Two-dimensional Active Tuning of an Aluminum Plasmonic Array for

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Two-dimensional Active Tuning of an Aluminum Plasmonic Array for Full-Spectrum Response Ming Lun Tseng, Jian Yang, Michael Semmlinger, Chao Zhang, Peter Nordlander, and Naomi J. Halas Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02350 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 6, 2017

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Two-dimensional Active Tuning of an Aluminum Plasmonic Array for Full-Spectrum Response Ming Lun Tseng1,4†, Jian Yang2,4†, Michael Semmlinger1,4†, Chao Zhang1,4, Peter Nordlander1,2,4, and Naomi J. Halas1,2,3,4* 1

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Department of Electrical and Computer Engineering, 2Department of Physics and Astronomy,

Department of Chemistry, and 4Laboratory for Nanophotonics, Rice University, Houston, Texas 77005, United States

† These authors provided equally important contribution to this work. * Correspondence and requests for materials should be addressed to N.J.H. (email: [email protected]).

Abstract Color pixels composed of plasmonic nanostructures provide a highly promising approach for new display technologies, capable of vivid, robust coloration and incorporating the use of low-cost plasmonic materials, such as aluminum. Here we report a plasmonic device that can be tuned continuously across the entire visible spectrum, based on integrating a square array of aluminum nanostructures into an elastomeric substrate. By stretching the substrate in either of its two dimensions, the period and therefore the scattering color can be modified to the blue or the red of the at-rest structure, spanning the entire visible spectrum. The unique two-dimensional design of this structure enables active mechanical color tuning, under gentle elastic modulation with no more

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than 35% strain. We also demonstrate active image switching with this structure. This design strategy has the potential to open the door for next-generation flexible photonic devices for a wide variety of visible-light applications. Keywords: stretchable plasmonics, full-color tuning, aluminum plasmonics, active image switching, full color display, dynamic color display The generation of colors and images with pixels consisting of subwavelength optical resonators has recently become an important topic of research.1-10 Such pixels can have greatly improved spatial resolution10 and color vibrancy5, 11 when compared with conventional approaches. They have recently been demonstrated in applications such as imaging, printing,10 colorimetric sensing,12 color filtering,13 and counterfeiting countermeasures.7 However, current approaches are still quite limited in dynamical tuning range; a pixel that can be freely and easily tuned across the entire visible spectrum is an unmet technological challenge. Metallic nanostructures produce strong, vivid colors due to the collective oscillation of their conduction electrons.14 Many approaches for tuning the color of plasmonic and metamaterial surfaces have been reported.15-23 For example, liquid crystal,15, 24, 25 and molecular plasmonics26, 27 techniques have been used to develop color-tunable devices. Among them, using flexible polymers such as polydimethylsiloxane (PDMS) as stretchable substrates can be a good strategy. PDMS is known for being elastic, inert, nontoxic, and nonflammable. Developing a tunable nanophotonic device based on PDMS substrate has become a very active research topic in recent years. This is because the mechanical deformation of photonic nanostructures fabricated on flexible substrates allows for simple, quick, and reproducible tuning of photonic devices.17-20, 22, 23, 28 This approach has been widely used in many applications of stretchable, flexible, and tunable photonic devices,


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such as solar cells29, iLEDs30, tunable filter31, and hemispherical electronic eye cameras.32 Recently, PDMS has also been used for the development of a tunable metalens20. Even though there are several reports of stretchable plasmonic devices, current devices either cover only limited regions of the visible spectrum,18, 23, 33-36 suffer from large resonance linewidths22, 23, or report the device response in terms of extinction instead of scattering peaks. Although some near-infraredto-red wavelength tuning has been achieved by substantial unidirectional stretching of the substrate33-36, this approach is constrained by the elastic limit of the substrate material. Large strains can lead to hysteresis37, and by stretching beyond the elastic limit, a substrate is likely to become deformed and degrade with repeated use.37, 38 Here we report a full color tunable plasmonic device composed of a two-dimensional periodic array of rectangularly shaped aluminum nanostructures. The working principle is shown in Figure 1. While the individual aluminum nanostructures show broadband scattering spectra across the visible spectrum (Fig. S1), when patterned into a two-dimensional array, their linewidth becomes substantially narrowed by far-field diffraction.39-41 The scattering spectrum depends on the excitation and collection angles (Fig. S3) as well as the period of the array, and can be tuned by stretching the substrate to modulate the interstructure distances. To achieve full color tuning without approaching the elastic limit of the substrate, the device was designed to exhibit green light in its relaxed state (Fig. 1a). When stretched along the long edge of the nanoparticles (y-axis), the lateral interparticle spacing along the x-axis is decreased, blue-shifting the scattering spectrum (Fig. 1b). Conversely, stretching along the short edge (x-axis) increases the lateral spacing between the nanostructures. The corresponding increase in array period redshifts the peak position (Fig.1c). When the collection cone is restricted, the scattering color will be highly sensitive to the lattice spacing. In this way, it can be tuned gently across the entire visible spectrum. This results in a


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highly tunable plasmonic device with vivid colors, while keeping the required amount of mechanical deformation of the elastomeric substrate material to a minimum.

Figure 1 | Working principle of the full-spectrum stretchable plasmonic device. Top: schematic of the color change of the device under different stretching conditions. Bottom: corresponding schematic of the two-dimensional nanoparticle array. (a) two-dimensional array in its relaxed state; (b) when stretched along its y-axis, and (c) when stretched along its x-axis. Px=Py=400nm, W=100 nm, L=130 nm. The height of the aluminum nanostructures is 35 nm. When the device is observed under a dark-field optical setup, in the initial state, the color of the scattering light is green. When it is stretched along the short axis of the nanostructure, the color of the scattering light is turned to red due to the change of the lattice. On the other hand, when the device is stretched along the long axis of the nanostructure, due to the decrease of the period along the short axis of the device, the color of the scattering light is blue shifted.


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The plasmonic nanostructures were fabricated in a multistep process. Initially, the aluminum nanoparticles on bilayer dielectric pillars of hydrogen silsesquioxane (HSQ) and poly(methyl methacrylate) (PMMA) were fabricated on an n-type silicon substrate. This was achieved by using electron beam lithography, followed by development, oxygen plasma etching, and aluminum evaporation. To fabricate such a plasmonic device for large area applications, several other fabrication methods could be adopted, including nanoimprint33 and nanostencil lithography42 techniques. The array size was 50μm × 50μm. The arrays were subsequently transferred to a low-modulus, adhesive PDMS substrate (Fig. 2a). A top-view scanning electron microscope (SEM) image of the as-fabricated aluminum nanoparticle/HSQ/PMMA multilayered structure is shown in Fig. 2b. An SEM image of the silicon substrate after PDMS curing and stripping is shown in Fig. 2c, confirming that the aluminum nanoparticles have been transferred to the PDMS substrate.


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Figure 2 | Fabrication of the full-spectrum stretchable plasmonic device. (a) Schematic of the sample fabrication process. E-beam lithography was performed on the HSQ/PMMA layer on the n-Si substrate, followed by development, oxygen plasma etching and Al evaporation. The nanostructures sitting atop the HSQ/PMMA pillars were then transferred to the PDMS. SEM images of the Al-coated Si template (b) before and (c) after PDMS stripping. Scattering spectra of the plasmonic device for a range of stretching ratios were measured using dark-field spectroscopy (Fig. 3) and a laser-driven white light source. The wavelength and polarization of the incident light were controlled with a high-resolution monochromator and a broadband polarizer. The beam was s-polarized and incident on the plasmonic array at an incident angle of ~ 68o. The scattering intensities at a range of various wavelengths were collected with an objective in reflection mode and recorded with a charge-coupled device (CCD) detector. The collection angle of the objective ranged from nominally 17.4 o to 19.1o.


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Figure 3 | Measurement setup for the stretchable plasmonic device. The experimental and simulated scattering spectra as well as the dark-field CCD images under different stretching conditions are shown in Fig. 4. In the relaxed state, the device shows a cyan color, as shown in the corresponding CCD image (Fig. 4a). The corresponding scattering maximum is at nominally 495nm (Fig. 4b). As the strain was increased from 0% to 32% along the short axis of the nanoparticles (as depicted in the top panel of Fig. 4c), the color of the device gradually turned to green (strain: 10%), yellow (13%), and red (23% and 32%). Corresponding to the color change, the scattering peak of the device was red-shifted from 495nm to 645nm during the horizontal stretching process. Conversely, the color of the device was changed to blue and then to purple by stretching the device up to 31% along the long axis of the nanoparticles (as depicted in Fig. 4c). In this vertical stretching process, the scattering peak was shifted to nominally 440nm. Overall, the experimental and simulated spectra are in good agreement: the small variations between the observed and the simulated spectra could be due to sample imperfections (see Supporting Information Section II) or minor surface buckling33. All scattering spectra show very sharp profiles with a full width at half maximum of less than 20 nm. The peak position blue-shifts only about half as far as it red-shifts for comparable stretching in the two respective directions.


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This is because horizontal contraction is nonlinear with respect to vertical expansion (see Supporting Information Section IV). With both effects combined, the scattering window and the color of the plasmonic device was tuned across the visible spectrum while keeping the maximum strain at less than 35%. All strains are well below the elastic limit for most PDMS fabrication protocols (see Supporting Information Section VI). In order to test the repeatability of our device, we stretched it in multiple intervals up to one hundred times (see Figure S13). The device used here is composed of aluminum nanostructures with periods (x, y) = (400 nm, 400nm). The peak position varies only slightly (up to 15nm), which is most likely due to inaccuracy in returning to exactly the same stretching condition each time. According to FDTD simulation results, at the scattering peak, around 5.6% of the source energy at that wavelength was collected by our detector. To improve this efficiency, the unit cell could be replaced with a more complex nanostructure (e.g., metal/insulator/metal multilayered nanoantenna22) to enhance the interaction between the device and incident light. More discussions about scattering efficiency, peak linewidth and elastic limit can be found in Supporting Information Section I, IV, and VI, respectively. The chromaticity, or color purity, corresponding to the spectrum of the plasmonic device at different stretching ratios was analyzed by convolving each spectrum with the Commission internationale de l'éclairage (CIE) 1931 color matching functions. The calculated results are labeled on the CIE 1931 chromaticity diagram shown in Fig. 5. The white triangle plotted on the chromaticity diagram corresponds to the limit of the standard red–green–blue (sRGB) color gamut. Notably, all the colors fall outside of this triangle, indicating that the plasmonic device can produce much more vivid colors than the sRGB color gamut. With all these stretching conditions combined, 76% of the area of the CIE diagram can be covered. Using only three points (23% and 4% along the x-axis, and 14% along the y-axis), 58% of the diagram, and almost the full sRGB triangle can


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be covered. To fully cover the sRGB triangle, a fourth stretching condition (e.g., 10% along the xaxis) can be included. Alternatively, this could be achieved with only three points if a stretching condition between 4% and 10% was used.

Figure 4 | Full color tuning on a single plasmonic device. (a) CCD images and (b) experimental spectra of the device at different stretching conditions. Note that the axes in (a) are the same as the ones in Fig. 3. (c) Schematic of the two-dimensional stretching method for full-color tuning. (d) The corresponding simulated scattering spectra.


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Figure 5 | Full color tuning. CIE 1931 chromaticity diagram overlaid with the sRGB gamut (white line). Device colors with different stretching ratios are shown as well. Here x and y are the standard color coordinates. Several results on image tuning have been recently reported. For example, polarizationsensitive nanostructures13 and liquid crystals15 have been used for tuning the color of plasmonic patterns. Even though the erasing and restoring of an optical image consisting of plasmonic pixels has recently been demonstrated7, continuous switching of optical images through mechanical deformation has not yet been achieved. In addition to full color tuning, dynamic, image switching can also be realized with our stretchable plasmonics device presented here. The working principle


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of the device is illustrated in Fig. 6a. The device contains patterns which consist of plasmonic nanoparticle arrays with different periods. When illuminated with a white light source, the patterns scatter different colors due to their differing periodicities. However, when the wavelength window of the incident light is narrow, only the pattern containing the array with the matching period can be observed. To demonstrate this, three adjacent patterns of the letters O, W, and L, with different periods were fabricated and transferred onto a PDMS substrate. SEM images of these three patterns prior to transfer are shown in Fig. 6b. The patterns (O, W, L) with corresponding periods (x, y) = (429nm, 348nm), (400nm, 400nm), and (348nm, 429nm) lead to different scattering peak positions at 430nm, 495nm, and 530nm, respectively, for the device in its relaxed state. When the patterns are illuminated with a white light source, the colors of the letters O, W, and L are purple, cyan, and green, respectively. As shown in the middle panel of Fig. 6c, when cyan light with a wavelength window of 465-505 nm is used as an excitation source, only pattern W can be observed clearly in the relaxed sample, because it is the only one pattern with its scattering peak located in this excitation window. When stretching the whole device horizontally by 15%, all the scattering peaks of the three patterns are red-shifted. Now, only pattern L can be observed clearly in the darkfield image, since the scattering peaks of the other two patterns are both beyond the wavelength window of the incident light. Similarly, only pattern O can be observed clearly when the sample is stretched vertically by 26% (Fig. 6c). For an excitation wavelength of 490nm, the ratio of the maximum scattering intensity of the bright letter to the two dark letters under different stretching conditions is 9.6, 9.1, and 29.7 for the cases of O, W, and L, respectively (see Supporting Information Section V). Even though we have restricted the number of images to three in this case, in principle this number could be greatly increased. By judicial choice of pattern period, switching


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between many different images should be possible. The narrow linewidth of the peaks allows for switching between devices of many different periodicities. In conclusion, we have presented a practical stretchable plasmonic device that can achieve both full color tuning as well as active, multiple image switching. The narrow collection cone leads to sharp scattering spectra of the 2D aluminum nanostructure arrays yield highly vivid colors that can be easily tuned across the visible spectrum. Furthermore, active switching between multiple optical images is also made possible. In both cases, two-dimensional stretching limits the amount of elastic strain required for these effects to less than 35%. This is ideal for integration into compact microelectromechanical systems (MEMS) based photonic devices. Overall, this full-color plasmonic device has a wide and diverse range of possible applications, such as highly sensitive surface stress or colorimetric sensors, ultracompact MEMS-based spectrometers, active color filters, as well as new types of real-time displays.


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Figure 6 | Dynamic image switching. (a) Schematic of image switching based on stretchable plasmonic patterns. (b) SEM images of as-fabricated patterns (O, W, L) before they were transferred onto the PDMS substrate. Magnified SEM images of the respective patterns are shown in the right panel. (c) CCD image of the plasmonic patterns under white light illumination. (d) Dynamic, multilevel image switching between dark field images scattered from plasmonic patterns under different stretching conditions.


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Methods Design of plasmonic device. To achieve full color tuning, the building blocks of the nanoparticle array should have a large scattering efficiency across the whole visible regime. Aluminum is ideal for this because its high bulk plasma frequency makes its plasmonic resonance tunable from the ultra-violet down to the infrared43, 44. To cover the entire visible spectrum, the isolated aluminum nanoparticles were designed to have a broad resonance centered at around 550nm (Figure S1). In a second step, to produce vivid colors, the far-field interference among the unit cells in the lattice was used to suppress all the scattered light outside the desired wavelength window. The wavelength threshold can be calculated by the diffraction formula: 𝜆𝑚𝑎𝑥 = 𝐷𝑥 𝑛(sin 𝜃𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 + sin 𝜃𝑐𝑜𝑙𝑚𝑎𝑥 ) 𝜆𝑚𝑖𝑛 = 𝐷𝑥 𝑛(sin 𝜃𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 + sin 𝜃𝑐𝑜𝑙𝑚𝑖𝑛 ) Here, 𝜆𝑚𝑎𝑥 /𝜆𝑚𝑖𝑛 is the maximum/minimum wavelength that interferes constructively within the collection angle of the objective. 𝐷𝑥 is the period in the x-direction, 𝑛 is the refractive index of the media, 𝜃𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 is the incident angle, and 𝜃𝑐𝑜𝑙𝑚𝑎𝑥 /𝜃𝑐𝑜𝑙𝑚𝑖𝑛 is the maximum/minimum collection angle of the objective. Therefore, the color scattered from the plasmonic device can be tuned by varying the period. Fabrication of the plasmonic device. To fabricate the aluminum nanoparticle arrays, layers of PMMA 950 A2 resist (thickness~70 nm) and HSQ (DOW CORNING, XR-1541-002, thickness~70 nm) were spin-coated on an n-type silicon substrate (resistivity: 0.01~0.02 Ω⋅cm ), and baked at 250 °C for 2 mins. An e-spacer layer was then coated on the sample before e-beam exposure. Electron beam patterning was performed by using an e-beam lithography system (FEI Quanta 650) at an acceleration voltage of 30keV with a beam current of 40pA. After e-beam


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exposure, development was performed by immersing the substrate in a 2.3% TMAH (MF-319) solution at room temperature for 70s. Oxygen plasma etching (Fischione, 660W) was subsequently performed for 73s to directionally etch the PMMA layer. Aluminum nanoparticles sitting on the HSQ/PMMA pillars were made by e-beam evaporation of a 35nm layer of aluminum (base pressure: 5×10-7 Torr, evaporation rate: 0.1nm/s). There are several reasons to use bi-layered resist. The top HSQ layer serves as a mask in the oxygen plasma etching to define the geometry of the nanostructure. That is because the etching rate of HSQ is much lower in comparison with the underneath PMMA layer. Furthermore, the adhesion between PMMA and the silicon substrate is low. Therefore, with a bi-layered resist the aluminum nanostructures sitting on the HSQ mask can be transferred to the PDMS successfully. The PDMS substrate with thickness ~ 2 mm was prepared by mixing the weighing base and curing agent of a commercial material (Sylgard 184, Dow Corning) with a ratio of 35:1. The solution was then degassed and cured at room temperature and at 80°C for 12 and 2 hours, respectively. To transfer the aluminum nanoparticles from the silicon to the PDMS substrate, the donor was first stamped onto, and then stripped from the PDMS. Due to the good adhesion between aluminum and silicon, only the aluminum nanoparticles on the HSQ/PMMA pillars can be transferred to the PDMS. Spectral measurements. The optical measurements were performed using a hyperspectral darkfield microscopy setup (Fig. 3). A continuum laser driven light source (Energetic LDLS) was used in combination with a polarizer (s-polarization) and a 1200 g/mm scanning monochromator (Princeton Instruments Acton SP2150) to produce the excitation beam. It was weakly focused onto the sample at an incident angle of 68o. The scattered rays were then collected via a reflective objective (Edmunds Optics, 15X/0.28). A beam stop was added to restrict the collection cone from 17.4o to 19.1o. The objective then focused the light onto a CCD (Princeton Instruments PIXIS


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1024) for detection. Using this method, monochromatic images between 400 and 700nm were obtained. The wavelength increments for the spectral scans were 5nm. Finally, the scattering spectra were calculated by subtracting the sample’s scattering response from the substrate background, and dividing by a white calibration to correct for the light source spectrum. Acknowledgements We thank Adam Lauchner, Pratiksha Dongare, Liangliang Dong, and Michael McClain for their support and discussions. This research was financially supported by the National Science Foundation (NSF) grant ECCS-1610229, the Air Force Office of Scientific Research Multidisciplinary Research Program of the University Research Initiative (AFOSR MURI FA9550-15-1-0022), the Army Research Office (MURI W911NF-12-1-0407), Defense Threat Reduction Agency (HDTRA 1-16-1-0042), and the Welch Foundation under Grants C-1220 (NJH) and C-1222 (PN). MLT acknowledges postdoctoral fellowship support from the Ministry of Science and Technology, Taiwan (Grant No. 105-2917-I-564-019-). Supporting Information Available Supporting information includes sections on design of the plasmonic device for vivid color generation, influence of sample imperfections, size dependence, dynamic color tuning, dynamic image switching, elastic limit of PDMS, comparison with related work, and design of a MEMSbased stretchable device and Figures S1−S14. Author Contributions M.L.T., J.Y., and M.S. contributed equally to this work and conceived the project. J.Y. and M.L.T. designed the samples and performed the theoretical simulations; M.L.T., M.S., and C.Z. fabricated the samples; M.L.T. and M.S. performed the optical measurements. All authors analyzed the


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results and contributed to the preparation of the manuscript and discussions. P.N. and N.H. supervised the research. Competing financial interests: The authors declare no competing financial interests. References 1. Kristensen, A.; Yang, J. K. W.; Bozhevolnyi, S. I.; Link, S.; Nordlander, P.; Halas, N. J.; Mortensen, N. A. Nature Reviews Materials 2016, 2, (1), 16088. 2. Olson, J.; Manjavacas, A.; Basu, T.; Huang, D.; Schlather, A. E.; Zheng, B.; Halas, N. J.; Nordlander, P.; Link, S. ACS Nano 2016, 10, (1), 1108-17. 3. Proust, J.; Bedu, F.; Gallas, B.; Ozerov, I.; Bonod, N. ACS Nano 2016, 10, (8), 7761-7. 4. Bai, L.; Xie, Z.; Wang, W.; Yuan, C.; Zhao, Y.; Mu, Z.; Zhong, Q.; Gu, Z. ACS Nano 2014, 8, (11), 11094-11100. 5. Sun, S.; Zhou, Z.; Zhang, C.; Gao, Y.; Duan, Z.; Xiao, S.; Song, Q. ACS Nano 2017, 11, (5), 4445-4452. 6. Chen, T.; Reinhard, B. M. Advanced Materials 2016, 28, (18), 3522-3527. 7. Duan, X.; Kamin, S.; Liu, N. Nat Commun 2017, 8, 14606. 8. Genet, C.; Ebbesen, T. W. Nature 2007, 445, (7123), 39-46. 9. Zhang, J.; Ou, J.-Y.; Papasimakis, N.; Chen, Y.; MacDonald, K. F.; Zheludev, N. I. Opt. Express 2011, 19, (23), 23279-23285. 10. Kumar, K.; Duan, H.; Hegde, R. S.; Koh, S. C.; Wei, J. N.; Yang, J. K. Nat Nanotechnol 2012, 7, (9), 557-61. 11. Olson, J.; Manjavacas, A.; Liu, L.; Chang, W.-S.; Foerster, B.; King, N. S.; Knight, M. W.; Nordlander, P.; Halas, N. J.; Link, S. Proceedings of the National Academy of Sciences 2014, 111, (40), 14348-14353. 12. King, N. S.; Liu, L.; Yang, X.; Cerjan, B.; Everitt, H. O.; Nordlander, P.; Halas, N. J. ACS Nano 2015, 9, (11), 10628-10636. 13. Ellenbogen, T.; Seo, K.; Crozier, K. B. Nano Lett 2012, 12, (2), 1026-31. 14. Halas, N. J.; Lal, S.; Chang, W.-S.; Link, S.; Nordlander, P. Chemical Reviews 2011, 111, (6), 3913-3961. 15. Franklin, D.; Chen, Y.; Vazquez-Guardado, A.; Modak, S.; Boroumand, J.; Xu, D.; Wu, S. T.; Chanda, D. Nat Commun 2015, 6, 7337. 16. Xu, T.; Walter, E. C.; Agrawal, A.; Bohn, C.; Velmurugan, J.; Zhu, W.; Lezec, H. J.; Talin, A. A. Nat Commun 2016, 7, 10479. 17. Pryce, I. M.; Aydin, K.; Kelaita, Y. A.; Briggs, R. M.; Atwater, H. A. Nano Lett 2010, 10, (10), 4222-7. 18. Yang, A.; Hryn, A. J.; Bourgeois, M. R.; Lee, W. K.; Hu, J.; Schatz, G. C.; Odom, T. W. Proc Natl Acad Sci U S A 2016, 113, (50), 14201-14206. 19. Kamali, S. M.; Arbabi, E.; Arbabi, A.; Horie, Y.; Faraon, A. Laser & Photonics Reviews 2016, 10, (6), 1002-1008. 20. Ee, H. S.; Agarwal, R. Nano Lett 2016, 16, (4), 2818-23. 21. Zheludev, N. I.; Kivshar, Y. S. Nat Mater 2012, 11, (11), 917-924. 22. Song, S.; Ma, X.; Pu, M.; Li, X.; Liu, K.; Gao, P.; Zhao, Z.; Wang, Y.; Wang, C.; Luo, X.


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Two-dimensional Active Tuning of an Aluminum Plasmonic Array for

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