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Abstract
Spectroscopic ellipsometry was used for the generation and study of the hybrid TPP-SPP mode as a sensor probe for the real-time formation of amalgam structures on the surface of a plasmon active gold layer. The Au/Hg amalgam formation features and the mercury atoms’ penetration into the gold layer were determined by means of the experimental TIRE data and a regression analysis of a multi-layer model containing the index-profile amalgam layer. The hybrid TPP-SPP mode behavior of the coupled excitations provided more information about the mercury atoms’ penetration into the gold layer than the single TPP and SPP resonances did. The present study demonstrated the possibility of using the hybrid TPP-SPP mode to design advanced optical gas sensor technologies.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Surface electromagnetic wave based optical sensing has been widely used for the detection of adsorbed gas molecules on surfaces or at the solid-liquid interfaces in various biochemical interactions. Such surface plasmon polariton (SPP) optical sensors are among the most popular due to the high sensitivity which can be obtained because of the strong localization of the electric field at the metal/dielectric interfaces [1–3]. In SPP-type optical sensors, a glass prism as a coupler to achieve conditions of total internal reflection (TIR) is commonly used to excite the propagated SPP waves, which are transverse magnetic (TM) p-polarized. The exploitation of this SPP phenomenon requires the use of a semitransparent metal film (commonly gold or silver). Another type of surface electromagnetic wave are the Bloch surface waves (BSW), which can be excited at the surfaces of photonic crystals (PC), which are structures made of periodic dielectric multilayers [4]. Since the BSW exist at the interface between the periodic dielectric multilayer and the surrounding medium, they can be excited in both the TE (s-polarized) and TM polarizations. This is different from the SPP, which operate only in the TM mode [5].
Much attention has been given during the last decade to structures with covered thin metal layers placed on the top of the PC’s. Another type of surface mode, the so called Tamm plasmon-polariton (TPP) appears at the boundary between the photonic crystal and the metal layer [6,7]. Tamm plasmon-polaritons are optical states similar to the electron states proposed by I. Tamm [8], which can occur in the energy band gap on a crystal surface. The stop band of the PC resembles the energy band gap due to Bragg reflections in its periodic structure. In contrast to the SPP propagated surface electromagnetic waves, the TPP are non-propagating states and can be excited in both their TM and TE polarizations (due to the PC) similar to the BSW. In fact, the TPP is a standing wave, which is an interference phenomenon created by two surface waves being propagated in opposite directions [6]. The TPPs have an in-plane wave vector, which is less than the wave vector of light in a vacuum, which allows for their direct optical excitation. For the SPPs, this total internal reflection condition can be achieved only when the incident light reaches an in-plane wave vector equal to the surface plasmon resonance [9,10].
It has been shown [11] that both the TPP and the SPP modes can coexist on the same metal layer if suitable conditions (metal layer thickness and angle of incidence) for both excitations are satisfied and the coupling of these excitations results in the hybrid TPP-SPP mode. For TM polarized incoming light, both the TPP and the SPP are excited at different interfaces of the same metal layer, thus revealing the repulsive nature of these two resonances [11]. The hybrid TPP and the SPP modes can be excited by using a glass prism with a PC/gold layer structure attached to its base. At an appropriate angle of incidence (AOI) to the prism, both resonances can be excited, the TPP due to the Bloch waves in the PC and when the matching of the SPP in-plane wave vector is achieved through the prism coupler. The excitation of both the TM and the TE polarizations can be obtained by employing the spectroscopic ellipsometry technique in its total internal reflection geometry (TIRE) [12]. In fact, TIRE utilizes the analytical power of ellipsometry and increases the sensitivity of the analyzed surfaces by introducing the plasmonic effect into the operation scheme of the ellipsometer. The large sensitivity of TIRE to changes of the polarization states enables one to analyze in detail the structures and properties of the ultra-thin layers [13]. Moreover, the ability to perform real time, non-destructive measurements and kinetic analysis is very important in real sensing applications [2,14,15].
Recently, there has been large interest in the Tamm plasmons effect application for optical sensing by using spectroscopic ellipsometry [16], the perfect absorption due to optical Tamm states accompanied by a singular behavior of the phase revealed the improved sensitivity of optical sensors [17]. Refractive index sensor concept applying optical Tamm states were proposed by Zhang et al [18]. The mesoporous distributed Bragg reflectors were applied for generation of Tamm plasmons and sensing of analyte which can diffuse in to the PC from mesoporous layers [19]. Several publications have been mostly dedicated to numerical studies of the hybrid TPP-SPP mode in optical sensing applications when changing the refractive index of the ambient [20] or the optical magnetic field enhancement due to Tamm plasmons [21]. The features of the optical dispersion of the hybrid TPP-SPP mode have also been studied for possible designs of refractometric sensors which vary the refractive indexes of the dielectric ambient [20]. The hybrid TPP-SPP modes have also been experimentally generated in metal/semiconductor microstructures by exciting the TPP by the photoluminescence of quantum dots [22].
Nowadays the environmental pollution has had a growing influence on human health. One of the most dangerous chemical elements in the atmosphere is mercury because of its long residence time and the distances that it migrates. Modern industrial practices such as the combustion of coal, various technologies of plastics have long been identified as the source of mercury on a global scale [23]. All these processes are accompanied by the byproducts rich in toxic heave metals such as mercury. Another reason to study the amalgam formation was to use the material with high conductivity at optical frequencies, in order to have the possibility to tune the total dielectric function of the sensing layer through the presents of amalgam formation, which influence the coupling strength between the TPP and SPP resonances in the hybrid TPP-SPP mode.
The aim of these studies has been to investigate the kinetic features of mercury vapor adsorption onto gold surfaces by utilizing the hybrid TPP-SPP mode in TIRE configurations, thus demonstrating the advanced capabilities of such hybrid excitation in the design of plasmonic sensors.
2. Methods
The three types of samples were investigated. These consisted of structures supporting a single Tamm plasmon polariton (TPP), a single surface plasmon polariton (SPP) and a hybrid Tamm plasmon polariton / surface plasmon polariton (TPP-SPP). For the single TPP and the hybrid TPP-SPP, distributed Bragg gratings were formed on the tops of the substrates (SiO2 glass), which had 5 bilayers of ~130nm SiO2 and ~63nm TiO2. These were created by ion beam sputtering. The single SPP sample consisted of only gold film having a thickness of about 40 nm, which was deposited using the magnetron sputtering technique. The single TPP and the hybrid TPP-SPP samples were also coated with gold layers of the same thickness as the SPP. All the gold layers under investigation were polycrystalline.
The structure morphologies of the samples were examined using scattering electron microscopy (SEM). This was done using a dual-beam system Helios Nanolab 650 (FEI) equipped with an energy dispersive X-ray (EDX) spectrometer INCA X-Max (Oxford Instruments). In the first instance, the SEM images showed a smooth fine-grained pure Au layer structure with grain size of a few tens of nanometers. A SEM cross section image was made for the evaluation of the layer thicknesses (Fig. 1).
The spectroscopic ellipsometry (SE) experiments were conducted using a dual rotating compensator ellipsometer RC2 (J. A. Woollam Co., Inc.). The SE experiments were carried out in the 320 nm – 1700 nm spectral range. For the single TPP samples, a conventional ellipsometric configuration was used at an angle of incidence of 20° (Fig. 2). The angle of incidence (20° degree) was optimized in order to fulfil the excitation conditions of the TTP and to perform accurate spectroscopic ellipsometry measurements. For samples with the single SPP and with the hybrid TPP-SPP modes, experiments were conducted in a TIRE configuration using a glass BK7 prism (45°) at a AOI of 42.5° (Fig. 3, 4). The measured experimental data was then analyzed using the data acquisition software CompleteEase (J.A. Woollam Co., Inc.) in a multi-layer model. The simulated data were fitted to the experimental ellipsometric results using Levenberg-Marquardt algorithm for nonlinear regression and allowed to determine variable parameters in the model, such as thickness change during Hg adsorption, Hg concentration and depth-profile. The system analyzed consisted of a fused silica substrates, namely, a superlattice element of 5 bilayers with thicknesses of 130 nm/63 nm (SiO2 /TiO2). For the single TPP, gold film of 40 nm thickness was used. The single SPP samples consisted of the BK7 glass prism, a gold layer (40 nm) and a void. The hybrid TPP-SPP samples consisted of the BK7 glass prism, five bilayers of SiO2 /TiO2 (130 nm/63 nm) and a 40 nm gold layer.
Fig. 2 Experimental (dots) and calculated (solid curves) spectra of a single TPP sample before (red curves) and after (green curves) exposure to saturated mercury vapors. Inset represents the sample structure and the measurement configuration.
Fig. 3 Experimental (dots) and calculated (solid curves) spectra of a single SPP sample before (red curves) and after (green curves) exposure to saturated mercury vapors. The inset represents the sample structure and measurement configuration.
Fig. 4 Experimental (dots) and calculated (solid curves) spectra of the hybrid TPP-SPP sample before (red curves) and after (green curves) exposure to saturated mercury vapors. The inset represents the sample structure and the measurement configuration.
To form an amalgam surface layer, saturated mercury vapor with a concentration of 15 µg/m3 at room temperature was introduced into the Teflon chamber [2] containing the sensing gold layer. Ellipsometric parameters Ψ (λ)and Δ (λ) were then measured in a dynamic acquisition mode at an average rate of one spectra per 10 seconds. After the initial ~10 minutes of baseline measurements, saturated mercury gas was introduced into the sample chamber for approximately 100 minutes. Additionally, the sample with the thick (200 nm) gold layer attached to the glass substrate was incubated in the saturated mercury gas chamber for 48 hours in order to investigate the saturation of the mercury atoms into the thick gold layer. The variable angle ellipsometric spectra of the thick sample were measured after 24h and 48h. After introducing the mercury vapor, all the samples had a surface layer of the Au/Hg amalgam. The optical constants of the materials being used, namely Hg [24], BK7 [25], SiO2 [26], Au [27] and TiO2 [28] were taken from the literature.
3. Results and discussions
The spectroscopic ellipsometry measurements of the single TPP and the single SPP samples indicated that the Tamm plasmon polariton and surface propagated plasmon waves manifested themselves as dips in the Ψ(λ) spectra at 729 nm and 658 nm, respectively (Fig. 2 and 3). In the case of the single TPP sample, the external angle of light incidence was equal to 20°, while for the single SPP sample, the AOI was 42.5° in the prism coupler. For the single TPP sample, the fused silica substrate was set as the ambient material and according to the Snell law, the actual internal angle of light incidence for the fused silica glass / PC interface was adjusted via an angle offset of around 7° ± 0.4°. After the formation of the amalgam submonolayer onto the gold surface, the ellipsometric parameters Ψ and Δ blue shifted to 728 nm and 650 nm, respectively. A slight decrease of the resonance depths in Ψ and the suppression of the amplitude in Δ were observed for both excitations. The predicted behavior of the optical response for both excitations was determined by the total conductivity changes in the mixed gold / mercury layer due to the formation of the amalgam.
For the sample with the hybrid TPP-SPP mode, the behavior (Fig. 4) of the Tamm plasmon polariton component became opposite after the formation of the amalgam layer on the top of the gold layer. The TPP excitation moved to longer wavelengths, while the SPP moved to shorter wavelengths by the same blue shift (8nm) as in the case of the single SPP sample (Fig. 3). It should be noted that the excitation of the hybrid TPP-SPP leads to different positions of the TPP (584 nm) and the SPP (684nm) dip components (Fig. 4) when compared with the single TPP (729nm) and the single SPP (650nm), respectively. The TPP coupling with the SPP leads to the repulsion of both resonances. After the formation of the Au/Hg amalgam on the top of the gold layer, the coupling between the TPP and the SPP becomes weaker and both resonances move closer to each other.
It is reasonable to assume that the behavior of both TPP and SPP resonant components in the hybrid TPP-SPP mode were caused by two factors: the repulsion of the dispersion curves and the changes of conductivity in the plasmon active (gold) layer due to the formation of the amalgam. As noted above, the TPP and SPP resonances excited at different interfaces and were coupled inside the plasmon active layer. The penetration depth of both modes separately was about 25-30 nm into the gold layer [3].
The optical properties of the Au/Hg amalgam were determined from the thick gold layer, which had been incubated in the saturated mercury vapor chamber for 48 hours. The dielectric function of the Au/Hg amalgam was approximated as a homogeneous layer and described using the Bruggeman effective medium approximation (EMA). The EMA considers the Au/Hg amalgam film to be an isotropic physical mixture of two elements, Au and Hg, and homogenous on the scale of the wavelength. The dielectric function of the mixture was calculated from the volume fractions of its components, assuming that these retain their intrinsic optical properties. The evaluated volume fraction and thickness of the Au/Hg amalgam showed that the Au/Hg amalgam structure forms on the surface of the thick gold film, producing an increase of its thickness of 2.1 ± 0.2 nm (RMS = 1.1 nm). The volume fraction of the Hg in the EMA layer becomes 42% ± 1%. The thicker gold layer manifests better crystallinity and better adhesion, thereby blocking the further penetration of the mercury atoms deeper into the layer [29–32]. The same approach was used when analyzing the single TPP and the single SPP samples. The Au/Hg amalgam layer thicknesses and volume fractions of Hg obtained from the analysis were 8.8 ± 0.1 nm (RMS = 0.4 nm), 4.9 ± 0.1 nm (RMS = 2.3 nm) and 19% ± 1%, 14% ± 1% for single TPP and single SPP samples, respectively. Regression analysis of the single TPP and SPP samples also showed that the mercury atoms penetrated deeper into the gold layer by about 8 nm in the thin (~40 nm) gold films, and showed comparable lower volume fractions of Hg.
T. Morris and G. Szulczewski have shown that spectroscopic ellipsometry is sensitive to mercury diffusion into the polycrystalline gold film [30]. However, due to the low penetration depth of the light, only a simple approximation can be used in the analysis of the data. For Au/Hg amalgam formation, the EMA approximation masks the details of the depth profile. The TPP and SPP resonances excited separately are in principle sensitive to their interfaces, inner and outer, respectively. Meanwhile, monitoring of the optical responses in the hybrid TPP-SPP modes has shown that changes of the coupling strength between the TPP and SPP resonances during the kinetic process can give additional information about the amalgam formation and the structural changes on the surface. A more adequate optical model with the contribution of the index-profile was applied to fit the experimental ellipsometric data of the hybrid TPP-SPP resonances in order to account for the penetration of the Hg atoms into the layer. For the Au/Hg amalgam, a graded layer was used as an EMA mix of gold and mercury with two profiles, the linear and the exponential functions. However only the exponential profile gave a satisfactory estimation of the optical properties of the Au/Hg amalgam in the investigated wavelength range. Dynamic regression analysis, however, allowed for the reliable determination of the thickness variation and index profile, which are presented in Fig. 5a. The thickness variation shown in Fig. 5a exhibits the stepped Au/Hg amalgam growth process. For the first 40 min., the thickness increases exponentially. Later, after 30 min of amalgam formation, the layer thickness begins to grow linearly up to 90 min when the process was interrupted. This exponential growth corresponds to the Au/Hg amalgam formation, while linear part corresponds to the increased concentration (adsorption) of Hg atoms on the surface. The same similarities were observed in the dip position behavior shown in Fig. 5b. During formation of the Au/Hg amalgam surface layer, both resonances moved to shorter wavelengths, but after 30 min. the TPP dip started to move back. Finally, the TPP and SPP dips came closer to each other in the spectra by up to about 8 nm, indicating a weaker coupling between the resonances in the hybrid mode. The coupling strength of both resonances depends on the dispersion curves, which in turn are extremely sensitive to the total layer conductivity, which increases drastically with the increase of the Hg atoms on the surface of the gold layer. The determined index profile shown in the inset of Fig. 5a demonstrated high, about 80% Hg, concentration near the surface, which decreased exponentially down into the layer. Numerical calculations were also performed to study how coupling of TPP and SPP depends on the conductivity and thickness of Au/Hg layer. The influence of metal layer thickness on dispersion curve repulsion was shown earlier [11]. Our evaluation were performed using the same parameters of the model structure as in the experiment for corresponding real time measurements. Calculation results revealed, that main influence at first minutes (t = 20 min) of Hg adsorption on coupling between TPP and SPP resonances (Fig. 5b) was due to increase of about 1.5 times of real part of conductivity from 300 to 500 Ω−1cm−1 at 700 nm wavelength (Fig. 5a, green dots).
Fig. 5 a) Thickness variation of the amalgam evaluated from regression analysis of the hybrid TPP-SPP mode. The inset shows the depth-profile of the gold and mercury at t = 90 min. Green dots correspond to real part of conductivity of Au/Hg layer at 700 nm wavelength. 5 b) The experimental Ψ (λ) dip position dependence on the exposure time of the gold surface in the saturated mercury vapor.
The determined values of the parameters from all the investigated samples confirmed the assumption that the mercury atoms penetrate into the gold layer for about 5 nm and form an Au/Hg amalgam structure, thus furthering the optical signal changes related to the increased concentration of mercury atoms on the top of the sensor surface. Should be noted, that the hybrid TPP component not directly sensitive to the pure Hg, as this excitation formed on the other interface of the gold layer, however, weakening of coupling effect due to detuning of the SPP component from the optimal resonance conditions leads to the red shift of the TPP, which indicates Hg atoms adsorption on the surface. Very similar results of amalgam formation studies after 0.5-1 hour were reported earlier using structural methods such as XPS, secondary ion mass spectroscopy (SIMS), scanning Auger microscopy (SAM) and SEM [29,33].
The reported studies have shown that the saturation of the monitored signal takes place after about 30 min. and the penetration of Hg atoms deeper into the gold layer strongly depends on the adhesion and grain boundaries of the polycrystalline gold films [29,33]. It should be noted that in the case of the hybrid TPP-SPP mode, the behavior of the coupled excitations gives additional information about the penetration of the mercury atoms into the gold layer in comparison to the single TPP and SPP resonances.
4. Conclusions
The spectroscopic dynamic TIRE method was used for the generation and study of the hybrid TPP-SPP mode as a sensor probe for the real time formation of amalgam structures on the surface of a plasmon active gold layer. The mercury atoms penetration into gold layer was determined through the experimental data acquired through TIRE and the regression analysis of a multi-layer model with an index-profile amalgam layer. In particular, from the regression analysis of all studied samples and previously reported publications [2,29,30], we can conclude that mercury atoms penetrate about 5 nm into polycrystalline gold layers.
The present study has demonstrated the applicability of hybrid TPP-SPP mode to advanced optical gas sensors technologies. These results allow not only for the evaluation of the coupling strength of the TPP and SPP components of the hybrid mode in the plasmon active layer, but also provide some insight into the real time formation of the amalgam on the sensor surface.
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