Aluminum Plasmonics Based Highly Transmissive Polarization

Oct 27, 2014 - Nanophotonic devices enabled by aluminum plasmonics are saliently advantageous in terms of their low cost, outstanding sustainability, ...
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Letter pubs.acs.org/NanoLett

Aluminum Plasmonics Based Highly Transmissive PolarizationIndependent Subtractive Color Filters Exploiting a Nanopatch Array Vivek R. Shrestha,† Sang-Shin Lee,*,† Eun-Soo Kim,† and Duk-Yong Choi‡ †

Department of Electronic Engineering, Kwangwoon University, 20 Kwangwoon-ro, Nowon-Gu, Seoul 139-701, South Korea Laser Physics Centre, Research School of Physics and Engineering, Australian National University, Canberra ACT 0200, Australia

‡

S Supporting Information *

ABSTRACT: Nanophotonic devices enabled by aluminum plasmonics are saliently advantageous in terms of their low cost, outstanding sustainability, and affordable volume production. We report, for the first time, aluminum plasmonics based highly transmissive polarizationindependent subtractive color filters, which are fabricated just with single step electron-beam lithography. The filters feature selective suppression in the transmission spectra, which is realized by combining the propagating and nonpropagating surface plasmons mediated by an array of opaque and physically thin aluminum nanopatches. A broad palette of bright, high-contrast subtractive colors is successfully demonstrated by simply varying the pitches of the nanopatches. These subtractive color filters have twice the photon throughput of additive counterparts, ultimately providing elevated optical transmission and thus stronger color signals. Moreover, the filters are demonstrated to conspicuously feature a dual-mode operation, both transmissive and reflective, in conjunction with a capability to exhibit micron-scale colors in arbitrary shapes. They are anticipated to be diversely applied to digital display, digital imaging, color printing, and sensing. KEYWORDS: Nanoplasmonics, aluminum plasmonics, structural color filters, metallic thin films, electron-beam lithography

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polarization sensitivity, which is unfavorable for imaging or sensing applications in which unpolarized or arbitrarily polarized light is usually involved.18,19 The majority of previous works relying on plasmonic resonance preferentially adopted nanostructures made of noble metals, like gold (Au) and Ag. Unfortunately, devices in Au and Ag are hardly cost-effective owing to their incompatibility with the popular manufacturing scheme that incorporates complementary metal−oxide−semiconductor (CMOS) process. Moreover, for Au-mediated plasmonic resonance, interband transitions are supposed to cause a dissipative channel at wavelengths below 550 nm, while the nanostructures made of Ag are extremely vulnerable to color degradation resulting from oxidation and sulfidation.20,21 These factors practically prohibit the adoption of those precious metals as a material candidate for plasmonic filtering. Meanwhile, aluminum (Al) has gained ample interest as a promising, low-cost, and sustainable material for plasmonic color filters, thanks to its excellent optical properties with its interband transition lying outside the visible and near-UV spectral bands and thereby enabling strong plasmonic resonances. Al-based plasmonic devices offer colossal potential on account of their compatibility with the CMOS process, enabling cost-effective volume production.22−26 Additional

urface plasmon (SP) resonance in metallic nanostructures and nanoparticles has been playing a pivotal role in various fields, including sensing, imaging, color display, information processing, and energy harvesting, due to its unique and outstanding manipulation of light.1−4 The SP, referring to an oscillation of free electrons at a metal−dielectric boundary, is typically categorized into a propagating surface plasmon (PSP) and localized surface plasmon (LSP). The corresponding plasmonic resonances are termed PSP resonance (PSPR) and LSP resonance (LSPR).5 One of the most conspicuous applications empowered by such plasmonic resonance is a structural color filter, which is regarded as a crucial component for displays, image sensors, digital photography, and projectors. It offers the merits of good reliability, environmental friendliness, and durability, over conventional devices that use a chemical pigment or dye. Numerous plasmonic color filters based on extraordinary optical transmission (EOT) in a nanostructured opaque film, whose thickness is beyond the skin depth, were reported.6−12 Besides the EOT through such subwavelength metallic structure, a counterintuitive extraordinary low transmission (ELT) phenomenon in a translucent metallic nanostructure, where the metal thickness is comparable to or below the skin depth, has lately also been extensively studied.13−17 Plasmonic subtractive color filters (SCFs), permitting a transmission of 60%, were introduced by exploiting a one-dimensional (1D) grating on an ultrathin silver (Ag) film; however, they are subject to critical © XXXX American Chemical Society

Received: September 1, 2014 Revised: October 19, 2014

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Figure 1. (a) Schematic diagram of proposed color filters comprising a square array of Al NPs over a glass substrate. Incident white light is filtered into different visible colors, in accordance with the periodicity P of the array. (b) SEM images of the fabricated filters with different periods of 400, 340, and 260 nm from left to right, including their generated color images in the inset. (c) The measured transmission characteristics of the CMY color filters at normal incidence, with periods of 400, 340, and 260 nm, from top to bottom, for different polarization directions (ϕ).

(SEM) images of the prepared filters with different periods of 400, 340, and 260 nm, respectively, from left to right, where the duty ratio of the NP array, defined as the ratio of the width of the NP to the period (W/P), was simply set to 0.5. As observed in the SEM images, the proposed filters were confirmed to have been embodied with a square array of square shaped Al NPs, with a high-fidelity design. For the prepared devices of sizes 40 μm × 40 μm, bright CMY color images of high contrast were captured by using a digital microscope (Leica DM4000 M), as shown in the inset of the corresponding SEM images. Figure 1c depicts the corresponding measured transmission spectra of the prepared filters for different polarization directions. It was observed that, under normal incidence, the filters yield almost identical performance for all incident polarization directions, including ϕ = 0°, 45°, and 90°, where the transmission dips for the three CMY filters were located at λ = 631 nm (cyan), 540 nm (magenta), and 432 nm (yellow), respectively. A high transmission reaching up to 75% was attained in the visible range, which is highly remarkable in light of the fact that the Al film is three times thicker than the skin depth of the metal, allowing for no significant penetration of electromagnetic waves. Slight irregular mismatches among the transmission spectra for different polarizations are ascribed to unexpected structural asymmetry in the NP patterns, incurred by fabrication errors. It should be stated that such CMY SCFs are preferentially deemed to have twice the photon throughput of RGB additive counterparts, ultimately providing elevated optical transmission and thus stronger color signals.18 Details on device fabrication and optical characterization are given in the Supporting Information. For the proposed plasmonic SCFs, a broad palette of subtractive colors may be concocted by altering the period of the NP array to tailor the resonance wavelength and thus the output color. Toward that end, the transmission characteristics of the filters were scrutinized under normal incidence of a plane wave by means of a simulation tool based on the finite difference time domain (FDTD) method (FDTD Solutions, Lumerical, Canada). A unit cell with periodic boundary

optimistic aspects of Al encompass natural abundance, neutral tint, broad resonance, high stability, high tolerance to fabrication process, and good adhesion to diverse platforms.22−26 Taking into account both the aforementioned appeal of Al and the fact that SCFs have twice the photon throughput of red, green, and blue additive counterparts, ultimately providing elevated optical transmission and thus stronger color signals,18 it would be highly desirable to develop polarization-independent Al-based SCFs, which has not been done to date. In this paper, we demonstrate highly efficient polarizationindependent plasmonic SCFs, which capitalize on SP-mediated selectively reduced transmission, through a two-dimensional (2D) array of opaque but physically thin Al nanopatches (NPs) built on a glass substrate. The spectral positions of transmission dips are tailored by adjusting the periodicity of the NP array, in order to realize a basic palette of subtractive colors, in addition to the three primary subtractive colors of cyan, magenta, and yellow (CMY). The polarization dependence of the optical response of the filters has been meticulously investigated in conjunction with the mechanism underlying the selective suppression in transmission, via an array exploiting opaque metallic patches, in contrast to the previously reported color filters based on ELT for a nanostructured ultrathin metal film. We also address the possible applications of the proposed SCFs in ultrahigh resolution display/imaging, color printing, and reflection-mode color filters. The proposed color filters consist of a 2D periodic arrangement of Al NPs atop a glass substrate, where each patch has a footprint of W × W on the xy plane, a thickness of Hg (= 40 nm), and an identical period of P along the x and y directions, as illustrated in Figure 1a. The polarization direction of incident light is indicated by an angle ϕ of the electric (E) field with respect to the x direction. Light transmission may be selectively hindered at a resonance wavelength, depending on the structural parameters associated with the NP, so that incoming white light is efficiently filtered out to produce specific colors. Figure 1b displays scanning electron microscope B

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Figure 2. (a) Simulated (i) and measured (ii) transmission spectra of the filters when the period is varied from 260 to 440 nm, with the dashed trend line tracing the location of suppressed transmission. (b) Chromaticity coordinates corresponding to the simulated (i) and measured (ii) spectra in the CIE 1931 chromaticity diagram. (c) Comparison of transmission dip positions obtained by simulation (red dots) and measurement (blue dots). (d) Transmission-mode optical microscope images of the 40 μm × 40 μm sized filters, illuminated with unpolarized white light, showing bright colors with a high contrast from the respective filters, as the period of the NP array changes from 260 to 440 nm, in steps of 20 nm.

shown in Figure 2b,i and ii, respectively. The chromaticity coordinates for the simulated and measured spectra display good agreement with each other. In view of the variation in the chromaticity coordinates according to the periodicity of the Al NP array, the color emerging from the filters can obviously be customized by tuning the period, so as to acquire a palette of colors. Slight discrepancies in the coordinates obtained from the simulated and measured transmission spectra are imputed to the difference in absolute values of the transmission spectra arising from nonparallel incident light used for the spectral measurement, the surface roughness, and any nanofabrication defects, which were not fully reflected for simulations. One issue of concern may be the angular performance of the filters, which are elaborated in the Supporting Information (Figure S1). Figure 2c plots the locations of transmission dips estimated from the simulation (red dots) and measurement (blue dots) as the period of the NP array is varied from 260 to 440 nm, in steps of 20 nm, which exhibit appreciable correlation between each other. Figure 2d reveals the transmission-mode optical microscope images of the color generated by the fabricated devices, with different NP periods ranging from 260 to 440 nm in steps of 20 nm, when illuminated by unpolarized light. A wide variety of bright colors with a high contrast was realized in terms of the periodicity of the NP array, which is highly desirable for the extension of the operational range of conventional color filters, and the applications in display and multispectral imaging.18 It is remarked that the suppression of transmission around the resonance wavelength is slightly asymmetric, which might affect the purity of the colors. For instance, pure yellow color could not be achieved as indicated by the position of the chromaticity coordinates in Figure 2b. The purity of colors is presumed to improve with more symmetric resonance behavior.

conditions was considered so as to model an infinite array of NPs in the simulations, where a nonuniform mesh refined with a conformal mesh technique was used to mitigate field singularities at sharp edges and corners.27,28 The smallest mesh size in the calculation domain was as small as 4 nm. Simulations were carried out for various thicknesses and duty ratios of the Al NP. It was proved that the Al NP should be sufficiently thicker compared with its skin depth, in order to produce desired colored outputs by ensuring a highly suppressed transmission at resonance wavelengths. However, the suppression obtained from relatively thin Al NPs, the thickness of which is comparable to the skin depth, was not satisfactory. A thickness of 40 nm, which exceeds triple the skin depth of Al in the visible regime, was hence chosen with a duty ratio of 0.5, considering that the transmission is selectively suppressed at resonant wavelengths as intended yet guaranteeing high transmission efficiency at spectral bands away from the resonant wavelength. Figure 2a,i shows the calculated transmission spectra for an Al NP array, with the pitch ranging from 260 to 440 nm in increments of 20 nm, with the duty ratio fixed at 0.5. The optical response manifested a substantially lowered transmission at particular wavelengths according to the periodicity of the NP array, following a trend indicated by a dashed line, where the position of the diminished transmission shifts from λ = 432 to 675 nm in an approximately linear manner, with the array pitch varying from 260 to 440 nm. Figure 2a,ii presents the measured transmission characteristics, which reveal distinct prohibited transmission along a dashed trend line, which closely resembles that of the simulated results. On the basis of the simulated and measured transmission spectra of the filters as shown in Figure 2a, the corresponding chromaticity coordinates were calculated by using standard equations29,30 and plotted in a standard CIE (International Commission on Illumination) 1931 chromaticity diagram, as C

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Figure 3. (a) SEM image of a fabricated logo, with a footprint of 70 μm × 70 μm. (b) Transmission-mode optical microscope image of the logo, illuminated with unpolarized white light, exhibiting bright colors with a high contrast.

Aside from the realization of color filtering in square-shaped filters, we also endeavored to inspect the feasibility of the proposed technique for digital display/imaging of arbitrary colored patterns. A 70 μm × 70 μm sized logo with an ancient symbol “Swastika” was devised by incorporating a group of Al NPs with different periods of 400, 340, and 260 nm for different constituent parts, playing the role of creating the CMY colors. The SEM image of the completed logo is included in Figure 3a, where the inset shows a magnified view of a portion of the logo to illustrate the array of basic NPs. Figure 3b depicts the transmission-mode optical microscope image of the logo, shone with unpolarized white light, delivering bright colors with high contrast. The monotonically colored regions are visually uniform, thanks to the high fidelity of device fabrication to the design. Bright and distinct colors were observed, even at the site of sharp corners and edges of the pattern, supporting that the current color filtering scheme can be readily applied to ultrahigh resolution color display. Albeit the Al film is opaque, the physical thickness of the plasmonic color filters is comparable to that of the conventional color filters that resort to ultrathin metallic nanostructures.18 The proposed filters offer prominent features including simpler design and thinner thickness, as compared to other multilayered designs, such as metal−insulator−metal stacks.10,21,35,36 By exploring the optical properties of the structure with the help of the FDTD method based simulations, we elucidate the core physical mechanism underlying the selective spectral dips in transmission. As shown in Figure 4a, the contour map associated with the calculated transmission response, which is normalized to the area of a unit cell less the area of a single Al NP, is plotted in terms of the period of the NP array. The reflection and absorption spectra, which are similarly normalized to the NP area within a unit cell, are also shown in Figure 4b and c, respectively. As observed in Figure 4a, the location of the transmission dips, as implied by the solid white curve, shifts linearly with the period. The reflection and absorption are observed to be enhanced near the transmission dip. To scrutinize the relative contributions made by different phenomena pertaining to different metal−dielectric interfaces, such as PSPR, LSPR, and the Wood’s anomaly (WA), we overlay the respective curves obtained from the corresponding analytical equations or FDTD simulations as presented in Figure 4a through c. The analytical dispersion curves in connection with the PSP sustained at the metal−dielectric boundary are given by31

λPSP =

P 2

i +j

2

εm(λ)εd εm(λ) + εd

(1)

where εd and εm(λ) are the dielectric constants of the dielectric and metal layers, respectively, and i and j are integers signifying the order of resonance. The contribution of an LSP mode is estimated by calculating the spectral position of LSPRs for a single piece of Al NP constituting the array. These are represented by the solid red lines in Figure 4a−c. The simulated transmission dips and absorption/reflection peaks, which vary continuously from λ = 432 to 685 nm when the period increases from 260 to 440 nm, are positioned in between the red curve for the LSPR and the black curve for the PSPR. It is hence implied that both the LSP and PSP modes act collectively to inhibit the transmission around specific wavelengths. The narrow transmission peaks at shorter wavelengths, as observed in the FDTD based simulation results, are accounted for by the WA for the Al−glass interface (dashed pink line) and air−Al interface (dashed white lines) in Figure 4a−c, as in the case of diffraction gratings. Such WA is known to transpire when the diffraction order becomes tangent to the plane of the grating, as dictated by the equation:32−34 λWA =

P 2

i + j2

εd (2)

In addition, the PSPR for the air−Al interface (dashed black curve) was verified to assume a local minimum in the transmission spectra at shorter wavelengths, because of local enhancement in the absorption spectra. In an effort to prove the excitation of SPs responsible for the transmission dips, we then examined the electric-field intensity distribution through a vertical cross-section of an Al NP in an array with a period of P = 400 nm, formed on a glass substrate. It is deduced from Figure 4a−c that, for P = 400 nm, the Al− glass PSPR, as governed by eq 1, is found at 600 nm, the transmission dip is at 630 nm, and the LSPR predicted by FDTD simulations is at 690 nm. Figures 4d through f plot the 2D color map of the electric field intensity (E2) normalized to the incident electric field intensity (Einc2) at those wavelengths of 600, 630, and 690 nm. The electromagnetic field are observed to be remarkably reinforced and tightly confined at the Al−glass interface, thus indicating the occurrence of PSPR.8 Such field enhancement is attributed to the polarization of charges induced at the Al−glass interface by the electric field of an incoming radiation.38 The electric-field intensity distribution remains nearly invariant within the relatively broad spectral D

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Figure 4. Contour maps of the calculated (a) transmission spectra that are normalized to the area of the unit cell less the area of a metal NP, (b) reflection, and (c) absorption spectra, normalized to the area of an Al NP in the unit cell. Overlaid over the contour maps are the curves obtained from the corresponding analytical equations or FDTD simulations for the PSPR, LSPR, and WA. 2D color maps of the electric field intensity normalized to the incident electric field intensity (Einc) through a vertical cross-section of an Al NP array with a 400 nm period and a glass substrate at (d) λ = 600, (e) λ = 630, and (f) λ = 690 nm, which correspond to the PSPR, transmission dip, and LSPR, respectively.

As observed in Figure 4b, the reflection is selectively heightened at the positions of transmission dips. Thus, we have investigated the potential use of the designed color filters in the reflection mode, in light of their applications to color printing and reflective displays.35,36 A palette of reflective colors could be obtained for an NP array with different periods, and a reflection-mode high quality image of the aforementioned logo in response to incident unpolarized white light is also included in the Supporting Information, with more details (Figures S3 and S4). Taking into account that the demonstrated color palette available from the proposed filters is limited as compared to the previous reports, we may attempt different

region around the transmission minimum, including the 690 nm wavelength in relation to the LSPR, as anticipated by the FDTD simulation for a single NP. Accordingly, for an NP array of opaque Al films with a duty ratio of 0.5, the resonance modes exhibit the features of a hybrid of LSP and PSP modes. Though we discussed the situation for a typical NP array with P = 400 nm, similar observations could be made for other cases of NP arrays with different periods. Moreover, in a bid to identify the direction of resonant power flow, we monitored the magnitude of the Poynting vector along the z-direction, |Sz|, concluding that light is mostly reflected in the backward direction by the structure at λ = 600, 630, and 690 nm, as indicated in Figure S2. E

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angular performance of the filters, simulated direction of power flow around the transmission dip, color filtering in reflection mode and applications in color filtering, and a comparison of the performance between the devices made of Ag NPs, alongside related discussions and illustrative figures. This material is available free of charge via the Internet at http:// pubs.acs.org.

strategies, such as color mixing, to expand the palette.21,35 Our devices that simply entail one-step patterning of a single layer of physically thin Al film are however perceived to offer conspicuous advantages in terms of device fabrication, requiring no multiple layers of different materials. The use of Al in our devices overcomes the aforementioned shortcomings associated with the previous scheme based on Au.36 It should be noted that the proposed device featured the coloring operation both in transmission and reflection modes, unlike most of the conventional approaches, which were either focused on the reflection or transmission mode of operation. In summary, Al plasmonics based SCFs, exhibiting polarization-independent operation and high transmission reaching ∼75%, have been demonstrated that take advantage of a square array of optically thick Al NPs on a glass substrate, enabled by SP-induced suppressed transmission. By virtue of rigorous analysis, the selective spectral response was discovered to be stemming from the combined effect of the PSPR sustained at the Al−glass interface and the nonpropagating LSPR associated with an Al NP. The transmission dip concurred with the enhanced reflection. Both a colored logo and a palette of colors were successfully generated by the filters in both transmission and reflection modes. The proposed filter permitted a complementary operation, where the color available from the transmission and reflection modes originates from the subtractive and additive color filtering schemes, respectively. Thanks to its simple structure and efficient color tuning based on the periodicity of the Al NP array, the proposed color filter will be highly attractive for diverse applications, covering display, imaging, and color printing. The current work, which is chiefly concerned with polarization-independent plasmonic SCFs capitalizing on a 2D Al NP array, may be readily modified to embody a 1D metal grating structure, which consists of periodic air slots in a metal film,18,26 leading to both filtering and polarizing simultaneously, as desired by certain display applications.37 Although we have primarily resorted to the use of square NPs for the convenience of fabrication according to our available process conditions, the work may be readily attempted using nanodisks to achieve comparable performance. Aside from a square array as used here, a triangular array may be used for the NPs or nanodisks, giving rise to performance comparable to that reported in this work. It should be pointed out that a protective polymer layer may be coated over the NPs, causing a red shift in the resonance wavelength by a few tens of nanometers. This was confirmed via simulations for a 200 nm thick coating layer with a refractive index of ∼1.5. If the coating-induced red shift in the resonance wavelength is taken into account during the design, a resonance will occur at desired wavelengths by use of an NP array of shorter pitches than the pitches corresponding to the cases involving no protective layer. Finally, taking into consideration the aforementioned distinctive features rendered by the use of Al in plasmonic applications, and the capability of the proposed devices in accomplishing the high performance equivalent to the case of using noble metals, such as Ag as indicated in Figure S4, the proposed devices are assured of opening an avenue toward further research and development of high-quality plasmonic subtractive/additive color filters in Al.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

V.R.S. performed the design, optical characterization, and analysis of the device and wrote the manuscript; S.-S.L. supervised the analysis and cowrote the manuscript; E.-S.K. advised and supported in preparing the manuscript; D.-Y.C. fabricated the device and took the SEM images as well as the colored optical images. All authors discussed the results and implications and commented on the manuscript at all stages. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2013-067321, 2013-008672), and a research grant from Kwangwoon University in 2013. The work was partly supported by the Australian Research Council Future Fellowship (FT110100853, Dr. Duk-Yong Choi) and performed in part at the ACT node of the Australian National Fabrication Facility. The authors are grateful to Ms. Jun Cheng at Australian National University for her valuable support in fabrication.

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ABBREVIATIONS SP, surface plasmons; PSP, propagating surface plasmon; LSP, localized surface plasmon; PSPR, propagating surface plasmon resonance; LSP, localized surface plasmon resonance; EOT, extraordinary optical transmission; ELT, extraordinary low transmission; SCF, subtractive color filter; Ag, silver; Au, gold; CMOS, complementary metal−oxide−semiconductor; Al, aluminum; NP, nanopatch; CMY, cyan, magenta, and yellow; SEM, scanning electron microscope; FDTD, finite difference time domain; CIE, International Commission on Illumination; WA, Wood’s anomaly

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ASSOCIATED CONTENT

S Supporting Information *

Additional information including details on the experimental section including device fabrication and optical characterization, F

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