Strong Coupling Between Tamm and Surface Plasmons for Advanced Optical Bio-Sensing

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Article Strong Coupling between Tamm and Surface Plasmons for Advanced Optical Bio-Sensing

Zigmas Baleviˇcius Faculty of Electronics, Vilnius Gediminas Technical University, Naugarduko St. 41, 03227 Vilnius, Lithuania; [email protected]; Tel.: +37-068579015

Received: 12 November 2020; Accepted: 3 December 2020; Published: 5 December 2020

Abstract: The total internal reflection ellipsometry method was used to analyse the angular spectra of the hybrid Tamm and surface plasmon modes and to compare their results with those obtained using the conventional single SPR method. As such type of measurement is quite common in commercial SPR devices, more detailed attention was paid to the analysis of the p-polarization reflection intensity dependence. The conducted study showed that the presence of strong coupling in the hybrid plasmonic modes increases the sensitivity of the plasmonic-based sensors due to the reduced losses in the metal layer. The experimental results and analysis of the optical responses of three different plasmonic-based samples indicated that the optimized Tamm plasmons ∆Rp(TP) and optimized surface plasmons ∆Rp(SP) samples produce a response that is about five and six times greater than the conventional surface plasmon resonance ∆Rp(SPR) in angular spectra. The sensitivity of the refractive index unit of the spectroscopic measurements for the optimized Tamm plasmon samples was 1.5 times higher than for conventional SPR, while for wavelength scanning, the SPR overcame the optimized TP by 1.5 times.

Keywords: strong coupling; Tamm plasmons; surface plasmons; hybrid plasmonic modes; optical biosensors

1. Introduction Sensors based on optical signal interrogation are widely used for the detection of biochemical interactions at solid–liquid interfaces, for the monitoring of various chemicals adsorbed on surfaces [1,2] and for the evaluation of the refractive indexes of liquids [3]. For this purpose, surface plasmon resonance (SPR) optical sensors are most widely used [4,5]. Surface plasmon polariton (SPP) based optical sensors are able to detect the kinetics of various biochemical interactions such as antibody–antigen binding [4] and to determine, for instance, the adsorbed surface mass in monolayers of proteins [5]. The non-destructive, label-free, high sensitivity and real time monitoring ability of this optical signal makes this plasmonic based sensing technique an extremely attractive tool for the analysis of a wide range of surface science areas [6]. The high sensitivity of the optical signal registration is achieved due to the strong localization of the electric ﬁeld at the metal/dielectric interface when resonance conditions for such light–matter interaction are satisﬁed [7]. The SPP is a non-radiative electromagnetic waves, which can be excited by light through a glass prism or grating coupler which are necessary to match the in-plane wave vectors of the incident light and the plasmons in the metal layer. This rather strong light–matter interaction leads the properties of the dispersion relation (angular frequency (ω) and in-plane wavevector (k)) of the surface plasmon polaritons which are sensitive to the any perturbation of the refractive index changes on the SPP supported interfaces. However, the rather broad width of such SPP resonances, caused by the large absorption and scattering losses in the metal layer, limits the further improvement of the sensitivity of this type of optical sensor. These losses in the metals are determined by the imaginary part of their relative permittivity [8]. They shorten the

Coatings 2020, 10, 1187; doi:10.3390/coatings10121187 www.mdpi.com/journal/coatings Coatings 2020, 10, 1187 2 of 10 propagation length of the SPP waves and increase the width of the resonance. Thus, the appropriate dielectric function of the metal layer optimized for high sensitivity requires a larger negative real part and a smaller imaginary part of this dielectric function. This fundamental property of metals limits the further development not only of this sensing application [9], but also of similar developments in fields such as integrated photonic components, optical filters, lasing, and others [10]. Bloch surface waves (BSWs) can be generated at the surfaces of photonic crystals (PCs) and dielectrics [11]. As for the SPR, a coupler (glass prism or grating) is required for the excitation of such BSWs in order to satisfy the matching conditions of the in-plane wave vectors in the stop band of the photonic crystal [3,10]. The use, however, of only dielectric materials significantly narrows the width of these resonances as the dielectric function of such dielectric materials are weakly dispersive compared to metals. Due to weak dispersion of BSW the sensitivity to the refractive index changes are lower than for SPR. Advantages in sensitivity, however, are achievable due to the steeper slope of the Bloch surface wave resonances. Between the SPP and BSW, the intermediate type of electromagnetic surface waves which exist between the metal and photonic crystals are the so-called Tamm plasmon polaritons (TPP) [12]. These types of optical states are similar to the electron states proposed by I. Tamm [13]. In the case of photonic crystals, the Bragg reflections form a photonic stop band for the photons, which act as the energy band gap of the real crystals. The TPPs share the optical properties of both the SPP and the BSW. The resonant oscillations of the free electrons in the metal, like for the SPPs, are involved in the TP excitation and are non-propagating states which can be excited in both the TM and TE polarizations, like the Bloch surface waves on the interfaces between the PC and the dielectric [14]. Another important feature of the TPPs are that they have an in-plane wave vector which is less than the wave vector of light in a vacuum. Thus, a direct optical excitation of these TPPs is possible, contrary to the SPPs, where the light wave vector is always smaller than the SPP. In cases where the conditions for these excitations are satisfied for both the resonances of the SPPs and TPPs, a hybrid plasmonic mode appears [15]. Such situations can be realized whenever a glass prism is optically connected to a PC on which a thin metal layer is deposited. In such systems, the TPP and SPP resonances exist in a broad range of angles of light incident (AOI) to the prism. At certain AOIs to the prism corresponding to the same wavevectors at which both the SPPs and TPPs can be excited, the reflectance spectrum is changed, and the dispersion relations of both excitations are modified. In most cases, these changes in the reflectance and dispersion relations indicate a strong coupling regime between the TPPs and SPPs. Recently much attention has been paid to the study of strong coupling between these plasmon active metal nanostructures and the various emitters such as excitons in semiconductors, in dyes or in photochromic molecules [16–18]. The main feature of this strong coupling regime is that the energy exchange between the plasmons and the emitters occurs during a coherent time [19]. This means that in such coupled systems, the coupling strength between the plasmons and the emitters exceeds the damping rate and as a result, hybrid modes are created [20]. These hybrid modes can be achieved when the interactions are sufficiently strong, and the energy spectrum is modified, compared to the spectral positions of single resonances in non-coupled systems. In fact, these hybrid modes are polaritons formed partially from plasmons and emitters [21]. However, the existence of hybrid Tamm and surface plasmon modes were also demonstrated [15]. The presence of strong coupling between the TPP and the SPP lead to the anti-crossing of their dispersion curves and, as a result, narrow the width of their resonances. It has been shown that in its optimized hybrid TPP-SPP mode, the dispersion of the SPP branch can have reduced losses [22]. The splitting gap between the TPP and the SPP components in this new hybrid mode corresponds to their coupling strength [23]. The changes in the gap between the TPP and the SPP reflectance minimums and the widening or narrowing of the resonances indicate variations of their coupling strength. Coatings 2020, 10, x FOR PEER REVIEW 3 of 11 Coatings 2020, 10, 1187 3 of 10 electromagnetic waves. The sensitivity of this method is higher compared to that of conventional ellipsometryThe excitationor SPR [25,26 of these]. In hybridfact, TIRE TPP–SPP utilizes plasmonic the analytical modes power require of conditions ellipsometry of total and internal increases its sensitivityreflection. by These introducing states are alsothe polarizationSPR effect into sensitive. the operation Detailed analysisscheme ofof the the polarization ellipsometer. properties inStudi suches TIR employing setups can the be achievedstrong coupling by using total effect internal between reflection the ellipsometryTPP and SPP (TIRE) in the [24 ]. spectroscopic TIRE is a modetechnique have been combining performed spectroscopic before, where ellipsometry changes and in the the analysis resonant of surface wavelength plasmon were electromagnetic registered [22]. However,waves. most The sensitivity commercially of this available method is plasmonic higher compared sensors to use that the of conventional p-polarized ellipsometryintensity at or fixed wavelengthsSPR [25,26 and]. In scanning fact, TIRE angles utilizes of the incidence. analytical This power study of ellipsometry demonstrates and increasesthat increased its sensitivity sensitivity by to the pintroducing-polarization the reflection SPR effect due into theto the operation narrowing scheme of ofthe the resonance ellipsometer. width caused by strong coupling between TPPStudies and employing SPP can be the achieved strong coupling not only effect for between the wavelength, the TPP and but SPP also in thefor spectroscopicthe angle of modeincidence have been performed before, where changes in the resonant wavelength were registered [22]. However, scanning. most commercially available plasmonic sensors use the p-polarized intensity at fixed wavelengths and scanning angles of incidence. This study demonstrates that increased sensitivity to the p-polarization 2. Materials and Methods reflection due to the narrowing of the resonance width caused by strong coupling between TPP and SPPThree can samples be achieved were not investigated.only for the wavelength, These consisted but also for of the two angle structures of incidence supporting scanning. optimized hybrid Tamm-surface plasmons modes and a commercially available Xantec surface plasmon 2. Materials and Methods resonance (SPR) chip. The first two slightly different photonic structures of hybrid plasmonic modes were optimizedThree samples for enhanced were investigated. sensitivity These of these consisted particular of two excitation structures supportingcomponents optimized (Tamm hybrid plasmons or surfaceTamm-surface plasmons). plasmons For the modes optimized and a commercially surface plasmon available (SP) Xantec components surface plasmon in their resonance hybrid (SPR) modes, chip. The first two slightly different photonic structures of hybrid plasmonic modes were optimized for distributed Bragg gratings consisting of 6 bilayers of ~120 nm TiO2/~200 nm SiO2 and a 30 nm TiO2 enhanced sensitivity of these particular excitation components (Tamm plasmons or surface plasmons). layer on top were formed on BK7 glass substrates. For the optimized Tamm plasmon component, the For the optimized surface plasmon (SP) components in their hybrid modes, distributed Bragg gratings distributed Bragg grating consisted of five bilayers of ~120 nm TiO2/~200 nm SiO2 formed on a BK7 consisting of 6 bilayers of ~120 nm TiO2/~200 nm SiO2 and a 30 nm TiO2 layer on top were formed on glassBK7 substrate. glass substrates. These Bragg For gratings the optimized were Tammcreated plasmon by ion beam component, sputtering. the distributed Afterwards, Bragg thin grating gold (~40 nm) consistedlayers were of five deposited bilayers of on ~120 both nm photonic TiO2/~200 crystal nm SiO 2structuresformed on b ay BK7 magnetron glass substrate. sputtering. These Bragg The SPR samplegratings consisted were createdof a gold by film ion beam having sputtering. a thickness Afterwards, of about thin 45 goldnm (~40and nm)about layers 2 nm were underlayer deposited of Ti for betteron both adhesion. photonic crystal structures by magnetron sputtering. The SPR sample consisted of a gold film havingThe total a thickness internal ofreflection about 45 nmellipsometry and about 2(TIRE) nm underlayer experiments of Ti forwere better then adhesion. conducted using a dual rotating compensatorThe total internal ellipsometer reflection ellipsometryRC2 (J.A. Woollam (TIRE) experiments Co., Inc., Lincoln, were then NE, conducted USA). The using spectroscopic a dual ellipsometryrotating compensator experiments ellipsometer were carried RC2 out (J.A. in Woollam the 400 Co.,–10 Inc.,00 nm Lincoln, spectral NE, range. USA). TheFor spectroscopic all the samples ellipsometry experiments were carried out in the 400–1000 nm spectral range. For all the samples with the supported hybrid plasmonic modes and the SPR, the experiments were conducted in a TIRE with the supported hybrid plasmonic modes and the SPR, the experiments were conducted in a configuration. These used a half-cylindrical BK7 glass prism in the range of angle of incidence (AOI) TIRE configuration. These used a half-cylindrical BK7 glass prism in the range of angle of incidence 60°–75° connected via a refractive index matching fluid with investigated samples (Figure 1). To (AOI) 60◦–75◦ connected via a refractive index matching fluid with investigated samples (Figure1). compensateTo compensate for the for light the lightbeam beam focusing focusing effect effect due due to to the the semi semi-cylindrical-cylindrical prism, prism, lenses lenses with with a 40 a mm 40 mm focalfocal length length were were used used for for the the pre pre-focusing-focusing of of the the incoming and and reflected reflected light light to theto the detector. detector.

Figure 1. Cont.

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FigureFigure 1. 1. SamplesSamples structures structures and and measurements measurements configurations configurations for for single single SPR SPR ( (AA)) and and for for optimized optimized Figure 1. Samples structures and measurements configurations for single SPR (A) and for optimized TammTamm plasmons plasmons and and optimized optimized surface surface plasmons plasmons samples samples (B) and (B the) and custom-built the custom chamber-built connectedchamber Tamm plasmons and optimized surface plasmons samples (B) and the custom-built chamber connectedwith glass with prism glass and prism sensor and chip sensor (C). chip (C). connected with glass prism and sensor chip (C). AA liquid liquid handling handling system system with with a a custom custom-built-built Teflon Teflon chamber chamber was was used used in in which which the the surfaces surfaces of of allallA theliquid the sampl samples handlinges were were systemplaced. placed. Thiswith This chamber chambera custom was was-built filled filled Teflon with with deionized chamber deionized water,was water, used which which in waswhich was then then the changed changed surfaces of all thetoto sampldeionized deionizedes were water/ethanol water placed./ethanol This (50%/50%) (50% chamber/50%) mixture, mixture, was filled whose whose with refractive refractive deionized index index water, is is higher higher which than than was that that then of of pure purechanged to deionizeddeionizeddeionized water/ethanolwater. TheThe measuredmeasured (50%/50%) experimental experimental mixture, data data whose of of the the ellipsometricrefractive ellipsometric index parameters param is higheretersΨ(λ Ψ)than and(λ) and∆ that(λ) Δ were of(λ) pure deionizedwerethen then expressed water. expressed The as themeasured as p-polarized the p-polarized experimental intensity, intensity, using data using the of data the data acquisitionellipsometric acquisition software paramsoftware CompleteEaseeters CompleteEase Ψ(λ) and (J.A. Δ(λ) (J.A. Woollam Co., Inc.), and were then presented as the p-intensity map dependence on the wereWoollam then expressed Co., Inc.), as and the were p-polarized then presented intensity, as the using p-intensity the data map acquisition dependence software on the wavelength CompleteEase wavelength(λ) vs. angle (λ of) vs. incidence angle of ( θincidence) (Figures (θ2° )and (Figure3). Theses 2 and p-polarized 3). These p intensity-polarized maps, intensity in fact, maps, represent in fact, (J.A. Woollam Co., Inc.), and◦ were then presented as the p-intensity map dependence on the representthe dispersion the dispersion relations ofrelations the investigated of the investigated plasmonic plasmonic excitations. excitations. Furthermore, Furthermore, a fixed resonant a fixed wavelength (λ) vs. angle of incidence (θ°) (Figures 2 and 3). These p-polarized intensity maps, in fact, resonantwavelengths wavelengthλ = 800s λ nm = 8 (SP),00 nmλ (SP)= 715, λ = nm 715 (TP), nm (TP) and, λand= 820λ = 8 nm20 nm (Hybrid (Hybrid TP-SP) TP-SP) were were chosen chosen to representtodemonstrate demonstrate the dispersion the optimizedthe optimized relations sensitivity sensitivity of the properties investigated properties in the angular inplasmonic the spectra angular excitations. of spectrathe p-intensity of Furthermore, the polarizations p-intensity a fixed resonantpolarizationsfor SPR wavelength andTamm for SPRs plasmonλ and= 800 Tamm nm polaritons (SP) plasmon, λ and= 7polaritons15 hybrid nm (TP) TPP-SPP and, and hybrid mode λ = TPP 820 (Figure- SPPnm (Hybrid4mode). (Fig TPure-SP) 4). were chosen to demonstrate the optimized sensitivity properties in the angular spectra of the p-intensity polarizations for SPR and Tamm plasmon polaritons and hybrid TPP-SPP mode (Figure 4).

Figure 2. Cont.

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Figure 2. Experimentally measured p-intensity dispersion relations in deionized water solution of FigureFigure 2. Experimentally 2. Experimentally measured measured p p--intensityintensity dispersiondispersion relations relations in deionizedin deionized water water solution solution of the of the the single SPR mode in a Cr/Au (50 nm) structure (A), the optimized Tamm plasmon mode (B) singlesingle SPR SPR mode mode in ina Cr/Au a Cr/Au (50 (50 nm) nm) structure structure ((AA),), the the optimized optimized Tamm Tamm plasmon plasmon mode mode (B) generated (B) generated generatedin the in photonic the photonic structure structure of five bilayers of ﬁve (TiO bilayers2/SiO2 (TiO(120 2 nm/200/SiO2 (120 nm))/Au nm / (40200 nm) nm)) and/Au optimized (40 nm) and in the photonic structure of five bilayers (TiO2/SiO2 (120 nm/200 nm))/Au (40 nm) and optimized optimizedsurface surface plasmon plasmon mode (SP) mode in (SP)hybrid in hybridplasmonic plasmonic mode consisted mode consisted from six frombilayers six (TiO bilayers2/SiO (TiO2 (1202/ SiO2 surface plasmon mode (SP) in hybrid plasmonic mode consisted from six bilayers (TiO2/SiO2 (120 (120 nmnm/200/200 nm))nm))/40/40 nm nm TiO TiO2/Au2/Au (40 (40 nm) nm) (C ().C The). The white white dashed dashed lines lines indicate indicate the cross the cross-section-section at fixed at ﬁxed nm/200 nm))/40 nm TiO2/Au (40 nm) (C). The white dashed lines indicate the cross-section at fixed wavelengthwavelength which which were wer chosene chosen to to presentpresent the the angular angular dependence dependence of plasmonic of plasmonic excitations excitations on the p on- the wavelength which were chosen to present the angular dependence of plasmonic excitations on the p- p-polarizedpolarized intensity. intensity. polarized intensity.

FigureFigure 3. Experimentally 3. Experimentally measured measured p-intensity p-intensity dispersion dispersion relations relations in in deionized deionized water/ethanol water/ethanol (50%/(50%/50%)50%) mixed mixed solution solution of of the the single singleSPR SPR modemode in in a a Cr/Au Cr/Au (50 (50 nm) nm) structure structure (A), ( Athe), optimized the optimized TammTamm plasmon plasmon mode mode (TP) (TP) (B(B)) generatedgenerated in in the the photonic photonic structure structure of five of bilayers ﬁve bilayers (TiO2/SiO (TiO2 (1202/ SiO2 (120 nmnm/200/200 nm))nm))/Au/Au (40 (40 nm) nm) andand optimized optimized surface surface plasmon plasmon mode mode (SP) (SP) in hybrid in hybrid plasmonic plasmonic mode mode consisted from six bilayers (TiO2/SiO2 (120 nm/200 nm))/40 nm TiO2/Au (40 nm) (C). The white dashed consistedFigure 3. from Experimentally six bilayers (TiO measured2/SiO2 (120 p-intensity nm/200 nm)) dispersion/40 nm TiO relations2/Au (40 in nm) deionized (C). The water/ethanolwhite dashed lines (50%/50%) indicate mixed the cross-section solution of at the ﬁxed single wavelength SPR mode which in werea Cr/Au chosen (50 tonm) present structure the angular (A), the dependence optimized ofTamm plasmonic plasmon excitations mode (TP) on the (B) p-polarized generated intensity.in the photonic structure of five bilayers (TiO2/SiO2 (120 nm/200 nm))/Au (40 nm) and optimized surface plasmon mode (SP) in hybrid plasmonic mode consisted from six bilayers (TiO2/SiO2 (120 nm/200 nm))/40 nm TiO2/Au (40 nm) (C). The white dashed

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lines indicate the cross-section at fixed wavelength which were chosen to present the angular Coatings 2020, 10, 1187 6 of 10 dependence of plasmonic excitations on the p-polarized intensity.

Figure 4. Experimental spectra of p-intensity dependence on the AOI for single SPR (A), optimized Tamm plasmon (TP) (B), and optimized surface plasmon (SP) (C) samples before (red curves) and after Figure(black 4. curves) Experimental deionized spectra water of changed p-intensity to solution dependence of deionized on the waterAOI /forethanol single (50% SPR/50%). (A), optimized The spectra Tammcorresponds plasmon to (TP) the cross-sections(B), and optimized in the Figuressurface3 plasmon and4. (SP) (C) samples before (red curves) and after (black curves) deionized water changed to solution of deionized water/ethanol (50%/50%). The 3. Resultsspectra corresponds and Discussions to the cross-sections in the Figures 3 and 4. It has been shown before that by applying spectroscopic ellipsometry in its total internal reflection 3. Results and Discussions configuration and the so-called total internal reflection ellipsometry (TIRE), a higher sensitivity to theIt refractive has been index shown and before the attached that by surface applying mass spectroscopic can be achieved ellipsometry compared with in its that total produced internal by reflectionstandard configuration commercially available and the surface so-called plasmon total internal resonance reflection biosensors ellipsometry [27]. This improved (TIRE), a sensitivity higher sensitivitywas obtained to the mainly refractive due index to the and ability the attached to directly surface measure mass can the be phase achieved difference compared between with s- that and producedp-polarized by reflected standard waves commercially in the vicinity available of the surface SPR, where plasmon the phases resonance of the biosensorsreflected waves [27]. change This improveddrastically sensitivity [25]. Moreover, was obtained TIRE hasmainly a better due sensitivityto the ability to to even directly the ellipsometric measure the parameterphase differenceΨ than betweenthe standard s- and p-polarized p-polarized intensity reflected measurement waves in the which vicinity is obtained of the by SPR, the where SPR [24 the]. The phases TIRE of with the a reflectedhybrid plasmonic waves change mode of drastically Tamm and [25]. surface Moreover, plasmons TIRE were has applied a better for saturated sensitivity gas [to28 ], even graphene the ellipsometricinfluence to parameter the strong Ψ coupling than the [29 standard] and biosensing p-polarized [22 ],intensity where the measurement strong coupling which eff isect obtained between bythese the twoSPR excitations [24]. The TIRE was employed. with a hybrid However, plasmonic commercially mode of available Tamm and SPR surface biosensors plasmons normally were use appliedsimpler for optical saturated schemes, gas [28], where graphene only the influence p-polarized to the intensity strong of coupling the SPR [29] phenomenon and biosensing is registered [22], whereand which the stronguse only coupling a single wavelength effect between as the these light source. two excitations In this study, was the employed. TIRE method However, was used commerciallyfor the analysis available of the optical SPR biosensors properties normally and sensitivity use simpler features optical of the Tamm-surface schemes, where plasmon only the hybrid p- polarizedmodes. Theirintensity p-polarized of the SPR intensity phenomenon for a single is registered wavelength and which and the use p-polarized only a single signal wavelength intensity as of thethe light hybrid source. Tamm-surface In this study, plasmon the TIRE mode method were thenwas comparedused for the with analysis the single of the SPR optical used properties in standard andoptical sensitivity biosensors. features As of mentioned the Tamm-surface before, three plasmon samples hybrid were modes. used: theTheir single p-polarized thin gold intensity layer for for the a SPRsingle and wavelength the two photonic and the crystalsp-polarized (PC) /signalAu structures intensity with of the optimized hybrid Tamm-surface sensitivity for plasmon Tamm plasmons mode and for surface plasmons in their hybrid plasmonic modes, respectively. Coatings 2020, 10, 1187 7 of 10

In order to determine the wavelength at which the resonant effect of the plasmonic excitation was most efficient, the dispersion relations for all three samples were first measured experimentally and presented as maps of the p-polarized intensity of wavelength dependence on the angle of incidence (AOI). Figure2A represents the single SPR excitation and Figure2B,C, the hybrid Tamm-surface plasmon modes for slightly different multi-layered structures. As can be seen, the dispersion relation of the single SPR lies in the 650–700 nm spectral range for angles of incidence 70–75◦, while for the hybrid plasmonic modes, the SPP branch moves down to the longer wavelengths and lies at about 800 nm for the same AOI. Such modification of dispersion relation for SPP branch caused by strong coupling between Tamm optical states and surface plasmon polariton in the hybrid plasmonic mode. Such behavior is the result of the strong coupling effect between the Tamm plasmons and the surface plasmons, together with a narrowing of the excitation line of the SP branch in this AOI range, compared with that of a single SPR. The narrowing of the dispersion relation line indicates the lower losses of such SP in its hybrid mode. Attention should also be paid to the Tamm plasmon branch in its hybrid plasmonic mode which lies between 600–800 nm in a wide range angle of incidence and has an even narrower width than the SP branch. The cross-section at the resonant wavelength is marked with dashed lines in each spectra of the dispersion relation. In Figure2, the presented dispersion relations correspond to the optical responses to the ambient medium of deionized water, while in Figure3, the dispersion relations for all three samples are presented when responding to a mixture of deionized water (50%) and ethanol (50%) which changes the refractive index of the ambient medium. The cross-sections (dash lines) were made at the same resonant wavelength as with the pure deionized water. The cross-sections at a particular resonant wavelength give the angular spectra of the p-polarized intensity of the corresponding excitations (Figure4A–C). The black curves correspond to the optical responses in the vicinity of the plasmonic excitations for deionized water and the red curves show the changes in the optical response of the system when the refractive index changes due to the mixture of deionized water (50%) and ethanol (50%). As can be seen from Figure4B, the sample optimized for the Tamm plasmons manifested itself at the AOI = 62.5◦ and λTP = 715 nm for the deionized water and AOI = 64.5◦ for the mixture of water/ethanol (50%/50%), respectively. This corresponded to a shift of 2 degrees for the refractive index change δn = 1.3442 1.3298 = 0.0144. It should ◦ (λ=715 nm) − be noted that the full width at half maximum (FWHM) for the Tamm plasmon excitation in the hybrid mode was 0.88◦. In the case of the multilayer sample for optimized surface plasmons in their hybrid plasmonic modes, the angular shift was 1.7◦, i.e., from 66.4◦ up to 68.1◦ (Figure4C) and the FWHM = 1.56 to λ = 820 nm, δn = 1.3425 1.32979 = 0.0146. The same measurements ◦ SP (λ=820 nm) − were performed with the commercially available SPR chip consisted of a thin (~45 nm) gold layer without any additional multi-layered structures. The SPR shift to the larger AOI due to changes of the refractive index of the ambient (liquid) was 3.5◦, i.e., from 64.9◦ up to 68.5◦ (Figure4A) and the FWHM = 3.5 for the λ = 800 nm, δn = 1.3428 1.3282 = 0.016. However, a much ◦ SPR (λ=800 nm) − wider FWHM was registered for the SPR. This was 3.6◦, which indicated significantly higher metal losses for these plasmonic excitations compared with the Tamm plasmons or the surface plasmons in their hybrid plasmonic modes. The narrowing of the width of the hybrid plasmonic resonances due to such reduced losses leads to the increased values of the p-intensity reflectance for the TP δRTP 0.75 and SP δRTP 0.87 compared with the single SPR, which indicated that this was ≈ ≈ δRTP 0.16. As a result, these numbers led to a corresponding sensitivity of the refractive index ≈ ∆Rp(TP) 0.75 1 ∆Rp(SP) 0.87 1 ∆Rp(SPR) 0.16 1 unit of δn = 0.0144 = 52 RIU− , δn = 0.0146 = 60 RIU− and δn = 0.016 = 10 RIU− . The experimental results and analysis of the optical responses of these three different plasmonic based samples showed that the optimized Tamm plasmons ∆Rp(TP) and optimized surface plasmons ∆Rp(SP) samples produced five and six times better responses than the conventional surface plasmon resonance ∆Rp(SPR). Meanwhile, the conventional SPR overcame the optimized hybrid plasmonic modes for the registration of changes of the angle of incidence. These were by TP = 1.75 and SP = 2 times, respectively. Coatings 2020, 10, x FOR PEER REVIEW 8 of 11 optimized surface plasmons ∆푅푝(푆푃) samples produced five and six times better responses than the conventional surface plasmon resonance ∆푅푝(푆푃푅). Meanwhile, the conventional SPR overcame the Coatingsoptimized2020 ,hybrid10, 1187 plasmonic modes for the registration of changes of the angle of incidence. These8 of 10 were by TP = 1.75 and SP = 2 times, respectively. As variable angle spectroscopic ellipsometry gives the possibility of analysing the optical responseAs variable by using angle a full spectroscopic analysis of the ellipsometry polarized giveslight, the the possibility p-polarized of analysingintensity spectra the optical dependence response byon the using wavel a fullength analysis was also of the presented polarized (Figure light, 5 theA,B p-polarized). It should be intensity noted that spectra focusing dependence on only onthe the p- wavelengthpolarized light was is also related presented to the (Figurestudied5 A,B).plasmonic It should excitations, be noted w thathich focusing are generated on only only the p-polarizedin this state lightof light is polarisation. related to the As studied was noted plasmonic above [22], excitations, the surface which plasmon are generated polariton only component in this statein its ofhybrid light polarisation.plasmonic mode As was produces noted abovea higher [22 sensitivity], the surface than plasmon was achievable polariton componentwhen using in the its single hybrid SPR plasmonic for the modebovine produces serum albu a highermin protein sensitivity covalent than immobilization. was achievable when using the single SPR for the bovine serum albumin protein covalent immobilization.

Figure 5. Experimental spectra of p-intensity dependence on the wavelength for single SPR (A) and optimizedFigure 5 Experimental Tamm plasmon spectra (TP) (ofB) p samples-intensity before dependence (red curves) on andthe wavelength after (black curves)for single deionized SPR (A water) and changedoptimized to Tamm solution plasmon of deionized (TP) ( waterB) samples/ethanol before (50% /(red50%). curves) and after (black curves) deionized water changed to solution of deionized water/ethanol (50%/50%). Thus, this study was focused on achieving a higher sensitivity for the Tamm plasmon component in its hybridThus, plasmonic this study mode. was The focused AOI was on optimized achieving for a the higher TP and sensitivity the conventional for the SPR Tamm samples plasmon were chosencomponent so that in theits hybrid highest plasmonic sensitivity mode. would The be achievedAOI was for optimized both. The for single the TP SPR and of the commerciallyconventional availableSPR samples chip were manifested chosen so itself that as the producing highest sensitivity a dip in thewould p-polarized be achieved intensity for both. spectra The single at 680 SPR nm andof the AOI commercially= 70◦. After available changing chip the manifested deionized itself water as withproducing a mixture a dip of in water the p/-ethanolpolarized (50% intensity/50%), thespectra resonance at 680 nm dip and red AOI shifted = 70 up°. After to 754 changing nm (Figure the5 A).deionized For the water Tamm with plasmon a mixture optimized of water/et structure,hanol the(50%/50%), plasmonic the dip resonance was at thedip 663red nmshifted for AOIup to= 75466◦ nmand (Figure after increasing 5A). For the the Tamm refractive plasmon index optimized of liquid, thestructure, dip moved the plasmonic to a longer dip wavelength, was at the 663 i.e., nm up for to 712AOI nm = 66 (Figure° and after5B). Afterincreasing changing the refractive the deionized index waterof liquid, with the the dip water moved/ethanol to a longer mixture, wavelength, the increased i.e., up refractive to 712 nm index (Figure of the 5B liquid). After shifted changing in both the plasmonicdeionized water excitations with the to longerwater/ethanol wavelengths. mixture, For the the increased single SPR, refractive the red index shift wasof the 72 liquid nm and shifted this correspondedin both plasmonic to the excitations p-intensity to longer changes wavelengths. in δRp = 0.36 For. Forthe single the Tamm SPR, plasmonthe red shift optimized was 72 nm sample, and thethis spectral corresponded changes to were the smaller p-inten ~50sity nm. changes However, in δ푅 such푝 = 0 a.36 red. shift For theinduced Tamm a change plasmon in the optimized relative p-intensitysample, the of spectral about δ changesRp 0.54 were. For smaller both resonances, ~50 nm. However, the diﬀerences such a in red the shift refractive induced index a change of the liquidin the ≈ wasrelative the p same-intensityδn = of0.015. about Thus, 훿푅푝 as 0. can54. For be seenboth fromresonances, Figure 5theB, thedifferences red shift in of the the refractiv Tamme plasmon index of componentthe liquid was was the signiﬁcant same δn and = 0.015. the δ ThusRp value, as can was be evaluated seen from from Figure the simulation 5B, the red of shift the redof the shift Tamm with theplasmon smaller component changes of was the significant refractive indexand the of liquid,훿푅푝 value but takingwas evaluated into account from the the experimental simulation of red the shift red ∆R shift with the smaller changes of the refractive index of liquid, but takingp(TP) into0.54 account the1 as a reference. The evaluated sensitivity to the refractive indexes were δn = 0.015 = 36 RIU− experimental∆Rp(SPR) red0.36 shift as a reference.1 The evaluated sensitivity to the refractive indexes were and δn = 0.015 = 24 RIU− , respectively. Meanwhile, the spectral shift of these resonances gave ∆푅푝(푇푃) 0.54 −1 λ ∆푅푝(푆푃푅) 0.36 −1 λ ⁄ = = 36 푅퐼푈 and∆ ( TP) 50 ⁄ = = 24 푅퐼푈 , ∆ respectively.(SPR) 74 Meanwhile, the the correspondingδ푛 0.015 sensitivities δn = 0.015 δ=푛 32660.015nm/RIU and δn = 0.015 = 4933 nm/RIU. ∆ 50 Thesespectral evaluations shift of these led resonances to the following gave estimationsthe corresponding of the sensitivitysensitivities of the(TP hybrid)⁄ = plasmonic= 3266 modes 푛푚/ δ푛 0.015 and the conventional∆ SPR74 to the RIU. For the optimized Tamm plasmon sample, the p-intensity 푅퐼푈 and (SPR)⁄ = = 4933 푛푚/푅퐼푈. These evaluations led to the following estimations of the measurement wasδ푛 1.50 times.015 higher than for the conventional SPR. For the wavelength scanning, however, the SPR overcame the optimized TP by 1.5 times.

Coatings 2020, 10, 1187 9 of 10

4. Conclusions Summarizing, the TIRE method was used for the analysis of the angular spectra of the hybrid Tamm and surface plasmon modes and their comparison with those produced by conventional single SPRs. A more detailed analysis was made of the p-reflection intensity dependence on the AOI because such types of measurements are very common in commercial SPR devices. The conducted study showed that the presence of strong coupling in the hybrid plasmonic modes increases the p-polarized intensity sensitivity of such excitations due to the reduced losses in the metal layers. It should be noted that conventional SPR shows better sensitivity for AOI and wavelength scanning, while the registration of p-intensity changes gives better values for the optimized plasmonic modes. The angular spectra of the hybrid plasmonic modes with the strong coupling effect was analyzed for the first time in our knowledge. The strong coupling effect in the hybrid plasmonic modes allow one to control and tune the dispersion relation of the plasmonic excitations for corresponding purposes. Since modern coating technologies allow the production of nanometric structures with high precision, this produces the possibility of designing nanophotonic devices with advanced properties.

Funding: This research received no external funding. Acknowledgments: The authors thank for technical support in the photonic crystals preparation T. Tolenis and A. Valaviˇcius. Conﬂicts of Interest: The author declares no conﬂict of interest.

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