Aluminum Plasmonic Metasurface Enabling a Wavelength-Insensitive

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Aluminum Plasmonic Metasurface Enabling a WavelengthInsensitive Phase Gradient for Linearly Polarized Visible Light Song Gao, Wenjing Yue, Chul-Soon Park, Sang-Shin Lee, Eun-Soo Kim, and Duk-Yong Choi ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00783 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 4, 2017

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Aluminum Plasmonic Metasurface Enabling a Wavelength-Insensitive Phase Gradient for Linearly Polarized Visible Light Song Gao,† Wenjing Yue,† Chul-Soon Park,† Sang-Shin Lee,*,† Eun-Soo Kim,† and Duk-Yong Choi‡ †

Department of Electronic Engineering, Kwangwoon University, 20 Kwangwoon-ro, Nowon-Gu,

Seoul 01897, South Korea ‡

Laser Physics Centre, Research School of Physics and Engineering, Australian National

University, Canberra ACT 2601, Australia

ABSTRACT: For conventional phase-gradient metasurfaces, the phase tuning is mostly fulfilled by the exploitation of a nanoantenna- or a Pancharatnam-Berry-phase-based nanoresonator. The former scheme typically suffers from a dispersive phase gradient, while the latter is dispersionless yet functions only for circularly polarized light. The precise reproduction of such sophisticated nanoscale structures, which is essential for applications in color printing, color displays, and image sensing, is especially challenging in the visible band. In this work, an Al plasmonic

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metasurface, which enables a wavelength-insensitive phase gradient for linearly polarized light throughout the visible spectral band, is proposed and embodied. The unit cells constituting the metasurface capitalize on a trapezoidal Al nanoantenna that supports a gap-surface plasmon. The incident light, of which the spectrum ranges from 400 nm to 700 nm, is highly angle resolved via an anomalous reflection, providing a splitting angle as large as 42o in view of the corresponding captured color images. It is presumed that the demonstrated wavelength-insensitive phase gradient will lead to a well-defined planar wavefront, thereby substantially suppressing the crosstalk between the different wavelengths at a given angle. Last, the proposed trapezoidal Al nanoantenna has been meticulously inspected in terms of its phase variations that are associated with the plasmonic resonance mode. KEYWORDS: Metasurface, dispersionless phase gradient, visible spectrum splitting, aluminum plasmonic, gap plasmon resonator

M

etasurface, which refers to a two-dimensional metamaterial system based on an array of subwavelength nanoantennas or nanoresonators, has gained enormous attention due

to its conspicuous features that flexibly tailor optical properties in terms of the polarization, intensity, and phase.1,2 Such a metasurface is categorically superior to the conventional bulky optical components, and paves the way for the construction of integrated photonic systems. Recently, various types of phase-gradient metasurfaces, which can induce a space-variant phase profile, were explored.3 Their applications encompass meta-holograms4-8, optical-vortex generation9-12, waveplates13-15, metasurface lenses16-20, and anomalous light bending or spectrum splitting.21-27 With the intension of creating an arbitrary wavefront by means of the phasegradient metasurface, the phase tuning needs to preferentially fulfill a full range of 2π, which


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was mainly carried out by the exploitation of the nanoantenna resonances or the PancharatnamBerry phase. Specifically, for a V-shaped nanoantenna, which supports symmetric and asymmetric resonances, it only operates inefficiently for cross-polarized light.1,4,9,16,21 Concurrent resonances that are mediated by a magnetic and electric dipole10,15 were also attempted for silicon metasurfaces, yet the spectral range that permits a 2π phase variation is severely limited. For the case based on the plasmonic resonator of a metal-insulator-metal (MIM) configuration22,25-29, the operation is only available in the reflection mode so as to provide efficiencies up to 80% in the near infrared region.22 For the scheme tapping into the Pancharatnam-Berry phase, the metasurface causes no wavefront distortion, allowing for a wavelength independent or dispersionless phase gradient over a broad spectral band;6,7,19,24 however, this approach is only valid for circularly polarized light, thereby demanding a relatively complicated working system. It should be noted that a dispersionless phase-gradient metasurface exhibits a wavelength-invariant phase gradient24, in contrast to an achromatic metasurface that is adopted to steer multiple wavelengths toward the same direction or position.18,20 From the perspective of spectrum splitting for linearly polarized light, in particular, a linear phase variation that leads to a constant phase gradient should be induced along the unit cells that constitute the metasurface. For this purpose, diverse phase-gradient metasurfaces were introduced by the exploitation of a unit cell that consists of an array of discrete nanoresonators, which is variant in lateral dimension or shape, and it possibly renders a linear phase variation at certain wavelengths.21-23,25,27,30 A planar wavefront is obtained so as to prohibit crosstalk between the different wavelengths for a given angle of splitting. Nevertheless, it is most likely that the phase gradient inevitably hinges on the wavelength due to the intrinsic dispersion of the nanoresonator elements, so that the wavefront may become uneven and arbitrarily distorted26,30.


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Moreover, the precise reproduction of a multitude of miniaturized nanoresonators, as is required for the applications that include color displays, color printing, and image sensing, is extremely challenging in the visible band. As an alternative to the unit cells involving a group of discrete nanoresonators, a quasi-continuous metasurface that capitalizes on a unit cell that contains a single nanoantenna of the MIM configuration was extensively studied.26,31-35 Since it is made of noble metals like Au or Ag, the metasurface is unfortunately incompatible with the cost-effective complementary metal-oxide-semiconductor process; furthermore, its operation barely spans a part of the visible band, considering that Au and Ag have relatively low plasma frequencies and undesirable dispersion properties.5,26 Meanwhile, for the metasurfaces that were previously suggested for a beam splitting over the entire visible band, the angle of splitting is still tightly limited. As a consequence, such devices are rarely suitable for the implementation of wide-angle polarization beam splitters, ultrathin optical spectrometers, and light detection and ranging devices.3,26,36 In this paper, a wavelength-insensitive phase-gradient metasurface, which is feasible for linearly polarized light covering the whole visible band, is proposed and embodied. The metasurface relies on a unit cell that is based on a trapezoidal plasmonic nanoantenna of the Al-SiO2-Al configuration, which guides a gap surface plasmons (GSP) mode. The incident visible light can be efficiently angle resolved by virtue of the anomalous reflection that is initiated by the dispersionless metasurface so as to exhibit an approximately planar wavefront. The proposed metasurface will be a viable alternative to the conventional dispersionless metasurfaces such as the resorting to the Pancharatnam-Berry phase, which are necessarily dependent on circularly polarized light. For the proposed Al plasmonic phase-gradient metasurface drawing upon a unit cell of the MIM configuration, the schematic is illustrated in Figure 1. Each unit cell consists of a


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trapezoidal Al nanoantenna that is integrated with a similar metallic substrate via a dielectric insulator in SiO2. The substrate is optically thick, so that light transmission is prevented. With a relatively high plasma frequency compared with conventional noble metals like Au and Ag, Al is perceived as a prominent plasmonic material that is adequate for dealing with the whole visible spectrum.5,37,38 In response to incident light with the electric field polarized along the y-direction, the phase that is in regard to the reflected light is expected to shift depending on the position along the x-direction, in accordance with the linearly varying width of the trapezoidal Al nanoantenna. The incident light accordingly undergoes an anomalous reflection at an angle of θr. As listed in Figure 1, the structural parameters for the unit cell are determined to obtain a full 2π phase shift in the longitudinal direction so as to maximize the angular range for the targeted anomalous reflection. For the proposed structure, the thicknesses of the metallic nanoantenna, the insulator layer that serves as a spacer, and the substrate are H1 = 20 nm, H2 = 50 nm, and H3 = 130 nm, respectively. For the unit cell the periods along the x- and y-directions are Px = 720 nm and Py = 200 nm, respectively. For the Al nanoantenna, the width is adjusted from W1 = 50 nm to W2 = 150 nm in a linear manner while the length is fixed at Lx = 500 nm. To rigorously characterize the reflection phase imparted by the unit cell in terms of the wavelength, finite-difference time-domain based simulations were conducted with the assistance of a tool (FDTD Solutions, Lumerical). The phase profile along the x-direction has been examined under a normal incidence for the three representative wavelengths of λ1 = 450 nm, λ2 = 550 nm, and λ3 = 650 nm. A full phase shift of 2π is obviously obtained, as implied in Figure 2a, where dashed blue, green, and red lines indicate the related linear phase variations, which translate into a constant phase gradient for an ideal dispersionless metasurface. The discrepancies between the calculated and the perfect phase variations are as small as 0.305 rad., 0.183 rad., and


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0.266 rad. for λ1, λ2, and λ3, respectively, which are comparable to the results in ref 32. Figure 2b shows the simulated spectral phase response for linearly polarized light. The phase contour lines in connection with the representative wavelengths, starting from x = 0, are all nearly linear and parallel to each other, signifying an approximately wavelength-insensitive phase gradient throughout the visible domain. As a comparison, as shown in Supporting Information Figure S1, the phase gradient becomes significantly wavelength dependent when the adopted metal for the unit cell is replaced with Au or Ag. For the proposed phase-gradient metasurface depicted in Figure 1a, it is presumed that the angle of reflection for normally incident light is governed by the modified diffraction equation that takes into account the generalized Snell’s law1,31 as follows: sin θr = m0

λ0 λ0 dΦ λ + = m 0 , where m = m0 + 1 , Px 2π dx Px

(1)

where θr is the angle of reflection, λ0 is the wavelength in free space, m0 is the conventional diffraction order, and dΦ/dx is the phase gradient induced by the metasurface along the longitudinal direction. Assuming that the unit cell exhibits a linear phase variation spanning 2π, the angle of reflection may be simply expressed via an upgraded diffraction order, denoted by m.31 It is known that a discrete metasurface fails to allow for the wavelength-insensitive phase gradient for linearly polarized light, because the phase variation is only linear at a single designed wavelength.26,27,31 Meanwhile, we have introduced a metasurface based on continuous trapezoidal nanoantennas in order to achieve a wavelength-insensitive phase gradient. The phase gradient induced by the plasmonic metasurface results in an increment in the diffraction order that accounts for anomalous reflection. Thus such a metasurface device is considered to functionally mimic a conventional blazed grating that usually taps into a sawtooth échelette profile. For a typical blazed diffraction grating, the efficiency, which peaks at the blaze wavelength, is known to inevitably drastically decline with the wavelength, thereby critically limiting its spectral band of operation while the operation is largely susceptible to the state of polarization.26 The efficiency which is mainly determined by the blaze angle and the blaze facet,


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is highly sensitive to the quality of the fabricated blazed grating.39,40 The spectral band of operation for the proposed metasurface can be efficiently broadened due to the wavelengthinsensitive phase gradient, unlike conventional blazed grating. Under the condition that 0.55 < λ0 / Px < 1 for the designed metasurface, there exist at most three diffraction orders (m = -1, 0, and 1). Figure 2c plots the normalized far-field intensity |E|2, defined as |Er|2/|Ei|2, in terms of the angle of reflection and the wavelength. |Er|2 is the peak E-field intensity of the anomalously reflected light that emerges from the metasurface while |Ei|2 is the case according to the reflection from a plain substrate that involves no metasurface. As implied by Figure 2c, the incident light is monitored to predominantly couple to the anomalous reflection in accordance with the diffraction order of m = 1, while the intensity contrast for the desired anomalous reflection, when compared with the other orders of m = 0 and m = -1 reaches up to 13 and 48 at the wavelength of 550 nm, respectively. The reflection angle estimated from equation 1 is marked with dashed lines, indicating that there are excellent correlations between the simulated and calculated results. The incident light, whose spectrum ranges from 400 nm to 700 nm, is anomalously reflected by the proposed phase-gradient metasurface so as to propagate at angles running from 34o to 76o. According to equation 1, the splitting angle depends on both the wavelength range and the pitch along the x-direction Px of the unit cell. By exploiting smaller Px, we could achieve an angular splitting range of 42o, which approximately corresponds to a twofold improvement as compared to those of the previous works for the same wavelength range.26,35 It was concretely validated that the proposed device facilitates an enhanced beam shaping in conjunction with a wavefront that causes no remarkable distortion throughout the visible band. For the reflected light, the wavefront was inspected with respect to an E-field component (Ey) that is the same as that of the incident light, with an awareness that the proposed metasurface is not susceptible to a polarization conversion. Figure 2d plots the profile for the reflected Ey components for the wavelengths of 450 nm, 550 nm, and 650 nm, indicating the reflection angles of 39o, 50o, and 65o, respectively, as marked by the colored arrows. The realized wavefronts of concern are evidently relevant to a plane wave even below λ = 480 nm, as opposed to the previous report26,35, which is thought to be undesirably affected by a dispersive phase gradient. The electric field distribution along the x-direction of the unit cell for the selected wavelengths is plotted in Supporting Information Figure S2. The field amplitude is observed to slightly fluctuate according to the wavelength.


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The designed phase-gradient metasurface was manufactured on an Al substrate by means of a lift-off process, occupying a footprint of 300 µm × 200 µm. Its optical response is then evaluated using the test setup depicted in Figure 3a. The sample is fixed in the middle of a rotation stage (RBB450A/M, Thorlabs), and it is then illuminated by a normally incident collimated beam that is emitted by a halogen lamp (HL-2000, Ocean Optics) via a linear polarizer. A detector, appended to a spectrometer (USB4000, Ocean Optics), is used to capture the light reflecting off the device. An SEM image of the fabricated pattern is shown in Figure 3b. The reflection response of the device was assessed as a function of the angle.22 The reflected signal needs to be normalized with respect to the reference signal that is reflected from a plain Al substrate involving no metasurface. In order to practically facilitate the reference measurement, the incident angle was chosen to be 30o. Figure 3c shows the contour plot of the measured reflection spectra in terms of the angle of reflection, wherein it exhibits an evolution trend similar to the simulation results shown in Figure 2c, proving that the whole visible light can be efficiently resolved. Figure 3d shows the measured spectra of the reflected beam when the reflection angle is altered from 35o to 70o in steps of 5o. For each of the reflection spectra, the full-width-at-half-maximum spectral bandwidth was measured to be in the vicinity of 30 nm. The reflection presented a peak efficiency of approximately 30% at around λ = 550 nm. It is reckoned that the rest of the launched light is primarily accounted for by the absorption incurred by the metasurface, as predicted by the simulation results mentioned in Figure 2c. The efficiency of anomalous reflection was approximately 40% for the proposed metasurface, which is lower than ~80% of the previous Ag-based case.26 The efficiency is estimated to improve by increasing the SiO2 thickness and by optimizing the trapezoidal Al strip to a certain extent as long as the wavelength-insensitive phase gradient is achieved.28 Captured color images for the reflected


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beams are also displayed in the insets of Figure 3d with the angle varying from 40o to 60o, where vivid colors of dark blue, green, yellow, and red are obviously evident. The measurement results agree well with the calculated results in Figure 3e. The increase in the measured spectral bandwidth is attributed to the imperfectly collimated incident light and the relatively large opening angle of the detector window.26 The slight red-shift for the spectrum might arise from the displacement of the specimen under measurement away from the center of the rotation stage, which results in larger spectral shifts for smaller reflection angles that correspond to shorter wavelengths.41 Taking into account that the incorporation of the trapezoidal nanoantenna of the MIM configuration that supports a GSP mode by the proposed phase-gradient metasurface, its operation is elaborately elucidated.28,29 The GSP is regarded as a kind of guided electromagnetic wave that propagates along the metal-insulator interface, and it is effectively localized in the proximity of the insulator region between the two metallic films. In the case where the GSP is subject to a reflection at the terminations of the MIM structure, a GSP mode that is equivalent to a reinforced standing wave might be resonantly created. The fundamental GSP-resonance mode usually manifests in the form of a magnetic dipole, which is equivalent to a substantially enhanced magnetic field inside of the insulator layer.28,29 For a symmetric truncated-MIM structure, which can play the role of a typical Fabry-Perot etalon, the resonance transpires under the condition, as follows: w

2π neff = qπ − φ λ

(2)

where w is the width of the resonator, λ the wavelength in free space, neff is the effective refractive index of the resonance mode, q is the resonance order, and φ is the additional phase shift incurred by the reflection at the terminations of the resonator structure. It is predicted that


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the reflected light will undergo a phase shift amounting to 2π around the resonance wavelength via the adjustment of the lateral dimension of the resonator.28 It was reported that an almostlinear relationship exists between the scattering cross section and the width of the metal in the case of the MIM configuration that is based on materials like Au or Ag.42 The scattering cross section is understood as the ratio of the scattered power through the closed surface to the powerper-unit area for the incident wave. A similar linear relationship can be apparently achieved between the scattering cross section and the width of the resonator for the proposed Al plasmonic metasurface, as shown in Supporting Information Figure S3. The GSP mode that is in connection with the proposed phase-gradient metasurface was investigated by scrutinizing the evolution of the field profiles at a number of representative positions in the unit cell, as marked in Figure 4a. Figures 4b and 4c show the E- and H-field distributions in the central YZ plane (x = 360 nm) for the resonant wavelength of 550 nm. The excitation of the GSP mode is guaranteed by the substantially strengthened magnetic field inside of the SiO2 insulator in combination with the reinforced electric field at either side of the Al nanoantenna, which is responsible for the development of an anti-parallel electric displacement current along the metal-dielectric interfaces, as indicated by the arrows.28,29 The GSP mode was also examined by virtue of the vertical |Ez| component of the electric field in the XY plane (z = 155 nm) in the middle of the spacer, as shown in Figure 4d. In light of the established Ez profile, the location of the excited GSP resonant mode moves progressively along the unit cell with the wavelength.28,43 The presence of the GSP-resonance mode plays a role in interpreting the 2π phase tuning of the proposed Al plasmonic metasurface, which prominently features a wavelength insensitive phase gradient. To achieve a constant phase gradient, the resonance characteristics should be


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linearly dependent on the lateral dimension of the trapezoidal Al nanoantenna.31 The dependence of the resonance wavelength on the position along the x-direction of the unit cell has been chiefly examined by plotting the field distribution along either side of the trapezoidal Al nanoantenna. Figure 4e shows the |Ez| profile with the wavelength as according to its monitoring along the intersection between the XY plane and the XZ plane (y = 150 nm). The GSP mode is definitely checked to red-shift as the width of the trapezoidal Al nanoantenna increases. A linear relationship is obtained as intended, and is indicative of the claimed constant phase gradient that is insensitive to the wavelength. In contrast to the proposed Al plasmonic metasurface, the cases of Au and Ag were practically evaluated to incur phase variations that are unavoidably vulnerable to the wavelength, as shown in Supporting Information Figure S4. In conclusion, a dispersionless phase-gradient metasurface was concocted by exploiting a unit cell that comprises a trapezoidal Al nanoantenna of the MIM configuration, which supports a highly localized GSP-resonance mode. The linearly polarized incident light, spanning the whole visible band, was safely angle-resolved via an anomalous reflection, delivering an impressive splitting angle of 42˚. The reflected light was validated to assume a well-defined planar wavefront, and it is invulnerable to the crosstalk between different wavelengths. The excited GSP mode was thoroughly identified by examining the magnetic and electric field profiles alongside the corresponding electric-displacement current distributions. Lastly, the wavelength-insensitive phase gradient was interpreted by scrutinizing the near-linear dependence of the GSP-resonance mode on the lateral dimension of the trapezoidal Al nanoantenna. In the near future, the proposed metasurface could be readily applied to a wide range of applications encompassing wide-angle polarization beam splitters, ultrathin optical spectrometers, light detection and ranging devices, and so forth.


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ASSOCIATED CONTENT

Supporting Information. Reflection-phase distribution and dependence of resonance feature on changing of metal-nanoantenna width for which Au- and Ag-based MIM configurations are used. Field distribution along the x-direction of the unit cell at selected wavelengths. Dependence of the scattering cross section and the width of the Al nanoantenna in the MIM configuration. “This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected]

Author Contributions S.G. conceived the idea, performed the simulations and measurements, analyzed the mechanism and wrote the manuscript; W.Y. and C.-S.P. assisted in the analysis of the mechanism and the spectral measurements; S.-S.L. supervised the analysis and co-wrote the manuscript; E.-S.K. advised in the preparing of the manuscript; and D.-Y.C. fabricated the device. All of the authors discussed the results and implications and commented on the manuscript at all stages. All of the authors have approved the final version of the manuscript.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2016R1A2B2010170 and 2011-0030079). The work was partly supported by the Australian Research Council Future Fellowship (FT110100853, Dr. DukYong Choi) and was performed in part at the ACT node of the Australian National Fabrication Facility.

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Independent Silicon Metadevices for Efficient Optical Wavefront Control. Nano Lett. 2015, 15, 5369-5374. (16) Aieta, F.; Genevet, P.; Kats, M. A.; Yu, N.; Blanchard, R.; Gaburro, Z.; Capasso, F. Aberration-Free Ultrathin Flat Lenses and Axicons at Telecom Wavelengths Based on Plasmonic Metasurfaces. Nano Lett. 2012, 12, 4932-4936. (17) Arbabi, A.; Horie, Y.; Ball, A. J.; Bagheri, M.; Faraon, A. Subwavelength-thick lenses with high numerical apertures and large efficiency based on high-contrast transmitarrays. Nat. Commun. 2015, 6, 7069. (18) Aieta, F.; Kats, M. A.; Genevet, P.; Capasso, F. Multiwavelength achromatic metasurfaces by dispersive phase compensation. Science 2015, 347, 1342-1345. (19) Tang, D.; Wang, C.; Zhao, Z.; Wang, Y.; Pu, M.; Li, X.; Gao, P.; Luo, X. Ultrabroadband Superoscillatory lens composed by plasmonic metasurfaces for subdiffraction light focusing. Laser Photon. Rev. 2015, 9, 713-719. (20) Li, Y.; Li, X.; Pu, M.; Zhao, Z.; Ma, X.; Wang, Y.; Luo, X. Achromatic flat optical components via compensation between structure and material dispersions. Sci. Rep. 2016, 6, 19885. (21) Ni, X.; Emani, N. K.; Kildishev, A. V.; Boltasseva, A.; Shalaev, V. M. Broadband Light Bending with Plasmonic Nanoantennas. Science 2012, 335, 427. (22) Sun, S.; Yang, K. Y.; Wang, C. M.; Juan, T. K.; Chen, W. T.; Liao, C. Y.; He, Q.; Xiao, S.; Kung, W. T.; Guo, G. Y.; Zhou, L.; Tsai, D. P. High-Efficiency Broadband Anomalous Reflection By Gradient Meta-surfaces. Nano Lett. 2012, 12, 6223-6229. (23) Zhang, X.; Tian, Z.; Yue, W.; Gu, J.; Zhang, S.; Han, J.; Zhang, W. Broadband Terahertz Wave Deflection Based on C-shape Complex Metamaterials with Phase Discontinuities. Adv. Mater. 2013, 25, 4567-4572. (24) Huang, L.; Chen, X.; Muhlenbernd, H.; Li, G.; Bai, B.; Tan, Q.; Jin, G.; Zentgraf, T.; Zhang, S. Dispersionless phase discontinuities for controlling light propagation. Nano Lett. 2012, 12, 5750-5755. (25) Qin, F.; Ding, L.; Zhang, L.; Monticone, F.; Chum, C. C.; Deng, J.; Mei, S.; Li, Y.; Teng, J.; Hong, M.; Zhang, S.; Alu, A.; Qiu, C. W. Hybrid bilayer plasmonic metasurface efficiently manipulates visible light. Sci. Adv. 2016, 2, e1501168. (26) Li, Z.; Palacios, E.; Butun, S.; Aydin, K. Visible-Frequency Metasurfaces for Broadband Anomalous Reflection and High-Efficiency Spectrum Splitting. Nano Lett. 2015, 15, 16151621. (27) Huang, Y.; Zhao, Q.; Kalyoncu, S. K.; Torun, R.; Lu, Y.; Capolino. F.; Boyraz, O. Phasegradient gap-plasmon metasurface based blazed grating for real time dispersive imaging. Appl. Phys. Lett. 2014, 104, 161106. (28) Pors, A.; Bozhevolnyi, S. I. Plasmonic metasurfaces for efficient phase control in reflection. Opt. Express 2013, 21, 27438-27451. (29) Pors, A.; Bozhevolnyi, S. I. Efficient and broadband quarter-waveplates by gap-plasmon resonators. Opt. Express 2013, 21, 2942-2952. (30) Lee, Y. U.; Kim, J.; Woo, J. H.; Bang, L. H.; Choi, E. Y.; Kim, E. S.; Wu, J. W. Electrooptic switching in phase-discontinuity complementary metasurface twisted nematic cell. Opt. Express 2014, 22, 20816-20827. (31) Zhang, L.; Hao, J.; Qiu, M.; Zouhdi, S.; Yang, J. K. W.; Qiu, C. W. Anomalous behavior of nearly-entire visible band manipulated with degenerated image dipole array. Nanoscale 2014, 6, 12303-12309.


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(32) Li, Z.; Huang, L.; Lu, K.; Sun, Y.; Min, L. Continuous metasurfaces for highperformance anomalous reflection. Appl. Phys. Express 2014, 7, 112001. (33) Li, Z.; Palacios, E.; Butun, S.; Aydin, K. Ultrawide Angle, Directional Spectrum Splitting with Visible-Frequency Versatile Metasurfaces. Adv. Opt. Mater. 2016, 4, 953-958. (34) Zhang, Y.; Zhou, L.; Li, J. Q.; Wang, Q. J.; Huang, C. P. Ultra-broadband and strongly enhanced diffraction with metasurfaces. Sci. Rep. 2015, 5, 10119. (35) Li, Z.; Aydin, K. Broadband metasurfaces for anomalous transmission and spectrum splitting at visible frequencies. EPJ Appl. Metamat. 2015, 2, 2. (36) Dregely, D.; Taubert, R.; Dorfmuller, J.; Vogelgesang, R.; Kern, K.; Giessen, H. 3D optical Yagi-Uda nanoantenna array. Nat. Commun. 2010, 2, 267. (37) Zheng, B. Y.; Wang, Y.; Nordlander, Peter; Halas, N. J. Color-selective and CMOScompatible photodetection based on aluminum plasmonics. Adv. Mater. 2014, 26, 6318-6323. (38) Knight, M. W.; King, N. S.; Liu, L. F.; Everitt, H. O.; Nordlander, P.; Halas, N. J. Aluminum for plasmonics. ACS Nano 2014, 8,834-840. (39) Ryba, B.; Förster, E.; Brunner, R. Flexible diffractive gratings: theoretical investigation of the dependency of diffraction efficiency on mechanical deformation. Appl. Opt. 2014, 53, 1381-1387. (40) Lee, C.; Hane, K.; Lee, S. The optimization of sawtooth gratings using RCWA and its fabrication on a slanted silicon substrate by fast atom beam etching. J. Micromech. Microeng. 2008, 18, 045014. (41) Skowron, A. Performance Qualification and Verification in Powder X-Ray Diffraction. In Practical Approaches to Method Validation and Essential Instrument Qualification; Chan, C. C., Lam, H., Zhang, X. M., Eds.; John Wiley & Sons: New Jersey, 2010; pp 377-389. (42) Sondergaard, T.; Jung, J.; Bozhevolnyi, S. I.; Della Valle, G. Theoretical analysis of gold nano-strip gap plasmon resonators. New J. Phys. 2008, 10, 105008. (43) Pors, A.; Nielsen, M. G.; Bozhevolnyi, S. I. Broadband plasmonic half-wave plates in reflection. Opt. Lett. 2013, 38, 513-515.


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Figures and captions: Figure 1. Proposed Al plasmonic phase-gradient metasurface based on a trapezoidal nanoantenna of the MIM configuration. Figure 2. (a) Simulated phase variations for reflected light along the x-direction of the unit cell for wavelengths of 450 nm, 550 nm, and 650 nm, marked in solid lines. Dashed lines indicate corresponding linear phase variation for a dispersionless metasurface. (b) Phase contour in terms of the wavelength and the position along the x-direction. Phase contour lines starting at x = 0 for λ = 450 nm, 550 nm, and 650 nm are marked by dashed lines. (c) Normalized far-field E-field intensity |E|2 = |Er|2/|Ei|2 in terms of the wavelength and the angle of reflection under the normal incidence. Calculated reflection angles derived from equation 1 are also indicated by dashed lines. (d) E-field (Ey) distribution for the reflected light, with the propagation being signified by an arrow for λ = 450 nm, 550 nm, and 650 nm. Figure 3. (a) Experimental setup for measuring the reflection characteristics of the completed metasurface. (b) SEM image of the prepared metasurface pattern. (c) Measured contour plot of the reflection spectra in terms of the angle. Dashed line indicates the theoretical results. (d) Measured reflection spectra for angles ranging from 35o to 70o, where the insets show the corresponding captured colors. (e) Simulated reflection spectra with respect to the angle. Figure 4. (a) Description of the observation points within the unit cell. (b) E-field and (c) Hfield distributions along the YZ plane (x = 360 nm). The excitation of a GSP-resonance mode for λ = 550 nm is evidenced by the reinforced magnetic field inside of the insulator in conjunction with the enhanced electric field at either side of the Al nanoantenna, which is relevant to the development of the anti-parallel electric-displacement current at the metal-dielectric interfaces. (d) Distribution of |Ez| for the excited GSP modes as observed along the XY plane (z = 155 nm) in the middle of the insulator for λ = 450 nm, 550 nm, and 650 nm. (e) Dependence of the GSP resonance in terms of |Ez| along the intersection between the XY plane (z = 155 nm) and the XZ plane (y = 150 nm). The corresponding spectra appear to gradually red-shift when the trapezoidal Al nanoantenna increases in width along the x-direction.


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Figure 1. Proposed Al plasmonic phase-gradient metasurface based on a trapezoidal nanoantenna of the MIM configuration. Figure 1 (300×300 DPI)


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Figure 3. (a) Experimental setup for measuring the reflection characteristics of the completed metasurface. (b) SEM image of the prepared metasurface pattern. (c) Measured contour plot of the reflection spectra in terms of the angle. Dashed line indicates the theoretical results. (d) Measured reflection spectra for angles ranging from 35o to 70o, where the insets show the corresponding captured colors. (e) Simulated reflection spectra with respect to the angle. Figure 3 (300×300 DPI)


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Figure 4. (a) Description of the observation points within the unit cell. (b) E-field and (c) Hfield distributions along the YZ plane (x = 360 nm). The excitation of a GSP-resonance mode for λ = 550 nm is evidenced by the reinforced magnetic field inside of the insulator in conjunction with the enhanced electric field at either side of the Al nanoantenna, which is relevant to the development of the anti-parallel electric-displacement current at the metal-dielectric interfaces. (d) Distribution of |Ez| for the excited GSP modes as observed along the XY plane (z = 155 nm) in the middle of the insulator for λ = 450 nm, 550 nm, and 650 nm. (e) Dependence of the GSP resonance in terms of |Ez| along the intersection between the XY plane (z = 155 nm) and the XZ plane (y = 150 nm). The corresponding spectra appear to gradually red-shift when the trapezoidal Al nanoantenna increases in width along the x-direction. Figure 4 (300×300 DPI)


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Table of Contents Graphic (300×300 DPI)


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Aluminum Plasmonic Metasurface Enabling a Wavelength-Insensitive

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