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1 Supporting Information: Achieving Strong Field Enhancement and Light Absorption Simultaneously with Plasmonic Nanoantennas Exploiting Film-Coupled Triangular Nanodisks Yang Li, Dezhao Li, Cheng Chi, and Baoling Huang* Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. The Hong Kong University of Science and Technology Shenzhen Research Institute, Shenzhen, China. These authors contributed equally to this work. *Corresponding Author Dr. Baoling Huang Phone: S1

2 Geometric parameters optimization In this study, we optimized the Ag disk thickness t disk and Al 2 O 3 spacer thickness t spacer, which mainly determined the plasmon coupling strength between two metal layers. As shown in Figure S1(a), with the increase of t spacer, the resonant wavelengths of all the PNs significantly blue-shifted due to the decrease of the effective index of the resonance mode in Al 2 O 3 spacer, while the absorption first increased until reaching near 100%, and then decreased. For thicker spacer, the resonance coupling effect is relatively weak, thereby the absorption of the structures is imperfect. For thinner spacer, the excitation of resonance within the spacer is inadequate. Therefore, the spacer thickness t spacer =10 nm is chosen as an optimal value for the NIR PN with triangular disks. Figure S1: Simulated absorbance spectra of PN-T as a function of (a) spacer thickness t spacer and (b) disk thickness t disk, respectively. The array period P and the edge length L t is 400 nm and 269 nm, respectively. Regarding the disk thickness t disk, there is a trade-off between strong light confinement along vertical direction using thicker disk and efficient coupling of light into the horizontal spacer layer using thinner disk. Dependence of PNs properties on the disk thickness t disk are shown in Figure S1(b). As the t disk increases, the resonant wavelength quite blue-shifts to short wavelength region, and subsequently became stable at around 1.62 µm. The absorption increases significantly with the blue-shifting of resonant S2

3 wavelength, reaching maximum when the wavelength is stable. According to the simulated results, a critical disk thickness t disk of 70 nm is chosen. Consequently, in this study, the optimal Ag disk thickness t disk of 70 nm and Al 2 O 3 spacer thickness t spacer of 10 nm are used in all the PNs. Figure S2: (a) Simulated absorption spectra for PN-T as a function of incident angle and wavelength. (b) Measured absorption spectra for PN-T as a function of polarization angle and wavelength. The array period P and the edge length L t is 400 nm and 269 nm, respectively. Absorbance spectra in visible and NIR regions S3

4 Figure S3. (a) Simulated and (b) measured absorbance spectra in 0.55 µm to 2.0 µm of PNs with different shaped disks at normal incidence. Magnetic field distributions at the third order mode for PN-T under (c) TM and (d) TE polarizations, and (e) PN-S under TM polarization. The color bars show the enhancement factors. Field enhancement in isolated triangular nanodisk and bowtie-shaped nanodisk As shown in Figure S4, the maximum local electric field enhancement E loc / E 0 of the uncoupled single triangular nanodisk is 43, while the maximum E loc / E 0 for the bowtie-shaped nanodisk reach provided by the in-plane near-field coupling. The gap size between the two tip-to-tip triangular nanodisks is 20 nm. In this study, for the film-coupled nanodisk system, the spacer thickness (the gap size between nanodisks and the metallic film) is 10 nm. According to the mechanism for out-of-plane coupling between the nanodisks and its image charges in the metallic film, 1-2 the effective gap size is twice of the spacer thickness (i.e., 20 nm). Consequently, the effective gap size for the out-of-plane near-field coupling in PN-T we consider in the manuscript is equal to that for the in-plane coupling in bowtie-shaped nanodisk here. Despite the identical effective gap size, PN-T shows a 1.34 times larger E loc / E 0 than that of the bowtie-shaped nanodisk with the same edge length and thickness. Figure S4: Electric distributions in x-y plane of (a) uncoupled single triangular nanodisk and (b) typical bowtie-shaped nanodisk with gap size of 20 nm under TE polarization. The edge-length and thickness of each single nanodisk is 269 nm and 70 nm, respectively. S4

5 Figure S5: Electric field distributions for MIM structured PNs of bowtie-shaped nanodisk under TE polarization. Electric field distribution in (a) x-y plane and (c) y-z plane, the gap size between two triangular nanodisks is 20 nm. Electric field distribution in (b) x-y plane and (d) y-z plane, the gap size between two triangular nanodisks is 10 nm. The edge-length and thickness of each single nanodisk is 269 nm and 70 nm, respectively. We investigate the film-coupled bowtie-shaped nanodisks, which combine the in-plane coupling in bowtie-shaped nanodisks with the out-of-plane coupling between the nanodisk and its image charges in the metallic film. Figure S4(a) and (c) show the electric field distributions of the PNs with the same in-plane and out-of-plane gap size of 20 nm. The electric field is strongly confined in both the in-plane and the out-of-plane gap; however, the stronger confinement locates at the out-of-plane gap with a maximum E loc / E 0 of 133, which is slightly lower than that for PN-T of the same single disk s size. When the in-plane gap between the two disks decreases to 10 nm, the in-plane near-field coupling becomes stronger as evidenced by the higher field enhancement in the in-plane gap. Interestingly, the highest E loc / E 0 still S5

6 appears at the out-of-plane gap despite the larger effective out-of-plane gap size (20 nm). Due to the increase of the in-plane field confinement, the highest E loc / E 0 for in-plane gap size of 10 nm (116) is smaller than that for 20 nm. Effect of oxidation on the optical properties of PN-T with Ag nanodisks Although Ag nanodisks have the advantages of low cost and strong local field enhancement over their Au counterparts, the chemical tarnishing of Ag nanodisks, such as oxidation under ambient conditions, is an issue that has to be considered in practical applications. There have already been several previous works on studying the oxidation process of Ag nanoparticles. 3-7 Qi et al. studied the effect of size on the oxidation process of Ag nanoparticles, demonstrating that nanoparticles with a diameter of 20 nm fabricated by electroless plating showed a faster oxidation process than those with a diameter of 35 nm produced by e-beam evaporation. 7 They attributed this dependence to the larger surface to volume ratio, and the resulted higher surface energy of smaller particles. Similarly, several recent studies also proved that small Ag nanoparticles (typically less than 50 nm) are prone to form an Ag 2 O layer (typically 0-2 nm) when exposed to atmosphere. 3-6 In this study, the edge-length of the smallest triangular nanodisks is 135 nm (P = 200 nm), which is much larger than those small particles discussed in previous works. Here we investigate the effect of oxide layer on the absorbance and field enhancement of PN-T with the smallest triangular nanodisks (P = 200 nm, L t = 135 nm), which has the largest possibility to be oxidized. Figure S6a shows the simulated absorbance spectra of PN-T covered with Ag 2 O (thickness of 0, 1, and 2 nm). The mesh sizes along x, y, and z directions are all set to 1 nm during the simulation. It should be noted here that the thickness of Ag 2 O on the bottom side of Ag nanodisks is set to zero because Ag nanodisks are directly placed on an Al 2 O 3 layer. Interestingly, both the resonant wavelength and the peak absorbance exhibit negligible dependence on the Ag 2 O thickness, while the absorption bandwidth is slightly broadened by adding 1 or 2 nm Ag 2 O. From the wavelength-dependent refractive index of Ag 2 O, one can observe that Ag 2 O is almost a dielectric materials (n real ~2.4, n imag ~0) in the wavelength above 0.5 µm. 5-6 As a result, the thin layer of Ag 2 O probably acts as an antireflection reflection coating, whose reflective index is between that of PN-T and air, to expand the absorption bandwidth. Regarding the local electric field enhancement, the 2 nm Ag 2 O do has S6

7 Fig an influence on the highest E loc / E 0 of PN-T, resulting a reduction by 13% from 211 to 183. Fortunately, several effective strategies have been proposed to prevent the Ag nanoparticle from oxidation, one of which is to introduce a monolayer protective barrier, such as thiols 7-8 and Al 2 O 3. 3 Figure S6. (a) The simulated absorbance spectra of PN-T (P = 200 nm) with Ag 2 O layer. The electric field distribution in x-y plane of PN-T (b) without Ag 2 O and (c) with 2-nm-thick Ag 2 O. REFERENCES (1) Mock, J. J.; Hill, R. T.; Degiron, A.; Zauscher, S.; Chilkoti, A.; Smith, D. R. Distance-Dependent Plasmon Resonant Coupling between a Gold Nanoparticle and Gold Film. Nano Lett. 2008, 8, (2) Nordlander, P.; Prodan, E. Plasmon Hybridization in Nanoparticles near Metallic Surfaces. Nano Lett. 2004, 4, (3) Baraldi, G.; Carrada, M.; Toudert, J.; Ferrer, F. J.; Arbouet, A.; Paillard, V.; Gonzalo, J. Preventing the Degradation of Ag Nanoparticles Using an Ultrathin a-al 2o3 Layer as Protective Barrier. J. Phys. Chem. C 2013, 117, (4) Han, Y.; Lupitskyy, R.; Chou, T. M.; Stafford, C. M.; Du, H.; Sukhishvili, S. Effect of Oxidation on Surface-Enhanced Raman Scattering Activity of Silver Nanoparticles: A Quantitative Correlation. Anal. Chem 2011, 83, (5) Kuzma, A.; Weis, M.; Flickyngerova, S.; Jakabovic, J.; Satka, A.; Dobrocka, E.; Chlpik, J.; Cirak, J.; Donoval, M.; Telek, P., et al. Influence of Surface Oxidation on Plasmon Resonance in Monolayer of Gold and Silver Nanoparticles. J. Appl. Phys. 2012, 112, (6) Yin, Y.; Li, Z. Y.; Zhong, Z.; Gates, B.; Xia, Y.; Venkateswaran, S. Synthesis and Characterization of Stable Aqueous Dispersions of Silver Nanoparticles through the Tollens Process. J. Mater. Chem. 2002, 12, (7) Qi, H.; Alexson, D.; Glembocki, O.; Prokes, S. M. The Effect of Size and Size Distribution on the S7

8 Oxidation Kinetics and Plasmonics of Nanoscale Ag Particles. Nanotechnology 2010, 21, (8) Scuderi, M.; Esposito, M.; Todisco, F.; Simeone, D.; Tarantini, I.; De Marco, L.; De Giorgi, M.; Nicotra, G.; Carbone, L.; Sanvitto, D., et al. Nanoscale Study of the Tarnishing Process in Electron Beam Lithography-Fabricated Silver Nanoparticles for Plasmonic Applications. J. Phys. Chem. C 2016, 120, S8

Supplementary Information

Electronic Supplementary Material (ESI) for Nanoscale. This journal is The Royal Society of Chemistry 2014 Supplementary Information Large-scale lithography-free metasurface with spectrally tunable super