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Mesoporous silica layers with open and accessible mesopores were fabricated on aluminum nanoparticle arrays. The system can support plasmonic-photonic hybrid modes that are radiatively coupled surface plasmon polaritons in the nanoparticles. The coupling is mediated either by diffraction in the plane of the array or by waveguiding in the dielectric silica layer covering the array. Upon irradiation of an array with visible light, these hybrid modes are excited and appear as sharp spectral dips in optical transmission, a manifestation of light trapping in the system. The porous nature of the layers can be taken advantage of, via pore infiltration, to facilitate control over the layer’s refractive index. We demonstrated tuning in the wavelength and spatial distribution of trapped light in the system.
© 2016 Optical Society of America
1. Introduction
A surface plasmon polariton (SPP) is a collective oscillation of electrons at a metallic surface coupled to a lightwave [1]. In plasmonic arrays, where metallic nanoparticles are arranged in a period comparable to the wavelength of light, light diffraction via the periodic structure can mediate the radiative coupling between the SPPs in neighboring nanoparticles. Under a specific condition wherein the diffracted light propagates in the plane of the array, i.e., a Rayleigh anomaly, SPPs in each nanoparticle oscillate in phase to provide a plasmonic-photonic hybrid mode known as surface lattice resonance (SLR) [2–12]. Another type of coupling between the SPPs is possible via waveguiding: when the array is on or embedded in a thin dielectric layer, the light propagating in the layer mediates the coupling. This type of hybrid mode is referred to as a quasi-guided mode. In both hybrid modes, the coherent oscillation of SPPs yields a collective response that is stronger than the simple sum of each SPP.
The distinctive difference of these hybrid modes from an isolated SPP is the field distribution. While an SPP is a localized mode bound on the metallic surface, these modes accompany the electric field that extends to the neighboring nanoparticles because of strong radiative coupling. From the viewpoint of efficient use of light, the plasmonic array is a good platform for light management: in such systems, the light energy is efficiently trapped in the plane of the array and/or the dielectric layer and can be utilized as energy for further reactions. These hybrid modes have proven useful for surface enhanced Raman scattering (SERS) [13], intensified fluorescence [14–16], and to increase the efficiency of solar cell [17] applications owing to their characteristic field distributions.
In this study, we constructed a unique system where the plasmonic array is covered by a dielectric layer with a variable index of refraction, so that the excitation conditions of the SLR and quasi-guided modes can be controlled externally. As a dielectric layer, we selected mesoporous silica (MPS): MPS is a type of porous material having well-ordered, open and accessible pores, typically less than 10 nm in diameter. It is organized by self-assembly of surfactants [18–21]. The combination of MPS into photonic nanostructures has been explored to actively tune optical resonances [22–27], but the combination of MPS with plasmonic arrays has never been reported. Our system consists of a transparent layer of MPS on top of an aluminum (Al) nanoparticle array. We demonstrate that the number of modes sustained in the system, as well as the spatial distribution of the light energy, can be tuned by the refractive index of the layer, nlayer. The porous nature of the layer is advantageous for further applications through the embedding of functional species inside the pores.
2. Experimental
2.1 Preparation of Al array
First, the Al thin film was sputtered on the silica substrate. A resist (TU2-170, thickness = 200 nm) was spin coated on the thin film and pre-baked for 5 min at 95 °C. Separately, a silicon mold consisting of a periodic square array of cylinders (diameter 150 nm, height 150 nm, array pitch 350 × 350 nm) was prepared by electron beam lithography (F7000s-KYT01, Advantest) and silicon deep etching (RIE-800iPB-KU, Samco). Then, the surface structure of the silicon mold was duplicated on the resist by nanoimprint lithography (EntreTM3, Obducat). The resultant sample was structured by reactive ion etching (RIE-101iPH, Samco). The fabrication area was 6 × 6 mm. The Al array before the deposition of an MPS layer will be referred to as the array without MPS (Fig. 1(a)).
Fig. 1 Sketches of the samples, (a)–(c). Al array without MPS (a), Al array with filled-MPS (b), Al array with open-MPS (c). (d) is an SEM top-view image of the MPS layer prepared on a flat substrate with the rubbing treatment. The MPS was heat treated at 400°C prior to the measurement to obtain a clearer image. (e) shows an SEM top-view image of the Al array without MPS. The size of the nanoparticles in (e) is ca. 140 (diameter) × 80 nm (height), which are periodically arranged in a square lattice with a pitch = 350 nm. The coordinate axes used in optical measurements and numerical simulations are also indicated. The inset to (e) shows a photograph of the full array with a 6 × 6 mm size. The structural color is observed. (f) shows XRD patterns for the Al array with filled-MPS (top panel) and that with open-MPS (bottom panel). The inset to (f) is a sketch of the cross sectional plane of the MPS with the hexagonal vectors used to assign the diffraction peaks. The patterns were measured with the X-ray being incident along the x-axis (perpendicular to the rubbing direction).
2.2 Preparation of an MPS layer on the Al array
The MPS layer was designed to comprise cylindrical pores packed hexagonally with the long axis being aligned parallel to the surface [28, 29]. First, polyoxyethylene(10)cetyl ether, C16H33(OCH2CH2)10OH, (Brij56, 1.737 g) and poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol), HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H, (Pluronic123, 2.523 g) were dissolved in 2-propanol (61.0 ml). Then, 0.1-M aqueous solutions of hydrochloric acid (1.92 ml), water (2.4 ml), and tetraethoxysilane (TEOS, 10.7 ml) were added sequentially. The mixture was stirred for 2 h and used as a precursor solution. Separately, a thin layer of polyimide (PI) was spin coated on the Al nanoparticle array, followed by a rubbing treatment. The rubbing treatment forced the direction of cylindrical pores to be perpendicular to the rubbing direction [28]. The Al nanoparticle array was dip-coated with the precursor solution to form an MPS layer. After evaporation of the solvent, the sample was soaked in a bath of ethanol at 78 °C for 12 h to dissolve and extract the surfactant inside the pores [30, 31]. Finally, the sample was dried at room temperature. We will refer to the Al arrays with MPS layers before and after the extraction as the array with filled-MPS (Fig. 1(b)), and the array with open-MPS (Fig. 1(c)), respectively.
2.3 Optical transmission
The zeroth-order optical transmission was measured as a function of the angle of incidence, θin. For the measurement, we used the collimated beam from a halogen lamp with a beam diameter of ca. 0.5 mm. The sample was mounted on a computer controlled rotation stage. The absolute zeroth-order transmission as a function of wavelength, λ, and θin, T(λ, θin), was obtained by normalizing the transmission of the incident light through the sample to that of the glass substrate. The incident light was polarized along the y-axis, and θin was varied in the zx-plane to put momentum into the x direction (see Fig. 1(e) for the coordinate axes).
3. Simulation
The optical transmission was simulated by using the finite-element method (COMSOL Multiphysics). The coordinate axes were set in accordance with the experiment. The model consisted of layers with different refractive indices; from the bottom upwards these layers were the silica substrate (thickness dsub = 600 nm, refractive index nsub = 1.46), a layer of cylindrical Al nanoparticles (154 (diameter in x) × 130 (diameter in y) × 80 nm (height in z), nAl from the spectroscopic ellipsometry measurement), a PI layer (dPI = 30 nm, nPI = 1.68), an MPS layer (dlayer = 370 nm, nlayer = 1.43 and 1.11), and air on the top (dair = 600 nm, nair = 1.00). Al nanoparticles were embedded in the PI layer to a depth 30 nm from the bottom. Periodic boundary conditions were applied in the lateral directions to simulate a square lattice with a period of 350 nm, and a perfectly matched layer was located on the bottom of the model. A plane wave was incident on the top, and the transmitted light was monitored to simulate T(λ, θin).
4. Results and discussion
Figure 1(d) shows a top-view SEM image of the MPS prepared on a flat substrate with the rubbing treatment. The alignment of the mesocylinders parallel to the surface can be seen. Figure 1(e) is a top-view SEM image of the array. Al nanoparticles having cylindrical shape with diameter of ca. 140 nm are arranged in a square lattice with a pitch a = 350 nm. The array exhibits a structural color owing to light diffraction, as seen in the inset. Figure 1(f) illustrates the X-ray diffraction patterns measured at small angles for the arrays with filled-MPS and open-MPS to identify the macroscopic alignment of the mesopores. The measurements were done with the X-ray being incident perpendicular to the rubbing direction. For the array with filled-MPS (upper panel), the peaks observed at 2θ = 1.12, 2.16, and 3.22° are assigned to the 100, 200, and 300 crystal planes, respectively. These peaks come from the hexagonal pack of cylindrical pores lying parallel to the substrate. The plane distance is calculated to be d100 = 7.9 nm. The peaks at 2θ = 1.49 and 2.98° are from the {110} and {220} planes, respectively (see the inset for the spatial relation of the planes). These planes do not contribute to the diffraction peaks under the ideal 2θ–θ scanning geometry, but generate extra peaks because of the finite X-ray divergence angle that is comparable to the small diffraction angles for the MPS [32–34]. The emergence of these peaks indicates that the incident X-rays are directed parallel to the mesochannels, i.e., the honeycomb domains are globally aligned with the long axis being perpendicular to the rubbing direction. For the Al array with open-MPS (Fig. 1(f), bottom panel), the peaks shift to larger angles and the pores shrink to d100 = 7.5 nm.
Figure 2 summarizes T(λ, θin) for the three types of samples. Figure 2(a) shows T(λ, θin) of the Al array without MPS. A dip is seen at λ = 510 nm and θin = 0°, owing to the excitation of SPPs. The light diffraction in the plane of the array (i.e. Rayleigh anomalies) occurs when kout|| = kin|| ± G, where kout|| ( = 2πn/λ) and kin|| ( = 2πn·sinθin/λ) are the components of the diffracted and incident wave vectors parallel to the surface, respectively, and is a reciprocal lattice vector, where m1 and m2 are the pair of integers defining a diffraction order. When kin|| does not have a y-component, the Rayleigh anomalies satisfy the relation
The dotted lines are plotted with n = 1.46, which causes a Rayleigh anomaly at λ = 511 nm and θin = 0°. With increasing θin, Rayleigh anomalies split into the diffraction orders ( + 1, 0), (–1, 0) and (0, ± 1), and the SPP dips are modulated by these diffraction lines. This modulation indicates the in-plane interaction between the SPPs via light diffraction, since excitation conditions of SPPs for each of the isolated nanoparticles are independent of θin. Figure 2(b) shows T(λ, θin) for the Al array with filled-MPS. The dip at θin = 0° red shifts to λ = 578 nm because of the increase in refractive index in the upper layer from nair = 1.00. The modulation in T(λ, θin) is more notable, since the mismatch in refractive indices surrounding the array is reduced to facilitate SLRs [11]. The dip at θin = 0° is extended along the (–1, 0) diffraction order, and one dip is developed along the (0, ± 1) diffraction orders. There is another dispersive dip that does not follow a Rayleigh anomaly; it intersects the (0, ± 1) line at around λ = 500 nm and θin = 10°, and red shifts with the increase in θin. Figure 2(c) shows T(λ, θin) for the array with open-MPS. The dip at θin = 0° blue shifts to λ = 550 nm, and the extra dip at around λ = 500 nm and θin = 10°, which is observed for the Al array with filled-MPS, is absent.Fig. 2 Wavelength, λ, and incident angle, θin, dependence of zeroth-order transmission, T(λ,θin), (a)–(c): the Al array without MPS (a), with filled-MPS (b), and with open-MPS (c). The incident light is polarized along the y-axis, and θin was varied to put momentum into the x-axis. Simulated T(λ, θin), (d)–(f): the Al array without MPS (d), with filled-MPS (e), and with open-MPS (f). Also shown in the figures are the Rayleigh anomalies with the refractive index of nsub = 1.46 (dotted line) and nair = 1.00 (solid).
Figure 2(d) shows T(λ, θin) simulated for the Al array without MPS. The position of the SPP dip and its variation with θin observed by the experiments mentioned above are reproduced satisfactorily. In the simulation for the arrays with filled- and open-MPS (Figs. 2(e) and (f)), nlayer was varied to fit to the experimental results while dlayer was fixed to 370 nm. The data in Figs. 2(e) and (f), calculated with nlayer = 1.43 and 1.11, respectively, confirm that the simulation results agree with the experimental data well. The volume fraction of the mesopores, V, in the layer was estimated using a simple equation
where nsilica is the refractive index of silica gel. We used nsilica = 1.37 from the ellipsometry measurement and obtained V = 0.7.
In order to look into the evolution of the dips in more detail, we compared T(λ, 14°) for the arrays with filled- and open-MPS (Fig. 3). This angle of θin = 14° was selected to clearly separate the SLRs and the quasi-guided modes. For the Al array with filled-MPS (represented by the black line), three local minima appear at λ = 505 nm, 535 nm and 628 nm, respectively. The dips at λ = 505 nm and 628 nm are assigned to SLRs associated with the diffraction orders (0, ± 1) and (–1, 0), respectively, and the dip at 535 nm is a quasi-guided mode. The array with open-MPS (red line) shows two dips at λ = 505 nm and 598 nm, which are assigned to SLRs associated with diffraction orders (0, ± 1) and (–1, 0), respectively, while the quasi-guided mode does not appear. Consistent with the color maps in Fig. 2, the dips in T(λ, 14°) shown by experiment in Fig. 3(a) can be reproduced semi-quantitatively in the simulation results (see Fig. 3(b)) by using nlayer = 1.43 and 1.11 for the array with filled-MPS and open-MPS, respectively.
Fig. 3 (a) shows the experimentally obtained T(λ, θin = 14°) for the Al array with filled-MPS(black line) and open-MPS(red). (b) shows the simulated T(λ, θin = 14°) for the same cases as (a) with nlayer being set to 1.43 (black) and 1.11 (red). The vertical lines denote the (0, ± 1) and (–1, 0) diffractions evaluated for the refractive index of the substrate, nsub = 1.46.
We specify the origin of the modes by simulating the field distribution. In Fig. 4, we plotted the spatial distribution of the light energy in the plane of incidence (see the sketch in the inset), at three local minima for the array with filled-MPS (upper panels in Fig. 4) and two dips for the array with open-MPS (bottom panels), respectively. For the array with filled-MPS, the light energy is distributed close to the plane of the array upon irradiation with λ = 505 nm and 628 nm (Figs. 4(a) and (c)), showing a characteristic of SLRs. The diffraction orders (–1, 0) and (0, ± 1) are orthogonal to each other, and the field distribution reflects this difference in the direction of diffraction. In contrast, the light energy is distributed inside the MPS layer when light with λ = 535 nm is incident on the array (Fig. 4(b)), confirming that this is a quasi-guided mode. For the array with open-MPS, this quasi-guided mode disappears, i.e., no dip appears, because nlayer ( = 1.11) is too low to support waveguiding. The SLRs survive, although the larger mismatch in the refractive index between the substrate (nsub = 1.46) and nlayer results in a weaker field accumulation. Note that the scale in Figs. 4(a), (b), and (c) is different from that in (d) and (e).
Fig. 4 Calculated spatial distribution of the squared magnitude of the electric field normalized to the incident field, |E|2/|E0|2, in the zx-plane, at a y position intersecting the middle of a nanoparticle. For all the cases covered, the incident angle, θin = 14° (see the sketch). Shown in (a)–(c), the distribution was calculated for the structure with nlayer = 1.43 at λ = 505 nm (a), 535 nm (b), and 628 nm (c), and, shown in (d) and (e), the distribution was calculated for the structure with nlayer = 1.11 for λ = 505 nm (d), and 598 nm (e). Note that the scale in (a), (b), and (c) is different from that in (d) and (e). White lines represent the boundaries of the materials.
5. Summary
We prepared plasmonic arrays covered with an MPS layer. By controlling nlayer, the plasmonic-photonic hybrid modes excited on the array can be controlled. Our system is thin (400 nm excluding the substrate), but sustains multiple modes giving sharp and deep dips in optical transmission. This feature is practically unachievable in dielectric photonic systems; well-defined photonic bandgaps require multiple stacks of layers/particles with optical thickness, which should easily be thicker than several micrometers. Sensing is one possible application that makes use of the refractive index-induced, drastic change in field accumulation inside the porous thin layers: the chemical modification of pores with molecular-selective adsorbents easily leads to sensing applications.
Acknowledgments
A part of this work was supported by the Kyoto University Nano Technology Hub and NIMS Nanofabrication Platform in “Nanotechnology Platform Project” sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Financial support from Grant-in-Aids for Scientific Research (B, No.16H04217) from MEXT was acknowledged. SM gratefully acknowledges support from the construction project for the consortium fostering science and technology personnel, “Nanotech Career-up Alliance (Nanotech CUPAL)”.
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