kind thanof nucleating alkanes) agen andt, allowswhich absorbs for shifting some ofpart the of phasethe temperature state. Moreover, the nanoparticles do not interact with the light propagated inside the taper’s structure but change the energy (with a higher heat capacity than alkanes) and allows for shifting of the phase state. Moreover, the nanoparticlesproperties of the do cladding not interact materials. with the light propagated inside the taper’s structure but change the propertiesthe ofnanoparticles the cladding do not materials. interact with the light propagated inside the taper’s structure but change the propertiesFor checking of the thecladding influence materials. of the applied nanoparticles for light parameters, the polarization For checking the influence of the applied nanoparticles for light parameters, the polarization propertiesFor havechecking been the checked. influence As of a thesource, applied the nanopaSantec rticlesTLC 210 for laserlight wasparameters, used for the a wavelengthpolarization of properties1550properties nm, have which beenhave corresponds been checked. checked. to As Asthe a a source,telecommunication source, the the Santec Santec TLC range TLC 210 and 210laser for laserwas polarization used was for used a wavelengthparameters, for a wavelength of the of 1550polarimeter1550 nm, nm, which whichPAX corresponds from corresponds Thorlabs to withthe the telecommunication telecommunicationmeasurement head range for range andIR wavelengthfor and polarization for polarization was parameters, used. Figure parameters, the 13 the polarimetershowspolarimeter the change PAX PAX fromof from ellipticity Thorlabs Thorlabs for the withwith C15 measurement and C16 alkanes head head fordepending forIR wavelength IR wavelength on the temperaturewas used. was used.Figure change. Figure13 13 shows theshows change the change of ellipticity of ellipticity for for the the C15 C15 and and C16 C16 alkanesalkanes depending depending on onthe thetemperature temperature change. change.

(a()a ) (b(b) )

FigureFigure 13. 13. Fluctuation Fluctuation of of ellipticity ellipticity for for C15 C15 ( (aa)) andand C16C16 (b)) forfor purepure and and with with NP NP mixture mixture at at15–40 15–40 °C °C Figure 13. Fluctuation of ellipticity for C15 (a) and C16 (b) for pure and with NP mixture at 15–40 ◦C for all microdevices. for all microdevices.for all microdevices. AsAs can can be be observed observed for for C15, C15, the the addition addition ofof NPs does notnot changechange the the polarization polarization properties, properties, As can be observed for C15, the addition of NPs does not change the polarization properties, especiallyespecially ellipticity. ellipticity. Nanoparticles Nanoparticles cause cause aa situationsituation where therethere isis no no change change of of the the heating heating and and especiallycoolingcooling ellipticity. value value in inellipticity. Nanoparticlesellipticity. For For C15 C15 cause with with andaand situation withoutwithout wherethe admixture,admixture, there isthe the no change change change of of ellipticity of ellipticity the heating and and and coolingazimuthazimuth value is in negligible.is ellipticity.negligible. For C15 with and without the admixture, the change of ellipticity and azimuth isFor negligible.For C16 C16 at ata lowera lower temperature temperature (below (below 20 20 °C),°C), the pure materialmaterial and and the the admixture admixture of ofNP NP in inthe the heating process do not propagate power that is confirmed by the results—ellipticity and, especially, Forheating C16 process at a lower do not temperature propagate power (below that 20 is◦ C),confirmed the pure by materialthe results—ellipticity and the admixture and, especially, of NP in the heatingazimuthazimuth process are are dothe the notonly only propagate measurement measurement power on on random thatrandom is conﬁrmednoisenoise as therethere by isis the nono results—ellipticitylight. light. In In the the cooling cooling process and,process especially,for for both types of materials to 15 °C, the state of polarization is maintained. This is strictly connected with azimuthboth aretypes the of onlymaterials measurement to 15 °C, the on state random of polariza noisetion as is theremaintained. is no light.This is Instrictly the coolingconnected process with for both types of materials to 15 ◦C, the state of polarization is maintained. This is strictly connected with the material properties and the value of hysteresis (for cooling process temperature in which power disappears is below 15 ◦C). Figure 14 shows the change of azimuth for the C15 and C16 alkanes depending on the temperature change. Micromachines 2020, 11, x 11 of 14

the material properties and the value of hysteresis (for cooling process temperature in which power disappears is below 15 °C). Figure 14 shows the change of azimuth for the C15 and C16 alkanes depending on the Micromachines 2020, 11, 1006 11 of 13 temperature change.

(a) (b)

FigureFigure 14. Change14. Change of of level level of of azimuth azimuth forfor didifferentﬀerent temperatures temperatures for for all allkinds kinds of microdevices of microdevices for pure for pure materialsmaterials and and with with NP NP mixtures mixtures C15 C15 ( a(a)) and and C16C16 ( b).).

As canAs becan noticed be noticed in a stabilizedin a stabilized temperature temperature value value of azimuth of azimuth for bothfor both alkanes alkanes and and their their mixtures lightmixtures power doeslight power not change does not its change value. its For value. C15, For the C15, value the value of azimuth of azimuth does does not not changechange during during the the temperature change. For C16, in the heating process, the value of azimuth increases. In this case, temperature change. For C16, in the heating process, the value of azimuth increases. In this case, the admixture of nanoparticles reduces this phenomenon. the admixtureAll observed of nanoparticles fluctuation reducesrefers only this to phenomenon. the climatic chamber properties. For all materials, the valueAll observed of ellipticity fluctuation is ±5°. refers only to the climatic chamber properties. For all materials, the value of ellipticity is 5 . ± ◦ 4. Conclusions 4. Conclusions In the study, the influence of the admixture of ZnS:Mn nanoparticles to n-pentadecane (C15) and n-hexadecaneIn the study, (C16) the influence has been ofpresented. the admixture As can be of noticed, ZnS:Mn the nanoparticles nanoparticles toshift n-pentadecane the temperature (C15) of and n-hexadecanethe change (C16)phase state has beenfrom state presented. to liquid As for can 3 °C be for noticed, both alkanes. the nanoparticles For the cooling shift process, the it temperature can be observed that the temperature shifts by ~3 °C for C16 to a higher temperature, and for C15, the of the change phase state from state to liquid for 3 ◦C for both alkanes. For the cooling process, temperature stays at the same point. These shifts can be a result of the higher heat capacity of the it can be observed that the temperature shifts by ~3 ◦C for C16 to a higher temperature, and for C15, mixture with nanoparticles compare to the pure material. It is noteworthy that it works for a wide the temperature stays at the same point. These shifts can be a result of the higher heat capacity of spectral range of the propagated light. It is possible to obtain a threshold temperature sensor by the mixtureadmixing with NP possess nanoparticles different materials compare properties. to the pure As material.can be noticed It is nanoparticles noteworthy do that not introduce it works for a widea spectralsignificant range change of theof polarization propagated parameters. light. It is Fo possibler C15, the to ellipticity obtain a with threshold the NP temperature admixture shows sensor by admixingthat it NPchanges possess only di inff aerent very small materials range properties. of less than As1% canfor the be cooling noticed and nanoparticles heating process, donot for pure introduce a significantmaterial observed change of the polarization change is over parameters. 5%. As can Forbe observed C15, the for ellipticity both alkanes with and the mixtures NP admixture with NP, shows that itthe changes azimuth only stays in at a verythe same small level range independently of less than to 1% temperature for the cooling and in and this heating case forms process, a stable for pure materialpolarizer observed in a wide the range. change Furthermore, is over 5%. it Asshould can be be mentioned observed that for in both C16 alkanes with nanoparticles, and mixtures we with NP, theobserve azimuth standard stays or at pure the effects same of level temperature independently influence to on temperature an SOP parameter and in of this light case which forms is not a stable polarizerobserved in afor wide the second range. alkane Furthermore, with the nanopartic it shouldle, beespecially mentioned in a heating thatin process. C16 with The admixture nanoparticles, of nanoparticles reduces the change of azimuth in this process to about 10° from 40° for a pure one. we observe standard or pure effects of temperature influence on an SOP parameter of light which is not For C15 with and without nanoparticles, azimuth stays at the same level. For all measurements, there observedis no visible for the fluctuation second alkane of polarization with the parameters nanoparticle, during especially the investigation, in a heating which process. makes The the admixturenoise of nanoparticlesratio of these microdevices reduces the changenegligible. of azimuth in this process to about 10◦ from 40◦ for a pure one. For C15 withIn all and cases, without the presented nanoparticles, solution azimuth can be used stays as at a thetemperature same level. sensor/detector For all measurements, in difficult to there is no visiblereach places. fluctuation Additionally, of polarization by a special parameters admixture during of nanoparticles, the investigation, it is possible which to create makes sensors the noise that ratio of thesegive microdevicesa signal that the negligible. temperature is exceeded above the allowable enabled to react and avoid the In all cases, the presented solution can be used as a temperature sensor/detector in difficult to reach places. Additionally, by a special admixture of nanoparticles, it is possible to create sensors that give a signal that the temperature is exceeded above the allowable enabled to react and avoid the damage of equipment or infrastructure. All the proposed solutions are compact, in line, and there is a possibility of using different kinds of lasers.

Author Contributions: Conceptualization: K.A.S. and I.J.; methodology: K.A.S.; formal analysis: K.A.S.; investigation: I.J., K.A.S., J.K., K.M.-P.; resources: I.J., K.A.S., K.M.-P.; data curation: I.J., K.A.S.; writing—original draft preparation: K.A.S.; writing—review and editing: I.J., K.A.S., K.M.-P.; visualization: J.K., I.J.; supervision: Micromachines 2020, 11, 1006 12 of 13

K.A.S.; project administration: K.A.S.; funding acquisition, K.A.S., K.M.-P. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: This work was financially supported by the Ministry of Science and Higher Education as a statutory activity of the Technical Physics Applications Department of the Military University of Technology UGB 22-756. The work concerning the synthesis of ZnS:Mn nanoparticles was financially supported by the state of North-Rhine–Westphalia in Germany and the European Union as a part of the project of the Evonik Degussa Science-to-Business Center Nanotronics. We would like to thank Michael Bredol from Fachhochschule Muenster—University of Applied Sciences, Germany and Anna Prodi-Schwab for supporting this part of work. Conflicts of Interest: The authors declare no conflict of interest.

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