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Article Numerical Investigation on Multiple Resonant Modes of Double-Layer Plasmonic Grooves for Sensing Application

Shuwen Chu 1, Qiao Wang 2,*, Li Yu 1, Huixuan Gao 1, Yuzhang Liang 2 and Wei Peng 2,* 1 School of Optoelectronic Engineering and Instrumentation , Dalian University of , Dalian 116024, China; [email protected] (S.C.); [email protected] (L.Y.); [email protected] (H.G.) 2 School of , Dalian University of Technology, Dalian 116024, China; [email protected] * Correspondence: [email protected] (Q.W.); [email protected] (W.P.); Tel.: +86-411-8470-6693 (W.P.)

 Received: 17 December 2019; Accepted: 5 February 2020; Published: 11 February 2020 

Abstract: A high-performance multi-resonance plasmonic sensor with double-layer metallic grooves is theoretically constructed by introducing a polymethyl methacrylate groove with a numerical simulation method. Multiple resonance can be generated at the oblique incidence, and the number and feature of resonant mode for sensing detection is different for various incident angles. Specifically, at the incident angle of 30◦, the reflection spectrum exhibits two resonant dips, in which the dip at the of 1066 nm has an extremely narrow line width of ~4.5 nm and high figure of merit of ~111.11. As the incident angle increases, the electric dipole mode gradually weakens, but the resonance and cavity resonance mode are enhanced. Therefore, for an incident angle of 65◦, three resonance dips for sensing can be generated in the reflection spectrum to realize three-channel sensing measurement. These double-layer plasmonic grooves have potential in the development of advanced biochemical surface plasmon polariton measurements.

Keywords: double-layer plasmonic grooves; multi-resonance sensing; surface plasmon polaritons

1. Introduction Since the discovery of extraordinary optical transmission through subwavelength arrays on opaque metal films [1], the design of metal nanostructures and analysis of involved physical mechanisms have attracted enormous attention, and these studies have tremendously contributed to the development of surface plasmonic [2–6]. In the past few years, many applications on the basis of surface plasmon nanostructures have been proposed, such as sensors, imaging [7–9], negative refractive effects [10–12], color filters [13–17], perfect absorbers [18–20] and the like. Surface plasmon sensors have developed into an advanced detection method because of their high sensitivity, wide detection range and easy miniaturization. The one dimensional groove array, a simple structure, has attracted attention as a micro-nano sensor, which is easy to integrate. However, structural parameters exert a significant impact on the performance of the device [21–24]. Compared to a single-layer groove structure, double-layer plasmonic grooves can more freely adjust the wavelength position. Meanwhile, it is more conducive to the coupling between various modes, generating new optical characteristics. Currently, most involving groove sensors has concentrated on the near- region. However, very few studies have been achieved on the visible region [25–28]. In addition, the resonant mode of a single-layer groove sensor possesses a relatively wide full at half maximum (FWHM), resulting in a low figure of merit (FOM) in the sensing detection and limiting its sensing application to some extent [29–36]. For example, Fu et al. theoretically explored nanohole arrays arranged in a hexagonal lattice with a sensitivity of 348 nm/RIU and a FOM of 34.8 [33]. Liu et al.

Nanomaterials 2020, 10, 308; doi:10.3390/nano10020308 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 308 2 of 12 proposed a network-type metasurface with multiple reflection bands, which achieved FWHM of 3 nm in the visible and near-infrared regions, and the FOM reached up to 68.57 [34]. Sharm et al. presented a self-referenced SPP sensor incorporating titanium oxide grating on thin film atop substrates in the optical communication band, and an average spectral sensitivity and Surface plasmon polaritons (SPP) curve width of 693.88 nm/RIU and 26.03 nm were achieved for the optimized values of grating variables [35]. Li et al. numerically and experimentally demonstrated a similar structure based on double-layered metal grating, but they tested only the reflection from the top surface with an FOM of 38 under normal incidence [36]. In order to overcome the shortcomings of a single-layer plasmonic groove structure in the sensing application, this paper proposes a novel structure in which multiple resonant modes of double-layer plasmonic grooves are induced by employing a prism. A double-layer groove structure is established with a polymethyl methacrylate groove array. The simulation results indicate that the designed structure possesses multiple resonances in the visible and near-infrared regions under an oblique incident angle. In the case of the incident angle 65◦, wavelength positions of two resonant modes redshift with the increase of the of the surrounding environment. However, the wavelength position of another resonant mode remains constant, and its intensity has a change. Notably, for the incident angle 30◦, the double-layer groove structure generates a sharp sub-radiant resonance with a narrow line width of 4.5 nm and a high FOM of up to 111.11. Additionally, there is a good linear approximation between wavelength position and ambient refractive index. Finally, the physical mechanisms of these resonances are further revealed by changing relevant structural parameters and analyzing corresponding electromagnetic field distribution.

2. Structures and Methods The SPP is an electromagnetic surface wave that has the largest field strength at the surface and an exponential decay field to the interface direction simultaneously. Exciting SPPs on the surface of metal and dielectric satisfies the following formula [37]: r 2π 2π 2π ε1ε2 sin θ + m = = KSPPs (1) λ0 P − λ0 ε1 + ε2 where λ0 and θ are wavelength and angle of incident , respectively, P is the period of structure, m is diffraction order, ε1 and ε2 are relative of the dielectric material and Kspps represents the of the SPP wave. When the incident light meets the resonance condition, it converts energy into SPPs that propagate along the metal–dielectric interface. Figure1a,b illustrate the schematic of the proposed double-layer metallic groove structure. Two sets of metallic grooves were perpendicularly separated by high refractive index polymethyl methacrylate (PMMA) groove. There were different nanowell widths and the same nanowell thickness between these two set of grooves. The design of this structure is a reform of the classical Kretschmann model [38,39]. The resonant modes of the double-layer groove were excited by using a prism. The reflection spectrum of the designed nanostructure under normal incidence is shown in Figure1c. It can be seen that there was no resonance response for transverse electric (TE) polarized light at normal incidence. Therefore, only the case of transverse magnetic (TM) polarized light is discussed in this paper. The related structural parameters are as follows: structure period (P), incident angle (θ), nanowell width of top layer metallic groove (w), thickness of nanowell (h) and PMMA thickness (g). The refractive index of PMMA, prism and ambient medium are represented by na (1.46), nb (1.513) and n (1.33), respectively. A TM-polarized wave (magnetic field along metal nanowells) irradiated through the structure from prism substrate. We analyzed optical characteristics of this sensor using COMSOL Multiphysics (5.3 Version, Comsol company, Stockholm, Sweden), commercial software based on finite element methods (FEM), to obtain the reflection spectra and electromagnetic field distributions. In this simulation, period boundary conditions were used along the x and y direction, Nanomaterials 2020, 10, 308 3 of 12 and perfectly matched layers were applied in the z direction. The accuracy of the numerical results was mainly tested by adjusting the size of triangular grids and relative tolerance of the solver. The size of non-uniform triangular grids was used with a maximum mesh size of 10 nm and minimum mesh size of 1 nm. The relative tolerance of the solver was set in the range from 0.001 to 0.000001. Moreover, the results were further confirmed using the finite difference time domain (FDTD) algorithm (Lumerical FDTD Solutions, Inc., Vancouver, Canada). A Drude–Lorentz model defined the frequency constant of gold as [40]:

2 2 ωp ∆εΩ εm(ω) = εr (2) − ω(ω + iγ) − ω2 Ω2 + iωΓ − 14 where ωp is the plasma frequency, collision frequency associated with energy loss γ is 9.874 10 × rad/s, oscillator strength Ω is 4.077 1015 rad/s, spectral width of Lorentz oscillator is 6.578 1015 × × rad/s, weighting of factor ∆ε is 1.090 and εr is 5.967. Moreover, the designed structure can be fabricated by a combination of electron beam evaporation and electron beam . The optical properties and sensing response of the fabricated structure can be measured with an angle-resolved macrospectroscopyNanomaterials 2020, 10, 308 system. 3 of 12

FigureFigure 1. 1.Schematics Schematics of theof the double-layer double-layer metallic metallic groove groove structure. structure. (a) Three-dimensional (a) Three-dimensional diagram diagram of the designedof the designed structure structure consisting consisting of two sets of two of metallic sets of groovesmetallic upongrooves the upon Kretschmann the Kretschmann prism. (b )prism. Grating (b) diagramGrating of diagram one unit of and one corresponding unit and corre structuralsponding parameters. structural parameters. (c) Typical optical (c) Typical spectra optical of the spectra grooves of forthe transverse grooves for magnetic transverse (TM) magnetic and transverse (TM) and electric transv (TE)erse polarized electric (TE) light polarized under normal light incidence. under normal incidence. 3. Results and Discussion InThe order related to investigate structural the sensingparameters performance are as follows: of the proposed structure double-layer period (P plasmonic), incident groove angle and (θ), thenanowell physical width mechanism of top la ofyer its metallic resonant groove modes ( underw), thickness different of incident nanowell angles, (h) and we PMMA firstly investigatedthickness (g). theThe influence refractive of index structural of PMMA, parameter prism changes and ambient on resonance medium modes. are represented Figure2 shows by na (1.46), the reflection nb (1.513) and n (1.33), respectively. A TM-polarized plane wave ( along metal nanowells) spectra of the double-layer plasmonic groove at θ = 65◦ and θ = 30◦ for different thickness (g) of the PMMA,irradiated with through structure the parameters structure fromP = 1000 prism nm, substrate.h = 100 nmWeand analyzedw = 50 optical nm. Three characteristics resonant dips of this A sensor using COMSOL Multiphysics (5.3 Version, Comsol company, Stockholm, Sweden), (λ1 = 710.5 nm), B (λ2 = 748.5 nm) and C (λ3 = 905 nm) were observed at 65◦, and there were two commercial software based on finite element methods (FEM), to obtain the reflection spectra and resonant dips D (λ4 = 788.5 nm) and E (λ5 = 1066.8 nm) at 30◦, although there were four resonances at distributions. In this simulation, period boundary conditions were used along the incident angle of 60◦ and three resonances for incident angle 30◦. However, the sensing performance the x and y direction, and perfectly matched layers were applied in the z direction. The accuracy of of the resonance at the wavelength of ~971.5 nm with incident angle 65◦ was not good. In addition, the numerical results was mainly tested by adjusting the size of triangular grids and relative tolerance of the solver. The size of non-uniform triangular grids was used with a maximum mesh size of 10 nm and minimum mesh size of 1 nm. The relative tolerance of the solver was set in the range from 0.001 to 0.000001. Moreover, the results were further confirmed using the finite difference time domain (FDTD) algorithm (Lumerical FDTD Solutions, Inc., Vancouver, Canada). A Drude–Lorentz model defined the frequency dispersion constant of gold as [40]:

ω 2 ΔΩε 2 εω()=− ε p − (2) m r ωω()+−Ω+Γiiγ ωω22

ω 14 where p is the plasma frequency, collision frequency associated with energy loss γ is 9.874 × 10 rad/s, oscillator strength Ω is 4.077 × 1015 rad/s, spectral width of Lorentz oscillator is 6.578 × 1015 Δε ε rad/s, weighting of factor is 1.090 and r is 5.967. Moreover, the designed structure can be fabricated by a combination of electron beam evaporation and electron beam lithography. The optical properties and sensing response of the fabricated structure can be measured with an angle-resolved macrospectroscopy system.

3. Results and Discussion In order to investigate the sensing performance of the proposed double-layer plasmonic groove and the physical mechanism of its resonant modes under different incident angles, we

Nanomaterials 2020, 10, 308 4 of 12 firstly investigated the influence of structural parameter changes on resonance modes. Figure 2 shows the reflection spectra of the double-layer plasmonic groove at θ = 65° and θ = 30° for different thickness (g) of the PMMA, with structure parameters P = 1000 nm, h = 100 nm and w = 50 nm. Three resonant dips A (λ1 = 710.5 nm), B (λ2 = 748.5 nm) and C (λ3 = 905 nm) were observed at 65°, and there were two resonant dips D (λ4 = 788.5 nm) and E (λ5 = 1066.8 nm) at 30°, although there were four resonances at the incident angle of 60° and three resonances for incident angle 30°. Nanomaterials 2020, 10, 308 4 of 12 However, the sensing performance of the resonance at the wavelength of ~971.5 nm with incident angle 65° was not good. In addition, the depth of resonant dip at ~830 nm with incident angle 30° wasthe depthvery small, of resonant resulting dip in at lower ~830 nm sensing with incidentperformance. angle 30Therefore,◦ was very the small, sensing resulting performance in lower of sensing these twoperformance. resonant modes Therefore, has not the sensingbeen considered performance in the of manuscript. these two resonant According modes to hasFigure not 2a, been the considered effect of thein thethickness manuscript. of PMMA According on each to resonance Figure2a, thedip e wasffect different. of the thickness As the ofthickness PMMA of on the each dielectric resonance layer dip increased,was different. dip AsA underwent the thickness a ofblue the shift, dielectric but the layer positions increased, of dipdips A B underwent and C hardly a blue transformed. shift, but the Whenpositions the ofincident dips B andangle C hardlychanged transformed. to 30°, it Whencan be the seen incident from angleFigure changed 2b that to the 30◦ ,position it can be and seen intensityfrom Figure of 2dipb that D thewere position nearlyand unchanged. intensity ofHowever, dip D were there nearly was unchanged. a blue shift However, at the wavelength there was a positionblue shift of atdip the E. wavelength After a comprehensive position of dip compar E. Afterison, a comprehensive the sharpness comparison,of the resonance the sharpness value was of betterthe resonance with PMMA value thickness was better g with= 250 PMMA nm. Unless thickness stated,g = the250 value nm. Unlessof PMMA stated, thickness the value will of remain PMMA constantthickness in will the remainfollowing constant text. in the following text.

Figure 2. g Figure 2. ReflectionReflection spectraspectra for for di ffdifferenterent thickness thickness of polymethyl of polymethyl methacrylate methacrylate (PMMA) (PMMA)with incidentg with angle (a) θ = 65 and (b) θ = 30 . incident angle (a◦) θ = 65° and (b)◦ θ = 30°. Figure3 shows the influence of the thickness h on reflection spectra with other structural parameters Figure 3 shows the influence of the thickness h on reflection spectra with other structural unchanged. As the thickness h increased, all dips had red shifts and the depths of them increased. parameters unchanged. As the thickness h increased, all dips had red shifts and the depths of them However, compared with other dips, the wavelength shift of dip C was relatively small in Figure3a. increased. However, compared with other dips, the wavelength shift of dip C was relatively small Figure3b illustrates that the thickness h had an important effect on the intensity of dip D. Therefore, in Figure 3a. Figure 3b illustrates that the thickness h had an important effect on the intensity of dip the thickness h was one of the crucial factors affecting the line width of dips C and E. When the thickness D. Therefore, the thickness h was one of the crucial factors affecting the line width of dips C and E. h was small, the evanescent passed through the gold layer and entered the adjacent dielectric When the thickness h was small, the evanescent waves passed through the gold layer and entered layer, which caused the energy of incident light to not be coupled into the surface maximally, affecting the adjacent dielectric layer, which caused the energy of incident light to not be coupled into the the excitation of surface plasmon. On the contrary, when the thickness h increased, the attenuation of surface maximally, affecting the excitation of surface plasmon. On the contrary, when the thickness the evanescent wave in the gold film layer increased, which generated the coupling between incident h increased, the attenuation of the evanescent wave in the gold film layer increased, which light, and the surface plasmon wave became weak. For dips B and D, the choice of different h had an generated the coupling between incident light, and the surface plasmon wave became weak. For obvious effect on the resonance intensity. For the case of dips A and C and E, the reflection intensity dips B and D, the choice of different h had an obvious effect on the resonance intensity. For the case became large and the FWHM increased with the increase of the thickness h, which is disadvantageous of dips A and C and E, the reflection intensity became large and the FWHM increased with the for wavelength modulation sensing. Therefore, the FWHM and reflection intensity are two important increase of the thickness h, which is disadvantageous for wavelength modulation sensing. Therefore, factors for the selection of structural parameters. In this paper, the thickness of gold layer was selected the FWHM and reflection intensity are two important factors for the selection of structural to be 100 nm in consideration of the sensing performance. parameters. In this paper, the thickness of gold layer was selected to be 100 nm in consideration of Figure4 depicts the influence of nanowell width w on reflectance spectra of the double-layer the sensing performance. plasmonic grooves. As shown in Figure4a, both dip A and dip C had a blue shift with increasing nanowell width w at θ = 65◦. The filling factor f is defined as the ratio of nanowell width w and structure period P (i.e., f = w/P). When nanowell width w became large, that is, the filling factor f increased, the electric field intensity around the upper layer groove was weakened, leading to the

appearance of the blueshift of above two dip. It was also found that the reflection intensity of dip B had an increase, but the reflection intensities of both dip A and dip C were gradually reduced. The above phenomenon reflects that the coupling mechanisms for three resonant dips are different. When the incident angle was reduced to 30◦, the reflection intensity of dip D became large as nanowell width w increased, which is opposite the case of dip E (Figure4b). Compared to the case of incident angle 65 ◦, the amount of wavelength shift was relatively small. The appearance of both dip B and dip D was the Nanomaterials 2020, 10, 308 5 of 12

Nanomaterials 2020, 10, 308 5 of 12

result of the vanishing wave excited by the underlying gold, and the width of w directly affected the strength of the coupling and the magnitude of reflection (Figures 6a and 7c). Furthermore, as nanowell widthFigurew increased, 3. The effect the of linewidth the thickness of dip of gold A remained layer h on almost reflection unchanged, spectra under but theincident linewidths angle ( ofa) 65° both dip NanomaterialsC andand dip (b) E202030°. became, 10, 308wide. 5 of 12

Figure 4 depicts the influence of nanowell width w on reflectance spectra of the double-layer plasmonic grooves. As shown in Figure 4a, both dip A and dip C had a blue shift with increasing nanowell width w at θ = 65°. The filling factor f is defined as the ratio of nanowell width w and structure period P (i.e., f = w/P). When nanowell width w became large, that is, the filling factor f increased, the intensity around the upper layer groove was weakened, leading to the appearance of the blueshift of above two dip. It was also found that the reflection intensity of dip B had an increase, but the reflection intensities of both dip A and dip C were gradually reduced. The above phenomenon reflects that the coupling mechanisms for three resonant dips are different. When the incident angle was reduced to 30°, the reflection intensity of dip D became large as nanowell width w increased, which is opposite the case of dip E (Figure 4b). Compared to the case of incident angle 65°, the amount of wavelength shift was relatively small. The appearance of both dip B and dip D was the result of the vanishing wave excited by the underlying gold, and the width of w directly affected the strength of the coupling and the magnitude of reflection (Figures 6a and FigureFigure 3. 3. TheThe effect effect of of the the thickness thickness of of gold gold layer layer hh onon reflection reflection spectra spectra under under incident incident angle angle ( (aa)) 65° 65◦ 7c). Furthermore, as nanowell width w increased, the linewidth of dip A remained almost andand ( (bb)) 30°. 30◦ . unchanged, but the linewidths of both dip C and dip E became wide. Figure 4 depicts the influence of nanowell width w on reflectance spectra of the double-layer plasmonic grooves. As shown in Figure 4a, both dip A and dip C had a blue shift with increasing nanowell width w at θ = 65°. The filling factor f is defined as the ratio of nanowell width w and structure period P (i.e., f = w/P). When nanowell width w became large, that is, the filling factor f increased, the electric field intensity around the upper layer groove was weakened, leading to the appearance of the blueshift of above two dip. It was also found that the reflection intensity of dip B had an increase, but the reflection intensities of both dip A and dip C were gradually reduced. The above phenomenon reflects that the coupling mechanisms for three resonant dips are different. When the incident angle was reduced to 30°, the reflection intensity of dip D became large as nanowell width w increased, which is opposite the case of dip E (Figure 4b). Compared to the case of incident angle 65°, the amount of wavelength shift was relatively small. The appearance of both dip B and dip D was the result of the vanishing wave excited by the underlying gold, and the width of w directly affected the strength of the coupling and the magnitude of reflection (Figures 6a and

7c). FigureFurthermore,Figure 4. 4. TheThe effect e ffasect of ofnanowell differ differentent nanowellwidth w widthwidth increased, wwon on resonant resonant the linewidth dips dips under under incidentof incident dip angle Aangle remained (a )(a 65) 65°◦ and and almost (b ) unchanged,(b30) ◦30°.. Inset Inset but in thein (b )(b clearlylinewidths) clearly shows shows of the both the change change dip ofC of lineand line widthdip width E ofbecame of dip dip E forE wide. for di ffdifferenterent nanowell nanowell width widthw. w.

FigureFigure 55 depicts the effect e ffect of of different different structural structural periods periods PP onon the the reflection reflection spectrum spectrum at at θθ == 65°65◦ andand 30°. 30◦ .Obviously, Obviously, all all dips dips had had red red shift shift as as period period PP increasedincreased (Figure (Figure 5a,c).5a,c). This This trend trend was was exactly exactly oppositeopposite of of nanowire nanowire width width ww.. According According to to surface surface plasmon plasmon ex excitationcitation condition condition of of plasmonic plasmonic groove,groove, the the increase increase of of the the period period caused caused a a red red shift shift of of the the wavelength wavelength of of resonance resonance dip. dip. This This shows shows thatthat the the appearance appearance of of resonant resonant dip dip is is related related to to the the excitation excitation of of SPP SPP mode. mode. As As can can be be seen seen clearly clearly from Figure5b,d, the resonant dip and the period satisfy a linear relationship. In addition, we also investigated the variation of FWHM with the period in Figure5e,f. When structure period P had a change in the range from 960 nm to 1040 nm, the FWHM of dip A varied from 18 nm to 20 nm, and there was a change from 25 nm to 17 nm for the linewidth of dip C. Moreover, compared to the case of incident angle 65◦, the FWHM of resonant dip at 30◦ was not sensitive to the change of structural period, which is favorable in the fabrication of proposed structure.

Figure 4. The effect of different nanowell width w on resonant dips under incident angle (a) 65° and (b) 30°. Inset in (b) clearly shows the change of line width of dip E for different nanowell width w.

Figure 5 depicts the effect of different structural periods P on the reflection spectrum at θ = 65° and 30°. Obviously, all dips had red shift as period P increased (Figure 5a,c). This trend was exactly opposite of nanowire width w. According to surface plasmon excitation condition of plasmonic groove, the increase of the period caused a red shift of the wavelength of resonance dip. This shows that the appearance of resonant dip is related to the excitation of SPP mode. As can be seen clearly

Nanomaterials 2020, 10, 308 6 of 12

from Figure 5b,d, the resonant dip and the period satisfy a linear relationship. In addition, we also investigated the variation of FWHM with the period in Figure 5e,f. When structure period P had a change in the range from 960 nm to 1040 nm, the FWHM of dip A varied from 18 nm to 20 nm, and there was a change from 25 nm to 17 nm for the linewidth of dip C. Moreover, compared to the case

Nanomaterialsof incident2020 angle, 10 ,65°, 308 the FWHM of resonant dip at 30° was not sensitive to the change of structural6 of 12 period, which is favorable in the fabrication of proposed structure.

Figure 5. The influence of structural period P on resonant dips. For the case of incident angle 65◦, (Figurea) reflection 5. The spectra, influence (b) of the structural relationship period between P on resonant wavelength dips. position For the case of resonant of incident dips angle and period65°, (a)

Preflection, and (e) thespectra, effect (b of) the period relationshipP on line between widthof wavelength dips A and po C.sition For theof resonant case of incidentdips and angle period 30 ◦P,, (andc) reflection (e) the effect spectra, of (periodd) the relationshipP on line width between of dips wavelength A and C. position For the ofcase resonant of incident dips andangle period 30°, (Pc,) andreflection (f) the spectra, effect of ( periodd) the Prelationshipon line width between of dip wavelength E. position of resonant dips and period P, and (f) the effect of period P on line width of dip E. We further made an investigation of above optical phenomenon of the proposed structure. FigureWe6a,c further illustrate made the an magnetic investigation field andof above electric op fieldtical distributionsphenomenon atof dipthe D,proposed respectively. structure. It canFigure be seen6a,c illustrate that the the magnetic magnetic field field was and mainly electric concentrated field distributions on the at interface dip D, respectively. between the It gold can groovebe seen and that the the prism, magnetic and field a portion was mainly of the energyconcentrated entered on the the dielectric interface layer.between The the field gold intensity groove wasand exponentiallythe prism, and attenuated a portion in of the the vertical energy direction entered and the periodicallydielectric layer. distributed The field in theintensity horizontal was direction.exponentially These attenuated phenomena in arethe consistent vertical direction with near-field and periodically distribution distributed of surface plasmon in the horizontal mode for the groove structure [23,41]. Observing the magnetic field distribution of dip E, it was completely di fferent from dip D in Figure6b. At dip E, the energy was localized at the interface between groove and air and the upper portion of the dielectric layer. The electric field distribution of Figure6d shows that electric dipole mode of upper gold nanowell is excited. Therefore, resonant dips came from mutual coupling of different modes. Furthermore, the near-field distribution of dip E was mainly localized on Nanomaterials 2020, 10, 308 7 of 12 direction. These phenomena are consistent with near-field distribution of surface plasmon mode for the groove structure [23,41]. Observing the magnetic field distribution of dip E, it was completely different from dip D in Figure 6b. At dip E, the energy was localized at the interface between groove and air and the upper portion of the dielectric layer. The electric field distribution of Figure

6d shows thatNanomaterials electric2020, 10dipole, 308 mode of upper gold nanowell is excited. Therefore,7 ofresonant 12 dips came from mutual coupling of different modes. Furthermore, the near-field distribution of dip E was mainlythe localized upper surface on the of the upper groove. surface So dip E wasof the more groove. sensitive toSo changes dip E in was refractive more index sensitive of ambient to changes in refractive indexenvironment of ambient than that environment of dip D. than that of dip D.

Figure 6. Near-field distribution at resonant dips when the incident angle is 30◦. Magnetic field distribution of (a) dip D and (b) dip E. Electric field distribution of (c) dip D and (d) dip E. Figure 6. Near-field distribution at resonant dips when the incident angle is 30°. Magnetic field distributionTo of further (a) dip understand D and (b the) dip physical E. Electric properties field of resonancedistribution dips of A,(c) B dip and D C atand incident (d) dip angle E. 65◦, their electromagnetic distributions are shown in Figure7. As shown in Figure7a,b, the electromagnetic field of dip A was mainly concentrated in the dielectric layer while a small portion was distributed in To furtherthe upper understand and lower surfacethe physical of the bottom-layer properties groove. of Theresonance near-field dips distribution of A, at B the and surface C at of incident the angle 65°, their electromagneticbottom-layer groove clearlydistributions conforms to are the characteristics shown in of Figure SPP mode. 7. The As near-field shown distribution in Figure in 7a,b, the electromagneticthe dielectric field layerof dip was A due was to zero-order mainly cavityconcentrat mode toed be excitedin the [37dielectric], which caused layer a portionwhile ofa thesmall portion was distributedelectromagnetic in the fieldupper to be coupledand lower to gold surface nanowell. Comparedof the bottom-layer with that of dip A,groove. it can be seenThe near-field from Figure7c,d that the field distribution at dip B was significantly weaker and mainly localized on distributionthe at PMMAthe surface slit and theof lowerthe bottom-layer surface of bottom-layer groove groove. clearly As shownconforms in Figure to 7thee,f, the characteristics near-field of SPP mode. The distributionnear-field at distribution dip C depicts thatin the electromagneticdielectric layer field was was mainlydue to concentrated zero-order on thecavity upper mode to be excited [37],surface which of thecaused bottom-layer a portion groove andof the the dielectricelectromagnetic layer, that is, field the SPP to mode be andcoupled the cavity to modegold nanowell. were coupled with each other. The introduction of top-layer nanowell caused the electromagnetic field Compared withto be localized that of in dip the dielectricA, it can layer, be whileseen thefrom field Figure intensity 7c,d of upper that nanowell the field was verydistribution weak, which at dip B was significantlyis weaker the reason and why themainly depth oflocalized the reflection on dip the was PMMA the largest. slit In and addition, the thelower electromagnetic surface of field bottom-layer groove. As wasshown nearly in distributed Figure 7e,f, on the the upper near-field surface of thedist double-layerribution at groove, dip C so depicts refractive indexthat the sensitivity electromagnetic at dip C was largest. field was mainly concentrated on the upper surface of the bottom-layer groove and the dielectric layer, that is, the SPP mode and the cavity mode were coupled with each other. The introduction of top-layer nanowell caused the electromagnetic field to be localized in the dielectric layer, while the field intensity of upper nanowell was very weak, which is the reason why the depth of the reflection dip was the largest. In addition, the electromagnetic field was nearly distributed on the upper surface of the double-layer groove, so refractive index sensitivity at dip C was largest.

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Figure 7. Near-field distribution at resonant dips when the incident angle is 65◦.(a) Magnetic field Figuredistribution 7. Near-field and (b) electricdistribution field at distribution resonant di atps resonance when the dip incident A. (c) Magneticangle is 65°. field (a distribution) Magnetic field and distribution(d) electric fieldand distribution(b) electric field at resonance distribution dip at B. resonance (e) Magnetic dip fieldA. (c) distribution Magnetic field and distribution (f) electric fieldand (distributiond) electric field at resonance distribution dip at C. resonance dip B. (e) Magnetic field distribution and (f) electric field distribution at resonance dip C. To quantitatively evaluate the sensing performance of the proposed structure, we analyzed two majorTo criteria quantitatively in sensing evaluate detection, the refractive sensing indexperfor (RI)mance sensitivity of the andproposed FOM. Here,structure, FOM we is definedanalyzed as twothe ratiomajor of criteria bulk refractive in sensing index detection, sensitivity refractive and full index width (RI) at halfsensitivity maximum, and i.e.,FOM. FOM Here,= S /FOMFWHM. is definedFigure8 aas shows the ratio the of evolution bulk refractive of reflection index spectra sensitivity for the and analyte full width on the at surfacehalf maximum, of structure i.e., surfaceFOM = S/FWHM. Figure 8a shows the evolution of reflection spectra for the analyte on the surface of with a refractive index range of 1.33–1.40 for the incident angle of 65◦. Figure8b,c illustrate the RI structuresensitivity surface of these with three a refractive resonant index dips, range which of is 1.33–1.40 defined as for the the change incident of angle resonant of 65°. wavelength Figure 8b,c or illustrateresonant the intensity RI sensitivity with respect of these to thethree bulk resonant RI changes. dips, which In this is structure,defined as the the RI change sensitivities of resonant were wavelength or resonant intensity with1 respect to the bulk RI changes. In this structure, the RI SA = 202.38 nm/RIU,SB = 2.35 RIU− and SC = 497.62 nm/RIU, respectively. They also had good linear sensitivitiesapproximations were in S theA =whole 202.38 RI nm/RIU, range. Compared SB = 2.35 RIU with−1dip and C, S theC = sensitivity497.62 nm/RIU, of dip respectively. A was significantly They alsosmaller. had Asgood shown linear in approximations Figure7, the electromagnetic in the whole fieldRI range. of dip Compared C was mainly with localized dip C, the in sensitivity the upper ofof dipgold A layer, was andsignificantly could suffi smaller.ciently contactAs shown the analyte,in Figure so 7, the the sensitivity electromagnetic was high. field Dip of A wasdip excitedC was mainlyby zero-order localized cavity in modethe upper of the of bottom-layer gold layer, groove,and could which sufficiently resulted in contact lower sensitivity.the analyte, Figure so the8d sensitivitydepicts the was FOM high. of both Dip dip A Awas and excited dip C. by The zero FWHM-order of cavity dip A couldmode beof asthe small bottom-layer as 10.5 nm, groove, while whichdip C wasresulted 8.5 nm. in Atlower these sensitivity. two resonance Figure dips, 8d asdepicts refractive the indexFOM of analyteboth dip increased, A and dip the FWHMC. The FWHMgradually of dip decreased, A could that be as is, small the FOMas 10.5 became nm, wh larger.ile dip TheC was maximum 8.5 nm. At of these FOM two at dip resonance A was 19.27,dips, asand refractive dip C was index 58.54. of Itanalyte was observed increased, that the at resonantFWHM gradually dip C, the decreased, FWHM and that FOM is, remainedthe FOM constantbecame larger.in the rangeThe maximum of 1.37–1.40. of FOM The design at dip ofA thewas structure 19.27, and not dip only C reducedwas 58.54. the It detection was observed limit but that also at resonantrealized multiple-channeldip C, the FWHM measurement. and FOM remained constant in the range of 1.37–1.40. The design of the structure not only reduced the detection limit but also realized multiple-channel measurement.

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FigureFigure 8. 8.Sensing Sensing performancesperformances ofof proposedproposed double-layerdouble-layer plasmonicplasmonic groovesgrooves atat incidentincident angleangle 6565°.◦. (a(a)) The The relationship relationship of sampleof sample refractive refractive index index (RI) and (RI) relative and intensityrelative ofintensity dip B. (bof) Thedip relationship B. (b) The ofrelationship sample RI andof sample resonance RI and wavelength resonance of wavelength both dips A of and both C. dips (c) Figure A and ofC. merits (c) Figure (FOM) of merits for resonant (FOM) modesfor resonant of both modes dips A of and both C. dips A and C.

InIn addition, addition, we we also also investigated investigated the the sensing sensing performance performance of of proposed proposed double-layer double-layer plasmonic plasmonic groovesgrooves at at incident incident angle angle 30 30°.◦. Furthermore,Furthermore, reflectionreflection intensityintensity atat dipdip DD changed changed significantly significantly with with 1 thethe increase increase ofof ambientambient RIs,RIs, andand thethe refractiverefractive indexindex sensitivity S D waswas about about 1.34 1.34 RIU RIU−1−, as, as shown shown in inFigure Figure 9a.9a. Comparing Comparing the the electric electric field field distributions distributions at at dip B and dipdip D,D, itit waswas found found that that both both of of themthem were were caused caused by by SPPs. SPPs. Owing Owin tog to the the orders orders of SPPsof SPPs being being diff erentdifferent when when plasmonic plasmonic grooves grooves are irradiatedare irradiated at diff erentat different incident incident angles, whichangles, is evidentwhich inis theevident electric in field the diagram,electric field the intensity diagram, index the sensitivityintensity index of dip sensitivity B was greater of thandip B that was of greater dip D. Comparedthan that withof dip the D. case Compared of incident with angle the 65 case◦, this of structureincident obtainedangle 65°, a highthis structure RI sensitivity obtained of SE =a 500high nm RI/ RIUsensitivity at incident of S angleE = 500 30 nm/RIU◦. Moreover, at incident the FWHM angle of30°. dip Moreover, E was extremely the FWHM narrow of dip and E couldwas extremely be stably na maintainedrrow and atcould 4.5 nmbe stably in the maintained RI range from at 4.5 1.3 nm to 1.4.in the Therefore, RI range FOM from was 1.3 upto 1.4. to 111.11 Therefore, and had FOM a goodwas up linear to 111.11 approximate, and had as a showngood linear in Figure approximate,9b. The aboveas shown two valuesin Figure indicate 9b. The that above the proposed two values structure indicate can achievethat the important proposed applications structure can in sensingachieve detection.important Comparedapplications with in sensing both dips detection. A and C,Comp theared FOM with of dip both E wasdips increasedA and C, bythe 5.8FOM times of dip and E 1.9was times, increased respectively. by 5.8 times Thus, two-channeland 1.9 times, measurements respectively. canThus, be two-channel achieved at themeasurements incident angle can 30 be◦, andachieved the structure at the incident can achieve angle three 30°, resonance and the struct measurementsure can achieve at the incidentthree resonance angle 65 measurements◦. at the incident angle 65°.

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FigureFigure 9.9. Sensing performances of of proposed proposed double-layer double-layer plasmonic plasmonic grooves grooves at at incident incident angle angle 30°. 30 (◦a.) (Thea) The relationship relationship of ofthe the intensity intensity of of dip dip D D and and sample sample RIs. ( (bb)) The relationshiprelationship ofof wavelengthwavelength positionspositions ofof dipdip EE andand samplesample RIs,RIs, andand correspondingcorresponding FOM. FOM. 4. Conclusions 4. Conclusions In summary, we have theoretically demonstrated a high sensitivity and multi-resonance sensor In summary, we have theoretically demonstrated a high sensitivity and multi-resonance sensor based on a double-layer metallic grooves configuration. When the incident angle is 65 , the reflection based on a double-layer metallic grooves configuration. When the incident angle◦ is 65°, the spectrum has three resonance dips, and the corresponding sensitivities are 2.35 RIU 1, 202.38 nm/RIU reflection spectrum has three resonance dips, and the corresponding sensitivities− are 2.35 RIU−1, and 497.62 nm/RIU in the RI range from 1.33 to 1.40, and FOM is up to 19.27 and 58.54, respectively. 202.38 nm/RIU and 497.62 nm/RIU in the RI range from 1.33 to 1.40, and FOM is up to 19.27 and The excitation of resonant dips mainly depends on the coupling between SPP mode and cavity mode. 58.54, respectively. The excitation of resonant dips mainly depends on the coupling between SPP Changing the angle to 30 , the metal groove achieves dual resonance measurement. The refractive mode and cavity mode.◦ Changing the angle to 30°, the metal groove achieves dual resonance index of the resonance dip is 500 nm/RIU at 1050 nm, FWHM maintains at 4.5 nm, and FOM attains measurement. The refractive index of the resonance dip is 500 nm/RIU at 1050 nm, FWHM 111.11. The occurrence of this narrow band is due to the coupling between SPP mode and dipole maintains at 4.5 nm, and FOM attains 111.11. The occurrence of this narrow band is due to the resonance. Moreover, by employing the coupling between different modes under oblique incidence, coupling between SPP mode and dipole resonance. Moreover, by employing the coupling between our proposed structure achieves the purpose of free switching between two or more resonant sensors. different modes under oblique incidence, our proposed structure achieves the purpose of free In addition to the simple preparation process, the groove has a high quality factor, effectively reducing switching between two or more resonant sensors. In addition to the simple preparation process, the the detection limit in biosensing. This work is valuable for the developing of multi-resonance and groove has a high quality factor, effectively reducing the detection limit in biosensing. This work is high-performance sensors. Notably, the optimization of the structure parameter in this paper aims to valuable for the developing of multi-resonance and high-performance sensors. Notably, the obtain resonant dip with narrow line width and large depth, which are robust in experiment fabrication. optimization of the structure parameter in this paper aims to obtain resonant dip with narrow line In addition to a robust preparation process, the groove has a high quality factor, effectively reducing width and large depth, which are robust in experiment fabrication. In addition to a robust the detection limit in biosensing. This work is valuable for the developing of multi-resonance and preparation process, the groove has a high quality factor, effectively reducing the detection limit in high-performance sensors. biosensing. This work is valuable for the developing of multi-resonance and high-performance Authorsensors. Contributions: Conceptualization, methodology and investigation: S.C.; data processing: S.C., L.Y and H.G.; validation: Y.L.; writing—original draft preparation: S.C.; writing—review and editing: S.C., Y.L., Q.W. and W.P.;Author project Contributions: administration: Conceptualization, W.P.; funding acquisition:methodology Q.W. and andinvestigat W.P. Allion: authors S.C.; data have processing: read andagreed S.C., L.Y to and the publishedH.G.; validation: version Y.L.; of the writing—original manuscript. draft preparation: S.C.; writing—review and editing: S.C., Y.L., Q.W. Funding:and W.P.; projectThis research administration: was funded W.P.; by funding National acquisition: NatureScience Q.W. and Foundation W.P. All authors of China have (NSFC) read and (Grant agreed Nos. to 61520106013the published and version 61727816) of the manuscript. and Fundamental Research Funds for the Central Universities (Grant Nos. DUT18TD05). Funding: This research was funded by National Science Foundation of China (NSFC) (Grant Nos. Conflicts of Interest: The authors declare no conflict of interest. 61520106013 and 61727816) and Fundamental Research Funds for the Central Universities (Grant Nos. DUT18TD05). References Conflicts of Interest: The authors declare no conflict of interest. 1. Ebbesen, T.W.; Lezec, H.J.; Ghaemi, H.F.; Thio, T.; Wolff, P.A. Extraordinary optical transmission through Referencessub-wavelength hole arrays. Nature 1998, 391, 667–669. [CrossRef] 2. Hu, C.; Pu, M.; Li, X.; Wang, M.; Feng, Q.; Luo, X. 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