Ultrahigh refractive index sensitivity via lattice-induced meta-dipole modes in flat metallic nanoantenna arrays Rithvik R. Gutha, Seyed M. Sadeghi, Ali Hatef, Christina Sharp, and Yongbin Lin
Citation: Appl. Phys. Lett. 112, 223102 (2018); doi: 10.1063/1.5031768 View online: https://doi.org/10.1063/1.5031768 View Table of Contents: http://aip.scitation.org/toc/apl/112/22 Published by the American Institute of Physics
APPLIED PHYSICS LETTERS 112, 223102 (2018)
Ultrahigh refractive index sensitivity via lattice-induced meta-dipole modes in flat metallic nanoantenna arrays Rithvik R. Gutha,1,2 Seyed M. Sadeghi,1,a) Ali Hatef,3 Christina Sharp,1 and Yongbin Lin2 1
Department of Physics and Astronomy, University of Alabama in Huntsville, Huntsville, Alabama 35899, USA Nano and Micro Device Center, University of Alabama in Huntsville, Huntsville, Alabama 35899, USA 3 Department of Computer Science and Mathematics, Nipissing Computational Physics Laboratory (NCPL), Nipissing University, North Bay, ON P1B 8L7, Canada 2
(Received 30 March 2018; accepted 15 May 2018; published online 29 May 2018) We investigate control of plasmonic-photonic coupling in flat metallic nanoantenna arrays. We demonstrate that when the nanoantennas are packed together along their short axis (transverse lattice constant) and the incident light polarization is along their long axis, they can support latticeinduced plasmonic resonance coupled to a super-photonic mode that densely fills the superstrate volume. Our results show that at a certain wavelength, this resonance joins the plasmonic tip modes of the nanoantennas, forming meta-dipole modes. These modes have field profiles similar to those of the natural plasmonic dipole modes of individual nanoantennas, but they occur at much shorter wavelengths and offer a very high bulk refractive index sensitivity (925 6 12 nm/RIU). We show that with an increase in the transverse lattice constant, such a sensitivity decreases as the metadipole modes disappear. Under this condition, the refractive index sensitivity supported by natural modes of the nanoantennas increases, as the plasmonic edge mode suppression caused by charge rearrangement decreases. Published by AIP Publishing. https://doi.org/10.1063/1.5031768
Gold metallic nanoantennas (mANTs) have been contributing a huge share to the field of nanotechnology and biomedicine because of their optical properties. mANTs with various shapes and sizes gave many breakthroughs in the fields of medicine, devices, and fundamental science because of their localized surface plasmon resonances (LSPRs) in the visible and infrared range.1–15 When mANTs are organized in an array, the LSPRs of individual mANTs can overlap with their neighboring mANTs’ LSPR leading to significant changes in the optical properties. For dimers, it has been reported that, on decreasing the interparticle distance, the LSPR peak caused by the dominating dipole red shifted.16,17 Because of their better sensitivity to refractive index changes in the surrounding environment of the mANTs, the coupled plasmons have been extensively used for biological and chemical sensing applications.18,19 As one would expect, the optical properties of coupled plasmons highly depend on the lattice separation and refractive index (RI) of the environment. While a lot of studies related to the impact of lattice separations have been performed, they have been limited to dimers,6,8,9,20–22 nanodisks,23,24 nanorods (NRs),14,25 and nanosquares.26 mANTs with large lateral dimensions, in comparison to their heights, are known as flat metallic nanoantennas (FmANTs). Previous research on plasmonic modes of FmANTs has shown that they can possess two kinds of multipolar resonances, peripheral and breathing resonances.27,28 The peripheral modes are concentrated on the edges and tips of the FmANTs, and the breathing modes are concentrated at the center of the FmANTs. Recently, we showed that optically saturated and unsaturated surface lattice resonances with distinct high and low refractive index sensitivities a)
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(RISs) can be achieved with an array of FmANTs.29 Additionally, we also investigated the biosensing capabilities of the FmANTs by functionalizing them with streptavidin conjugated quantum dots (QDs).30 In this paper, we investigate the impact of lattice separation along the transverse axis of FmANTs (Fig. 1, y-axis) on the development of the plasmonic modes that are imposed by the periodic arrangement of FmANTs, i.e., lattice-induced plasmonic modes (LIPMs). By means of both experimental and numerical studies, we demonstrate that when the incident light is polarized along the long axes of the FmANTs, LIPMs can efficiently couple with a super-photonic mode formed on top of the FmANT arrays. This mode fills a large portion of the volume of the superstrate close to the FmANTs. We show that at a certain wavelength, the LIPMs are merged with the plasmonic tip modes of the FmANTs, forming collective resonances called lattice-induced meta-dipole modes. Such resonances have similar profiles to those of the dipole modes of single FmANTs, but they are hybrid states that offer ultrahigh RIS, i.e., 925 6 12 nm/RIU. With the increase in the transverse lattice constant (ay), the LIPMs fade away and their
FIG. 1. SEM images of Sample 1 (a), Sample 2 (b), Sample 3 (c), and Sample 4 (d). The scale bar in (d) denotes 800 nm. The x-axis and y-axis in (a) represent the long and short axes of the FmANTs, respectively.
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RIS reduces. Our results also support suppression of natural LSPRs of FmANTs at lower wavelengths via repulsive charge arrangement caused by the array of FmANTs. This leads to reduction of RIS associated with LSPRs. With the increase in ay, however, RIS increases as these resonances mature, becoming similar to those in single FmANTs. These results are in contrast to the trend reported in Refs. 20 and 31 and offer novel understanding regarding RIS of patterned nanoparticles. We fabricated four samples containing arrays of FmANTs with the same lattice constants along the x-axis (ax ¼ 1000 nm) and different ay. Sample 1 [Fig. 1(a)] has a length (L) of 820 nm and a width (W) of 190 nm, and ay ¼ 250 nm. Sample 2 [Fig. 1(b)] has dimensions of L ¼ 832 nm, W ¼ 187 nm, and ay ¼ 300 nm. For Sample 3 [Fig. 1(c)], L ¼ 803 nm, W ¼ 207 nm, and ay ¼ 400 nm. Finally, Sample 4 [Fig. 1(d)] has dimensions of L ¼ 850 nm and W ¼ 235 nm with a lattice constant of ay ¼ 500 nm. The nominal thickness of the FmANTs was 40 nm. The transmission setup for measuring the extinction spectra included a halogen lamp, polarizer, microscope objective, collective lens, and spectrometer. The sample was placed between the microscope objective and the collecting lens. Two types of spectrometers were used, one for visible wavelengths and the other for near infrared wavelengths, providing a wide range from 400 to 1600 nm. The extinction spectra of these samples were measured first with air as the superstrate medium of the FmANTs and then with methanol (RI of 1.3284) and isopropyl alcohol or IPA (RI of 1.3772). The incident light was considered polarized along the long axis of FmANTs (x-pol). The results of the measurements of the extinction spectra of the samples when the superstrate was air (thick solid lines), methanol (dashed lines), and IPA (thin solid line) are shown in Figs. 2(a)–2(d). From Fig. 2(a), it can be seen that when ay ¼ 250 nm, the extinction spectrum contains two major peaks, peak A1 at 809 nm and peak B1 at 1121 nm. As we increase ay from 250 nm to 500 nm, peak A1 shifted to 898.8 nm [peak A2, Fig. 2(b)], 933.6 nm [peak A3, Fig. 2(c)], and 965.5 nm [peak A4, Fig. 2(d)]. Under the same conditions, peak B shifted to 1133 nm [peak B2, Fig. 2(b)], 1217 nm [peak B3, Fig. 2(c)], and 1295 nm [peak B4, Fig. 2(d)]. Upon addition of methanol, it can be seen that for the case of ay ¼ 250 nm, peak A1 red shifted by 55 nm, while peak B1 shifts by 292 nm. For the cases of ay ¼ 300, 400, and 500 nm, the red shifts of peaks A2, A3, and A4 are 73 nm, 97 nm, and 113.5 nm, respectively. Under these conditions, peaks B2, B3, and B4 are shifted by 286 nm, 208 nm, and 130 nm, respectively. Upon addition of IPA, A1, A2, A3, and A4 are red shifted by 64 nm, 84.6 nm, 115.4 nm, and 131.5 nm and peaks B1, B2, B3, and B4 by 339 nm, 315 nm, 243 nm, and 165 nm, respectively. This suggests that these sensors are very sensitive to small changes in RI, as the RI change from methanol to IPA is 0.0488. The results for the RIS of peaks A and B averaged over several regions for the Samples 1–4 and are shown in Fig. 3. It can be noticed that as ay increases from 250 nm to 500 nm, the RIS of peak A increases from 135 6 20 nm/RIU to 242 6 52 nm/RIU (open squares). On the other hand, for the case of peak B (open circles), the increase in ay significantly reduces the RIS. A key observation here is that the RIS of
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FIG. 2. (a)–(d) Measured extinction spectra of Samples 1–4, respectively. Thick solid, dashed, and thin solid lines refer to the cases when the superstrate was air, methanol, and IPA, respectively. (a0 )–(d0 ) The simulated extinction spectra of Samples 1–4 when the RI of the superstrate (n) is 1 (thick solid lines) and 1.33 (dashed lines).
peak B for ay ¼ 250 nm is 925 6 12 nm/RIU, the highest reported so far for an array of FmANTs with similar sizes. This value reduces to 405 6 25 nm/RIU as ay increases to 500 nm. We find that the RIS associated with the difference between refractive indices of methanol and IPA is 742 6 44 nm/RIU. This is consistent with the fact that this value is obtained after a significant shift of peak B1, wherein one expects RIS starts to saturate. To better understand the experimental results, we simulated the interaction of light with arrays of FmANTs with a height of 40 nm, ax ¼ 1000 nm, and different ay, as those in Samples 1–4 using COMSOL multiphysics. For this, we consider L ¼ 820 nm and W ¼ 190 nm for Sample 1 and L ¼ 800 nm and W ¼ 200 nm for Samples 2–4. The top corners of the FmANTs were set to a 3 nm radius small curvature to satisfy the structural replication of the fabricated FmANTs. The incident light polarization was set along the x-pol. A 2-D free triangular mesh was applied on the surface and sides of the gold FmANTs and the interfaces using the extremely fine feature on the COMSOL software. A 3-D free tetrahedral mesh was applied for the rest of the geometry. A minimum mesh size of 1.6 nm was applied. The RI of the
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FIG. 3. Measured RIS as a function of ay for peak A (squares) and peak B (circles). The simulation results for RIS associated with peaks A0 and B0 are shown with filled squares and circles, respectively. The horizontal dotted line refers to the RIS of peak Q of a single FmANT (Fig. 4).
substrate on which the FmANTs were considered was set to 1.51 (glass), and the superstrate RI was set to 1 and 1.33. The array of the FmANTs was achieved by adding periodic conditions to the four sides of a single FmANT. The results of simulation for the extinction cross section of a single FmANT for x-pol when the superstrate is air are shown in Fig. 4. Here, we can see two major peaks at 680 nm (peak P) and 965 nm (peak Q). The plasmonic field enhancement profiles of the modes associated with the optical features P and Q in the x-y plane are also shown in Fig. 4 (inset). These profiles were obtained by finding the ratio of the fields in the presence to those in the absence of the FmANT arrays. The x-y plane is located at the interface between the FmANT and the glass substrate. The results show that peak P is caused by a multipolar plasmonic mode
FIG. 4. Simulation extinction of single FmANT. The inset shows the mode profiles for peak P and Q along the x-y plane.
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with five antinodes while peak Q has three antinodes. Additionally, for these modes, we can see that there is significant field enhancement at the tips of the FmANT. The RIS associated with peak Q is 300 nm/RIU (Fig. 3, horizontal dotted line). Figures 2(a0 )–2(d0 ) show the results of the simulations corresponding to the cases considered in Figs. 2(a)–2(d), respectively. Consistent with the experimental data, for each ay, these results show two major peaks (A0 and B0 ) associated with peaks A and B. Variations of these peaks with ay and refractive index are also consistent with the experimental results. Fig. 3 (filled squares and circles) shows that the RIS associated with peaks A0 and B0 follows the same trend as peaks A and B (squares and circles), although their values are smaller. This can be associated with the mesh sizes and simplification of curvatures of the FmANTs around the corners. Note that peak A0 for ay ¼ 500 nm looks similar to peak Q in Fig. 4. There is, however, no feature in Fig. 4 similar to peak B0 , suggesting that this peak is unique to closely packed FmANTs. Peak P in Fig. 4 is similar to the small peaks seen at about 650 nm in Fig. 2 and is of no interest in this paper. Figure 5 shows the mode profiles associated with the main optical features and their peripheries seen in Fig. 2(a0 ) (circles) when RI of the superstrate is 1.33 (dashed line). The results show that for such an ay value, all resonances [(b), (c), and (e)] are basically LIPMs involved with coupling of sub-developed LSPRs (tip modes) with the interfering photonic fields in the superstrate. With the wavelength, the degree of the mode suppression of LSPRs increases while the photonic field forms multipolar-like patterns. A key observation here is that at 900 nm (peak A0 ), the photonic fields are only coupled to the plasmonic fields at the tips of FmANTs. Figures 5(d)–5(f) show the formation of LIPMs that are efficiently coupled with the photonic modes. At 1190 nm [Fig. 5(d)], such modes are closer to the center of the FmANTs. At 1335 nm [Fig. 5(e)], they get closer to the tips and couple with the tip modes (peak B0 ). With a further increase in the wavelength, LIPMs and photon modes in the superstrate start to fade away.
FIG. 5. Mode profiles for main optical features seen in Fig. 2(a0 ) (ay ¼ 250 nm). The color-coded bar covers field enhancement between 0 and 3.
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FIG. 6. Mode profiles at 1190 nm and 1335 nm are shown in (a) and (b), respectively, along different planes. The values associated with z represent the distance in nm from the substrate and base of the FmANT interjection along the z-plane.
To further understand the LIPM shown in Fig. 5(e), we scrutinize its mode profiles at 1190 nm [Fig. 6(a)] and 1335 nm [Fig. 6(b)] at different heights of the x-y plane relative to the substrate and FmANT interface (z). At 1190 nm, the LIPM is formed closer to the center of the FmANT as it is strongly coupled to the photonic modes in the superstrate [Fig. 6(a), x-z plane]. Its mode profile along the x-y plane at z ¼ 41 nm (right above the FmANT) shows that it is clearly differentiated from the tip plasmons. At z ¼ 39 nm (just below the top surface of the FmANT), the LIPM is concentrated at the edges of the FmANT and as we go deeper into the FmANT, their intensity starts to decrease (at z ¼ 30 nm and 0 nm). Finally, just below the FmANT (at z ¼ 1 nm), very faint LIPMs can still be noticed closer to the center of the FmANT. This tells us that the LIPMs are wrapped around the FmANTs with a stronger field at the top. This can be explained considering the fact that the curvature specifications applied at the corners of the FmANTs make it look thinner at the top and wider as we go deeper. At 1335 nm, the mode profile along the x-z plane shows that the LIPMs are more towards the tips of the FmANTs and are coupled to the tip plasmon modes. At z ¼ 41 nm, the x-y profile of this LIPM suggests the formation of a field profile similar to that of the dipole modes of individual FmANTs except that this
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happens at a much lower wavelength and is a hybridized state. Therefore, it is referred to as a meta-dipole mode. Again, as we go down into the FmANT, the meta-dipole modes are more concentrated on the peripheral of the FmANT (z ¼ 39 and 30 nm). For small z, the LIPM is suppressed and we are mostly left with the tip mode (z ¼ 0 nm). At z ¼ 1 nm, we see the residue of suppressed edge modes naturally supported by FmANTs. The profiles in the y-z plane show that at both the wavelengths, the super-photonic mode densely fills the superstrate volume. These profiles refer to the cuts along the y-axis in the middle of the FmANTs. Figure 7 shows the plasmonic field enhancement profiles associated with peaks B0 (a)–(d) and A0 (a0 )–(d0 ) for different ay. The results shown in Figs. 7(a)–7(d) show that for all values of ay in the x-y plane at z ¼ 0 nm, peak B0 is involved with plasmon tip modes and suppressed edge modes. In the x-z plane, however, we see the meta-dipole modes weaken with the increase in ay. In fact, for ay ¼ 500 nm, they nearly vanished. These results suggest that the coupling between the super-photonic modes in the superstrate with the metadipole modes is the reason behind the ultrahigh RIS of peak B0 . With the increase in ay, the RIS of the arrays is mostly via the plasmonic modes at the tips. This explains the trend seen in Fig. 3 (open circles). The results in Figs. 7(a0 )–7(d0 ) for peak A0 show that the increase in ay leads to dramatic changes in the LSPRs of FmANTs and their coupling with the photonic modes. The mode profiles in the x-y plane at z ¼ 0 nm show that as ay increases, the under-developed edge modes start to gain strength and become clearly visible. However, the results for the x-z plane show a significant evolution of photonic modes and their coupling to plasmonic modes. As shown in Fig. 7(a0 ), apart from the regular plasmon at the tips of the FmANTs, the edge plasmons which go deep into the substrate are very faint. As ay increases, the edge modes get stronger and start to couple with the superstrate photonic modes. These results suggest that at this wavelength, the arrays mostly lead to suppression of edge modes. As ay increases, this process becomes weaker, leading to higher RIS (Fig. 3, filled squares) close to that supported by single FmANTs (Fig. 3, horizontal dotted line). This phenomenon has never been reported before and gives us newer
FIG. 7. Mode profile of peaks B0 (a)–(d) and A0 (a0 )–(d0 ) for ay ¼ 250, 300, 400, and 500 nm. The RI of the superstrate is considered to be 1.33.
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understanding about the plasmonic-photonic coupling in the patterned nanoparticles. In conclusion, we investigated the impact of lattice separation on the plasmonic modes and their coupling to the photonic modes. Our results suggest that for x-pol, there are two possible plasmon-photon coupling mechanisms. In the first type, the plasmons at the tip of the FmANTs couple strongly to the photonic modes in the superstrate via LIPMs leading to a meta-dipole mode with the maximum RIS of 925 6 12 nm/RIU for ay ¼ 250 nm. In the second type, the plasmons on the edges of the FmANTs couple to the photonic modes in the superstrate. The strength of these edge modes increases with increasing ay as the anti-capacitive coupling is suppressed, thereby increasing the plasmon-photon coupling leading to increased RIS. This work was supported by U.S. National Science Foundation under Grant No. CMMI 1234823. 1
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