Design of Narrow-Band Absorber Based on Symmetric Silicon Grating and Research on Its Sensing Performance

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Article Design of Narrow-Band Absorber Based on Symmetric Silicon Grating and Research on Its Sensing Performance

Miao Pan 1, Huazhu Huang 1, Wenzhi Chen 1, Shuai Li 1, Qinglai Xie 1, Feng Xu 2,*, Dongwei Wei 2, Jun Fang 2, Baodian Fan 3 and Lihan Cai 4

1 College of Physics and Information Engineering, Quanzhou Normal University, Quanzhou 362000, China; [email protected] (M.P.); [email protected] (H.H.); [email protected] (W.C.); [email protected] (S.L.); [email protected] (Q.X.) 2 College of Chemical Engineering and Materials Science, Quanzhou Normal University, Quanzhou 362000, China; [email protected] (D.W.); [email protected] (J.F.) 3 College of Electronics and Information Science, Fujian Jiangxia University, Fuzhou 350108, China; [email protected] 4 School of Mechanical and Electrical Engineering, Putian University, Putian 351100, China; [email protected] * Correspondence: [email protected]

Abstract: In this paper, using the surface plasmon and Fabry–Pérot (FP) cavity, the design of a symmetric silicon grating absorber is proposed. The time-domain ﬁnite difference method is used for simulation calculations. The basic unit structure is a dielectric grating composed of silicon dioxide,

metal and silicon. Through the adjustment of geometric parameters, we have achieved the best of the symmetric silicon grating absorber. A narrowband absorption peak with an absorption rate greater

Citation: Pan, M.; Huang, H.; than 99% is generated in the 3000–5000 nm optical band, and the wavelength of the absorption Chen, W.; Li, S.; Xie, Q.; Xu, F.; peak is λ = 3750 nm. The physical absorption mechanism is that silicon light generates surface Wei, D.; Fang, J.; Fan, B.; Cai, L. plasmon waves under the interaction with incident light, and the electromagnetic field coupling of Design of Narrow-Band Absorber surface plasmon waves and light causes surface plasmon resonance, thereby exciting strong light Based on Symmetric Silicon Grating response modulation. We also explore the influence of geometric parameters and polarization angle and Research on Its Sensing on the performance of silicon grating absorbers. Finally, we systematically study the refractive Performance. Coatings 2021, 11, 553. index sensitivity of these structures. These structures can be widely used in optical filtering, spectral https://doi.org/10.3390/coatings sensing, gas detection and other fields. 11050553 Keywords: plasmon resonance; Fabry–Pérot resonance; symmetric silicon grating; refractive Academic Editor: Marcin Pisarek index sensor

Received: 24 February 2021 Accepted: 19 April 2021 Published: 8 May 2021 1. Introduction

Publisher’s Note: MDPI stays neutral In recent years, the resonance phenomenon of nanostructures has been widely studied with regard to jurisdictional claims in in periodic structures. The resonance mode can be excited by surface plasmon or photon, published maps and institutional afﬁl- which affects the absorption characteristics of the structure [1–4]. Among them, the ideal iations. absorber with a wide working wavelength is very useful in photon detection and light energy absorption [5–7], while the ideal absorber with narrow working bandwidth has great advantages in sensing, ﬁltering and selective thermal radiation [8–15]. The refractive index (RI) sensors based on perfect narrowband wave absorbers have broad application prospects because of their simple construction and high sensitivity [16–21]. Researchers Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. have proposed such a sensor with good performance [22,23]. In addition, photothermal This article is an open access article detection can also be realized by a narrowband ideal absorber [24]. distributed under the terms and To obtain narrowband perfect absorbers, many people have studied all-metal struc- conditions of the Creative Commons tures or three-layer or multi-layer metal insulator metal (MIM) structures [25–30]. However, Attribution (CC BY) license (https:// they are not compatible with the complementary metal-oxide-semiconductor (CMOS) pro- creativecommons.org/licenses/by/ cess and have high manufacturing costs [31]. Therefore, people are paying attention to 4.0/). the realization of narrowband perfect absorbers through the dielectric structure on the

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(CMOS) process and have high manufacturing costs [31]. Therefore, people are paying Coatings 2021, 11, 553 attention to the realization of narrowband perfect absorbers through the dielectric2 of 9 structure on the metal system [32–34]. Compared with the perfect metal absorber, this absorber can save manufacturing costs [35]. More important, it can avoid complicated metallithography system processes. [32–34]. Compared To produce with ultra-narrow the perfect absorption, metal absorber, we usually this absorber insert a can dielectric save manufacturinglayer between costs grating [35 ].and More metal. important, However, it can the avoidstructure complicated is relatively lithography complex. processes. Therefore, Toto produce simplify ultra-narrowthe structure, absorption,we can place we the usually dielectric insert on top a dielectric of the metal layer substrate between [36–40]. grat- ingFor and example, metal. Fitio However, et al. thesyst structureematically is studied relatively the complex. perfect absorption Therefore, spectrum to simplify of thedie- structure,lectric grating we can on place metal the structure dielectric [41]. on topLater, of theLiao metal et al. substrateproposed [ 36a –narrowband40]. For example, perfect Fitiowave et al.absorber systematically composedstudied of dielectric the perfect and absorption composite spectrum dielectric of on dielectric the metal grating substrate on metal[42,43]. structure At the same [41]. time, Later, they Liao ca etn provide al. proposed enough a narrowbandnarrow bandwidth. perfect wave absorber composedHere, of we dielectric propose and a compositenarrowband dielectric perfect on abso therber metal based substrate on symmetric [42,43]. At silicon the same (Si) time,grating. they We can realize provide perfect enough absorption narrow bandwidth. by placing Si grating on the metal film. Simulta- neously,Here, a we low-loss propose resonance a narrowband mode is perfect adopted, absorber and the based Si grating on symmetric is composed silicon of two (Si) Si grating.strips in We each realize unit perfectstructure. absorption Therefore, by th placinge absorber Si grating produces on the resonance metal film. absorption Simulta- at neously,the wavelength a low-loss of resonance 3750 nm, modeand the is absorptivity adopted, and is the as Sihigh grating as 99%. is composed The absorber of two can Si be stripsused infor each the unitspectral structure. detection Therefore, of signals. the Wh absorberen used produces as a sensor, resonance its sensitivity absorption (S) and at thefigure wavelength of merit of (FOM) 3750 nm, can andreach the 2780 absorptivity nm/RIU, is92.67. as high Many as 99%. researchers The absorber have reported can be usedthat forperiodic the spectral grating detection structures of signals.with metal When layers used have as a achieved sensor, its perfect sensitivity absorption (S) and of figurelight. of For merit example, (FOM) the can two-dimensional reach 2780 nm/RIU, aluminum 92.67. Many grating researchers absorber have based reported on the that die- periodiclectric waveguide grating structures structure with proposed metal layersby Zhou have et achievedal. can absorb perfect light absorption up to 99.16% of light. [44]. ForThe example, gold nanodisk the two-dimensional array absorber aluminumbased on metal–insulation–metal grating absorber based structure on the dielectric proposed waveguideby Zhang structureet al. can achieve proposed perfect by Zhou absorption et al. can in absorb three bands lightup [45]. to Moreover, 99.16% [44 ].the The absorber gold nanodiskis based arrayon the absorber dielectric based grating on metal–insulation–metal on the silver film proposed structure by Abutoama proposed by et Zhangal. [46]. etCompared al. can achieve with perfecttheir work, absorption the absorber in three we bands proposed [45]. Moreover,is simple in the structure, absorber has is based higher onsensitivity the dielectric and FOM, grating and on can the be silver made film with proposed advanced by AbutoamaCMOS technology et al. [46 [47].]. Compared with their work, the absorber we proposed is simple in structure, has higher sensitivity and2. Materials FOM, and and can Methods be made with advanced CMOS technology [47]. 2. MaterialsFigure and1A presents Methods the structure of a narrowband perfect absorber of a symmetric Si grating. It is made of a quartz substrate and a Si grid on the surface of the Au thin layer. Figure1A presents the structure of a narrowband perfect absorber of a symmetric Si The optical parameters of Si and Au are obtained from reference [48]. The grating period grating. It is made of a quartz substrate and a Si grid on the surface of the Au thin layer. is A1, and each period has a Si grating structure with two secondary periods (A2); (B) is The optical parameters of Si and Au are obtained from reference [48]. The grating period isthe A1, section and each of the period single-period has a Si grating grating. structure h1 is the withheight two of secondarythe Si gate. periodsh2 is the(A2); thickness (B) is of the Au thin layer. w is the thickness of the Si gate. After systematic optimization, the op- the section of the single-period grating. h1 is the height of the Si gate. h2 is the thickness oftimal the Au parameters thin layer. of wthe is absorber the thickness are as of follows: the Si gate. A1 = After3100 nm, systematic h1 = 630 optimization, nm, h2 = 90 nm, the w = 260 nm, A2 = 700 nm. For simulating the optical performance of the perfect absorber, optimal parameters of the absorber are as follows: A1 = 3100 nm, h1 = 630 nm, h2 = 90 nm, wwe = 260use nm the, A2FDTD = 700 method nm. For [49–51]. simulating The incident the optical light performance source is TM of the(The perfect direction absorber, of the weelectric use the field FDTD is along method the [X-axis)49–51]. plane The incidentlight wave, light and source the direction is TM (The of the direction light source of the is electricperpendicular field is along to the the gratingX-axis) surface. plane light The wave, X-axis and and the Y-axis direction directions of the lightare sourceperiodic isboundary perpendicular conditions; to the the grating Z-axis surface. direction The is Xa -axisperfectly and matchedY-axis directions layer boundary are periodic condi- boundarytion. conditions; the Z-axis direction is a perfectly matched layer boundary condition.

Figure 1. (A) Three-dimensional diagram of symmetrical Si grating structure and (B) section diagram of element structure.

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Coatings 2021, 11, 553 3 of 9 Figure 1. (A) Three-dimensional diagram of symmetrical Si grating structure and (B) section dia- Figuregram of 1. element (A) Three-dimensional structure. diagram of symmetrical Si grating structure and (B) section diagram of element structure. 3.3. ResultsResults 3. Results AsAs shownshown inin FigureFigure2 ,2, in in the the case case of of normal normal incident incident light light waves, waves, we we calculate calculate the the As shown in Figure 2, in the case of normal incident light waves, we calculate the datadata ofof reflectance, reflectance, transmittance, transmittance, and and absorbance. absorbance. From From Figure Figure2, we 2, canwe observecan observe that thethat data of reflectance, transmittance, and absorbance. From Figure 2, we can observe that transmittancethe transmittance is basically is basically zero, zero, and there and there is only is oneonly reflection one reflection inclination inclination within within the range the the transmittance is basically zero, and there is only one reflection inclination within the ofrange the studyof the spectrum. study spectrum. Then, we Then, use Awe = use 1-R-T A = to 1-R-T get the to reflectanceget the reflectance of the structure, of the struc- and range of the study spectrum. Then, we use A = 1-R-T to get the reflectance of the struc- theture, reflectance and the reflectance of the absorption of the peak absorption is more peak than 99%.is more This than reflection 99%. This peak reflection angle appears peak ture, and the reflectance of the absorption peak is more than 99%. This reflection peak atangleλ = 3750appears nm at with λ = the 3750 full nm width with at the half full (FWHM) width at = half 30 nm. (FWHM) The structure = 30 nm. can The be structure used as angle appears at λ = 3750 nm with the full width at half (FWHM) = 30 nm. The structure ancan infrared be used filter. as an infrared filter. can be used as an infrared filter.

FigureFigure 2.2. SpectrogramSpectrogram ofof reﬂectancereflectance (R), transmittanc transmittancee (T), (T), and and absorbance absorbance (A) (A) under under the the normal normal Figureincidence 2. Spectrogram of symmetrical of reflectance Si grating. (R), transmittance (T), and absorbance (A) under the normal incidenceincidence ofof symmetricalsymmetrical SiSi grating.grating. The symmetric Si grating divides the incident light into many orders. The wave The symmetricsymmetric SiSi gratinggrating dividesdivides thethe incidentincident lightlight intointo manymany orders.orders. TheThe wavewave vectorsvectors ofof differentdifferent ordersorders cancan be determined by the formula: k( k(i)i )= = k k0sinsinα α+ +2π 2iπ/A1.i/A1. In vectors of different orders can be determined by the formula: k(i) = k0sin0 α + 2πi/A1. In Inwhich which k0 and k and k(i) k(arei) the are wave the wave vectors vectors of incident of incident light and light order, and i order,= 0, ±1,i ±2...= 0, ±n,±1, α is± 2...the which k0 and0 k(i) are the wave vectors of incident light and order, i = 0, ±1, ±2... ±n, α is the ±incidentα angle. With proper parameters, the order (+1, −1) can connect− the plane propa- incidentn, is theangle. incident With angle.proper Withparameters, proper parameters,the order (+1, the −1) order can connect (+1, 1) the can plane connect propa- the gating surface plasmon waves. It can be seen that the coupling resonance between the planegating propagating surface plasmon surface waves. plasmon It can waves. be seen It that can bethe seen coupling that theresonance coupling between resonance the betweensurface plasmon the surface mode plasmon and the mode incident and thelight incident leads to light strong leads absorption. to strong Through absorption. the surface plasmon mode and the incident light leads to strong absorption. Through the Throughwave vector the wavematching vector conditio matchingn [52], condition the coupling [52], theprocess coupling can be process expressed can be as expressedEquations wave vector matching condition [52], the coupling process can be expressed as Equations as(1) Equations and (2): (1) and (2): (1) and (2): 2πλ n (++ 1)/ ( 1)=k( + 1)=k sin α + 2 πi / A1 2πn (+πλ1)eff/λ++(+ 1) = k( + +1) = k 0 sin α + π2πi/A1 (1)(1) eff2 neff ( 1)/ ( 1)=k( 1)=k 0 sin0 α + 2i / A1 (1) 2πλ(− n) (−− 1)/(− () = 1)=k(( − − 1)=k) = sin αα −− 2 πi / A1 2πneffπλ1eff/−−λ 1 k −1 k 00sin − π2πi/A1 (2)(2) 2 neff ( 1)/ ( 1)=k( 1)=k 0 sin α 2i / A1 (2) InIn whichwhich nneffeff representsrepresents thethe effectiveeffective refractiverefractive indexindex ofof thethe surfacesurface plasmonplasmon model.model. In which neff represents the effective refractive index of the surface plasmon model. λλ(+1)(+1) andand thetheλ λ((−−1) all mean resonance wavelength.wavelength. InIn thethe casecase ofof verticalvertical incidence, incidence,α α= = 0 0 λ(+1) and the λ(−1) all mean resonance wavelength. In the case of vertical incidence, α = 0 andand λλ(+1)(+1) == λλ((−−1),1), so so only only one one absorption absorption peak peak occurs. occurs. As As shown shown in Figure3 3,, itit is is the the and λ(+1) = λ(−1), so only one absorption peak occurs. As shown in Figure 3, it is the wavewave vectorvector matchingmatching diagram diagram of of the the structure. structure. wave vector matching diagram of the structure.

Figure 3. Wave vector matching graph.

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Coatings 2021, 11, 553 Figure 3. Wave vector matching graph. 4 of 9

To better understand the physical mechanism of the symmetrical gratings, electric andTo magnetic better understand fields are simulated the physical for mechanismthe peak wavelength of the symmetrical [53–55]. Figure gratings, 4 shows electric the anddistribution magnetic of fields the areelectric simulated field and for themagnetic peak wavelength field at λ = [ 533750–55 ].nm, Figure respectively.4 shows the The distributionelectric field of mainly the electric distributes field and in magnetic the upper field inner at λ =corner 3750 nm,of the respectively. horizontal The Si electricgrating, fieldand mainlythe magnetic distributes field mainly in the upperdistributes inner in corner the groove of the of horizontal the Si grating. Si grating, This is and because the magneticof the interaction field mainly of light distributes waves with in the free groove electrons of the on Sithe grating. surface, This and isthe because motion of of the the interactionsurface plasmon of light produces waves with surface free electronsplasmon onwa theves. surface, Surface and plasmon the motion resonance of the is surface caused plasmonby the coupling produces between surface the plasmon surface waves. plasmon Surface wave plasmonand the electromagnetic resonance is caused field of by light. the couplingThrough between the distribution the surface of plasmonelectric field wave and and magnetic the electromagnetic field, we can field analyze of light. that Through a strong therectangular distribution displacement of electric field current and magneticis formed field, betw weeen can the analyzecracks of that the a Si strong grating, rectangular which is displacementalso verified currentby the distribution is formed between of magnetic the cracks fields. of Both the Si of grating, them meet which the is alsoright-handed verified byspiral the distributionrule. of magnetic fields. Both of them meet the right-handed spiral rule.

FigureFigure 4.4. TheThe distribution distribution of of electric electric field ﬁeld (A) (andA) and magnetic magnetic field ﬁeld(B) on (B the) on XOZ the section XOZ sectionat λ=3750 at λnm= 3750 of the nm symmetric of the symmetric Si grating. Si grating.The grating The is grating outlined is outlined with white with lines. white lines.

ToTo systematically systematically study study the the influence influence factors factors of of Si Si grating grating structure structure on on absorption absorption performance,performance, we we use use the the control control variable variable method method to to change change the the geometric geometric parameters parameters of of thethe structure. structure. Figure Figure5 A,B5A,B shows shows the the influence influence of of period period change change and and Si Si grating grating spacing spacing onon the the absorption absorption performance performance of of Si Si grating. grating. FigureFigure5 A5A shows shows that that the the absorption absorption peak peak wavelengthwavelength shifts shifts to to red, red, and and the the absorption absorption peak peak increases increases first first and and then then decreases decreases with with thethe change change of of the the Si Si grating grating period. period. Simultaneously, Simultaneously, we alsowe also notice notice that thethat half-height the half-height and half-widthand half-width of the of absorption the absorption peak gradually peak gradually decrease decrease with the with increase the increase of the period of the length. period Thislength. is because This is thebecause longer the the longer period, the the period, smaller the the smaller neff. The the influence neff. The of influence the change of inthe thechange spacing in the width spacing of the width Si grating of the on Si the grating redshift on of the the redshift peak wavelength of the peak is muchwavelength smaller is thanmuch that smaller of the than period that length of the (In period Figure length5B). Similarly, (In Figure the 5B). absorption Similarly, peak the absorption first increases peak andfirst then increases decreases and withthen thedecreases change with of the the spacing change between of the spacing the two between secondary the gratings. two sec- Theondary half-height gratings. and The half-width half-height of theand absorption half-width peak of the decrease absorption with peak the increasedecrease of with the the Si gratingincrease spacing of the width, Si grating which spacing is because width, the smallerwhich is the because Si grating the spacing smaller is, the the Si stronger grating thespacing excited is, electricthe stronger field the is and excited the greaterelectric thefield displacement is and the greater current the is displacement generated, thus cur- wideningrent is generated, the half-height thus widening and half-width. the half-hei In addition,ght and half-width. we also note In that addition, the absorbance we also note of thethat absorption the absorbance peak does of the not decreaseabsorption significantly peak does (about not decrease 2%) when significantly the Si grating (about spacing 2%) iswhen 540 nmthe becauseSi grating the spacing Si grating is 540 spacing nm because corresponding the Si grating to the spacing maximum corresponding absorbance to changedthe maximum within absorbance a certain range, changed which within would a certain not have range, a great which impact would on the not absorbance. have a great impactFigure on 6theA,B, absorbance. shows the influence of the Si grating width and the Si grating geometric thickness parameters on the absorption performance of the Si grating. From Figure6A, we can observe that the absorption peak redshifts as the width of the Si grating increases. Moreover, the redshift distance is larger between the Si grating width of 200 nm and 260 nm. With the redshift of these absorption peaks, the half-height and width of these absorption peaks gradually increase. When the Si grating width is 300 nm, the half-height and width reach the maximum. These absorption peaks also appear to redshift as the thickness of the Si grating increases (As shown in Figure6B). Although these absorption peaks first increase and then decrease, there was no significant change in the absorption value. We note that at the thickness of the Si grating of 630 nm to 730 nm, the change in the absorption peak

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isFigure only 5. about Periodic 2%. parameters This also indicatesof symmetrical that Si the grating thickness (A) and of thespacing Si grating parameters does of not the changetwo signiﬁcantlysecondary gratings within (B a). certain height range.

Figure 6A,B, shows the influence of the Si grating width and the Si grating geometric thickness parameters on the absorption performance of the Si grating. From Figure 6A, we can observe that the absorption peak redshifts as the width of the Si grating increases. Moreover, the redshift distance is larger between the Si grating width of 200 nm and 260 nm. With the redshift of these absorption peaks, the half-height and width of these absorption peaks gradually increase. When the Si grating width is 300 nm, the half-height and width reach the maximum. These absorption peaks also appear to redshift as the thickness of the Si grating increases (As shown in Figure 6B). Although these absorption peaks first increase and then decrease, there was no significant change in the absorption value. We note that at the thickness of the Si grating of 630 nm to 730 nm, the change in FiguretheFigure absorption 5.5. PeriodicPeriodic peak parameters parameters is only of of symmetricalabout symmetrical 2%. ThisSi Si gratinggrating also (indicatesA ()A and) and spacing spacingthat parametersthe parameters thickness of the of of thetwo the two Si secondarygratingsecondary does gratingsgratings not change ((BB).). significantly within a certain height range.

the absorption peak is only about 2%. This also indicates that the thickness of the Si Figure 6. Parameters of the width of symmetrical Si grating (A) and thickness of symmetrical Si Figuregrating 6. doesParameters not change of the significantl width of symmetricaly within a Sicertain grating height (A) and range. thickness of symmetrical Si gratinggrating ((BB).).

FigureFigure7 7shows shows the the relationship relationship between between reflection reflection and and polarization polarization angle. angle. Transverse Trans- (TM)verse polarization(TM) polarization is 0◦. TEis 0°. polarization TE polarization is 90◦ ,is the 90°, electric the electric field isfield along is along the Y -axis.the Y-axis. The polarizationThe polarization of the of incident the incident beam isbeam changed is changed from TM from polarization TM polarization to TE polarization to TE polariza- with antion interval with an of interval 2◦. We canof 2°. see We that can with see the that increase with the of increase polarization of polarization angle, the extinctionangle, the angleextinction of reflection angle of anglereflection decreases. angle decreases. This means This that means the absorbance that the absorbance decreases decreases with the increasewith the ofincrease polarization of polarization angle at λangle= 3750 at λ nm. = 3750 This nm. is becauseThis is because the grating the grating in the wavein the

Coatings 2021, 11, x FOR PEER REVIEWabsorberwave absorber is a three-dimensional is a three-dimensional grating grating with an with asymmetric an asymmetric structure. structure. We also We note also that 6note of if 9 thethat polarization if the polarization angle is angle less thanis less 20 than◦, the 20 reﬂected°, the reflected inclination inclination still has still a good has extinctiona good ex- effect,tinction indicating effect, indicating that the absorberthat the absorber has a good has polarization a good polarization tolerance tolerance [56,57]. [56,57].

Figure 6. Parameters of the width of symmetrical Si grating (A) and thickness of symmetrical Si grating (B).

Figure 7 shows the relationship between reflection and polarization angle. Trans- verse (TM) polarization is 0°. TE polarization is 90°, the electric field is along the Y-axis. The polarization of the incident beam is changed from TM polarization to TE polarization with an interval of 2°. We can see that with the increase of polarization angle, the extinction angle of reflection angle decreases. This means that the absorbance decreases with the increase of polarization angle at λ = 3750 nm. This is because the grating in the wave absorber is a three-dimensional grating with an asymmetric structure. We also note that if the polarization angle is less than 20°, the reflected inclination still has a good extinction effect, indicating that the absorber has a good polarization tolerance [56,57].

Figure 7.7. Relation diagram of reﬂectancereflectance andand polarizationpolarization angleangle ofof SiSi grating.grating.

The perfect absorber also shows excellent sensing characteristics. Figure 8A shows the absorption peak spectrogram of the Si grating when the refractive index (RI) of the detection environment changes. We observe from the figure that the absorption peak appears to redshift with the increase of the environmental RI. Although the absorbance is decreasing, the absorbance of Si grating still remains 90% or above. However, at the same time, we notice that the half-height and width of the absorption peak remain basically unchanged, and the half-height and width are still around 30 nm. Figure 8B shows the absorption peak wavelength corresponding to the RI with the change of the RI of the surrounding environment. The diagonal data are obtained through our linear fitting. According to the formula, the S and FOM of the grating can be calculated [58–61]. The slope (K) of the line in Figure 8B is the S of the Si grating, whose value is 2780 nm/RIU. The FWHM of the Si grating structure is 30 nm, FOM = S/FWHM = 92.67. This indicates that the structure has high sensitivity and quasilinear response. The structure can be widely used in optical filtering, spectral sensing, gas detection and other fields.

Figure 8. (A) Absorbance spectrogram and (B) relationship between RI and the corresponding peak wavelength of symmetrical Si grating under RI change. K and S represent the slope of the straight line and the sensitivity of the structure, respectively.

4. Conclusions In summary, the main research is symmetrical Si grating, whose basic unit structure is that the base material is nonmetallic oxide Si dioxide, the middle layer is precious metal Au, and the top layer is two symmetrical Si grating. By adjusting the geometrical parameters, we achieve the optimal absorbance of the symmetrical Si grating absorber, that is, a narrow band absorption peak with an absorptivity greater than 99% in the optical band range of 300 nm to 500 nm, and the absorption peak wavelength is λ = 3750 nm. We discuss the optical response mechanism from the principle and the distribution of electromagnetic fields. The main reason why symmetrical Si gratings can interact with

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Coatings 2021, 11, 553 6 of 9 Figure 7. Relation diagram of reflectance and polarization angle of Si grating.

The perfect absorber also shows excellent sensing characteristics. Figure 8A shows The perfect absorber also shows excellent sensing characteristics. Figure8A shows the the absorption peak spectrogram of the Si grating when the refractive index (RI) of the absorption peak spectrogram of the Si grating when the refractive index (RI) of the detection detection environment changes. We observe from the figure that the absorption peak environment changes. We observe from the figure that the absorption peak appears to appears to redshift with the increase of the environmental RI. Although the absorbance is redshift with the increase of the environmental RI. Although the absorbance is decreasing, decreasing, the absorbance of Si grating still remains 90% or above. However, at the same the absorbance of Si grating still remains 90% or above. However, at the same time, we time, we notice that the half-height and width of the absorption peak remain basically notice that the half-height and width of the absorption peak remain basically unchanged, unchanged, and the half-height and width are still around 30 nm. Figure 8B shows the and the half-height and width are still around 30 nm. Figure8B shows the absorption peakabsorption wavelength peak wavelength corresponding corresponding to the RI with to the changeRI with ofthe the change RI of theof the surrounding RI of the surroundingenvironment. environment. The diagonal dataThe arediagonal obtained data through are obtained our linear through fitting. our According linear fitting. to the Accordingformula, the to S the and formula, FOM of the gratingS and FOM can beof calculatedthe grating [58 can–61 be]. Thecalculated slope (K) [58–61]. of the lineThe slopein Figure (K) 8ofB isthe the line S ofin theFigure Si grating, 8B is the whose S of the value Si grating, is 2780 nm/RIU.whose value The is FWHM 2780 nm/RIU. of the TheSi grating FWHM structure of the Si is grating 30 nm, structure FOM = S/FWHM is 30 nm,= FOM 92.67. = ThisS/FWHM indicates = 92.67. that This the structureindicates thathas highthe structure sensitivity has and high quasilinear sensitivity response. and quasilinear The structure response. can be widelyThe structure used in opticalcan be widelyfiltering, used spectral in optical sensing, filtering, gas detection spectral andsensing, other gas fields. detection and other fields.

Figure 8. (A) Absorbance Absorbance spectrogram and ( B) relationship between RI and the corresponding peak wavelength of symmetrical Si grating grating under under RI RI change. change. K K and and S S represent represent the the slope slope of of the the straight straight line and the sensitivity of the structure, respectively.respectively.

4. Conclusions In summary, thethe mainmain research research is is symmetrical symmetrica Sil Si grating, grating, whose whose basic basic unit unit structure structure is isthat that the the base base material material is nonmetallic is nonmetallic oxide Sioxide dioxide, Si dioxide, the middle the layermiddle is precious layer is metalprecious Au, metaland the Au, top and layer the is top two layer symmetrical is two symmetrical Si grating. BySi adjustinggrating. By the adjusting geometrical the geometrical parameters, weparameters, achieve the we optimal achieve absorbance the optimal of absorbance the symmetrical of the Si gratingsymmetrical absorber, Si grating that is, absorber, a narrow thatband is, absorption a narrow peakband withabsorption an absorptivity peak with greater an absorptivity than 99% ingreater the optical than 99% band in range the op- of tical300nm bandto range 500 nm, of 300 and nm the to absorption 500 nm, and peak the wavelength absorption peak is λ =wavelength 3750 nm. Weis λ discuss = 3750 nm. the Weoptical discuss response the optical mechanism response from mechanism the principle from and the the principle distribution and ofthe electromagnetic distribution of electromagneticfields. The main fields. reason The why main symmetrical reason why Si gratings symmetrical can interact Si gratings withlight can tointeract generate with a strong light response is that the Si gratings stimulate the generation of surface plasmon under the action of light, and the motion of the surface plasmon leads to the generation of surface plasmon wave and the coupling between the surface plasmon wave and the incident light results in surface plasmon resonance. In addition, when the period, spacing, thickness and height of symmetrical Si grating change, the absorption peaks all show redshift, among which the periodic change has the greatest influence on the redshift of the absorption peak wavelength. The change of the Si grating spacing has the greatest influence on the absorption peak efficiency and the half-height and width of the absorption peak, indicating that the structure is the most sensitive to the change of the Si grating spacing. Then, we obtain the polarization sensitivity of the symmetrical Si grating structure. With the increase of polarization angle, the extinction angle of reflection angle decreases. When the polarization angle is less than 20◦, the reflected dip angle still has a good extinction effect, indicating that the structure has a certain polarization tolerance. In the end, we obtained the sensitivity factor of the RI sensor S = 2780 nm/RIU and the FOM = 92.67. Finally, we designed a structure with high sensitivity and quasilinear response. The structure can be widely used in optical filtering, spectral sensing, gas detection and other fields. Coatings 2021, 11, 553 7 of 9

Author Contributions: Conceptualization, M.P., H.H. and W.C.; data curation, W.C., S.L. and Q.X.; formal analysis, F.X. and D.W.; methodology, F.X., D.W., J.F., B.F. and L.C.; resources, L.C.; software, W.C., S.L., Q.X. and D.W.; data curation, M.P.; writing—original draft preparation, M.P.; writing— review and editing, Q.X., F.X., D.W. and L.C. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by National Natural Science Foundation of China (21676222, U175252), the Natural Science Foundation of Fujian Province (2018J01559, 2019J01738, 2019J01737, 2019J01732, JT180380, JT180365), Major Program of Natural Science Foundation of Fujian Province for College Young Scholars (No. JZ160456), the PhD Research Startup Foundation of Quanzhou Normal University (G16059), the Quanzhou high level Talents Innovation and Entrepreneurship Project (2018C088R, 2020C004R, 2020C044R, 2018C128R). the Putian University’s scientific research project (2016072). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: No new data were created or analyzed in this study. Data sharing is not applicable to this article. Conflicts of Interest: The authors declare no conflict of interest.

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