Electrical Broad Tuning of Plasmonic Color Filter Employing an

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Electrical broad tuning of plasmonic color filter employing an asymmetriclattice nanohole array of metasurface controlled by polarization rotator Youngjin Lee, Min-Kyu Park, Seunguk Kim, Jeong Hee Shin, Cheil Moon, Jae Youn Hwang, Jun-Chan Choi, Heewon Park, Hak-Rin Kim, and Jae Eun Jang ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00249 • Publication Date (Web): 04 Jul 2017 Downloaded from http://pubs.acs.org on July 5, 2017

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Electrical broad tuning of plasmonic color filter employing an asymmetric-lattice nanohole array of metasurface controlled by polarization rotator Youngjin Lee,†,§ Min-Kyu Park,‡,§ Seunguk Kim,† Jeong Hee Shin,† Cheil Moon,∥ Jae Yun Hwang,† Jun-Chan Choi,‡ Heewon Park,‡ Hak-Rin Kim,*,‡ Jae Eun Jang*,† †

Department of Information and Communication Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Republic of Korea ‡ School of Electronics Engineering, Kyungpook National University, Daegu 41566, Republic of Korea ∥

Department of Brain and Cognitive Sciences Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Republic of Korea ABSTRACT: Wide range of color change in nanohole array structure on a metal film have been successfully demonstrated using asymmetric-lattice design of nanoholes and an electrically switching polarization rotator. Recently, some studies have been reported that various color states were obtained in a single unit cell structure using extraordinary optical transmission (EOT) of nano-patterned structure, which could be one of the most important solutions for achieving ultrahigh integration density in optoelectronic devices. However, because they used the interfacial refractive index or dielectric constant as controlling factors for the color tuning, they were not capable of inducing a changeable range of color with different primary color states. To overcome this limitation, in this study, an asymmetric-lattice nanohole array design was integrated with an electrically controlled polarization rotator, employing a twisted nematic (TN) liquid crystal (LC). This simple structure of nanohole arrays with a rectangular lattice enabled mixed color states as well as precisely designed two different primary colors, by modulating the polarization of the incident light. The color-tuning shift was greater than 120 nm. Since the surface plasmonic (SP) modes on both side, a top and a bottom interface, were matched better by the TN-LC layer assembled on the rectangular-lattice nanohole metal layer, the transmittance at the resonance peak wavelength was increased by 158% compared to that of the bare nanohole structure. The nanohole-array-on-metal-film simultaneously functions as an electrode, and this advantage, coupled with the low driving voltage of the TN-LC layer, can open new possibilities in applications to various optoelectronic device concepts. KEYWORDS: nanohole array, plasmonic resonance, asymmetric lattice, tunable color filter, liquid crystal

One

of the most interesting research topics in optoelectronic devices in recent times has been the effort to achieve

multicolor states in a single unit structure. This particular achievement would enable the further development of ultrahigh resolution and natural colors in display devices, or photoelectric sensors such as complementary metal–oxide– semiconductor (CMOS) image sensors. Cholesteric liquid crystal (ChLC) stack structures and photonic crystals have been investigated for this purpose.1–5 Unfortunately, the complicated device structure of the ChLC devices and the difficult fabrication method of three-dimensional (3D) photonic crystals, which is based on the “bottom-up” style nanotechnology, have limited its realization. As an alternative, a structural color filter based on the plasmonic effect in metal films can be an excellent solution for producing multicolor states in a unit cell, because of the existence of surface plasmon resonance, 6–10 which can induce extraordinary optical transmission (EOT) phenomena and color filtering effect. Nanohole arrays have been studied to achieve the color-filtering effect in several types of plasmonic structures, depending on the geometry of the metal nanohole arrays, because the structure can selectively filter and pass specific wavelengths. The resonance property can be changed by adjusting some of the geometrical factors of the metal nanohole-array structure, such as the thickness of the metal film, the shape of the holes, and the spacing between the

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holes. In this way, various selective color-filtering effects can be realized.11–19 However, when the structural factors are fixed, only one specific color is selected as the primary transmitted light. This static structure makes it difficult to obtain simply a multicolor function. Two methods can be considered to impart a dynamic characteristic to the color-filtering effect of the nanohole array structures. One is to use special materials with variable dielectric constants or refractive indices, and utilize the dielectric constant sensitivity of the plasmonic structures.20–29 This is possible because the plasmonic phenomenon is affected by the optical properties of the materials surrounding the nanohole array structure. Since liquid crystals (LCs) has the optically anisotropic properties, a combination of LCs and the metallic nanohole array can produce different color states depending on the optoelectrical state. However, typically, the range of changeable color is not enough to create two different primary-color states within the visible light range, for example, from red to green or green to blue (Figure S1, Supporting Information, SI). Even when some LCs, which have a relatively higher anisotropy in their refractive index (∆n ~ 0.4), have induced a meaningful color shift, the resonance wavelength shift was still less than 100 nm.23 In addition, it is not easy to apply these specific types of LCs to various devices, because of high driving voltage or poor material stability. Furthermore, to align the LCs precisely, an additional LC alignment layer is generally necessary on the nano-patterned plasmonic metal surface, which prevents the variation of the plasmonic interaction between the metallic nano-patterns and the refractive-index-changing LC. The other method is to utilize the polarization dependence of asymmetric plasmonic nanostructures. If the plasmonic nanostructures are fabricated with an asymmetric design in certain directions, in terms of the periodicity of the pattern or the pattern shape, different color resonance modes can be produced in a specific direction.30 Then, the mode can be selected by using the polarization state of the incident light. Some special device designs have been reported that can induce different primary-color states by changing the polarization states of the incident light.31–34 However, most of the results published so far have demonstrated color changes by mechanically rotating the polarizer, or by adopting an additional polarization controller as a separate optical layer. Therefore, these concepts are not appropriate for the various optoelectronic devices that require thin and compact structures, even if different color states can be easily achieved. To address these shortcomings, this work proposes an asymmetric-lattice nanohole array design with an electrically switched polarization rotator function, which employs a twisted nematic (TN) LC mode. To produce widely separated different primary-color states, simple rectangular-lattice nanohole arrays with different structural design factors corresponding to the x- and y-directional geometries fabricated on an aluminum (Al) film and changes in the optical characteristics of those designs were studied. By integrating the TN LC modes with the asymmetric-lattice nanohole array structure, we were able to tune electrically the polarization state of the incident light. The asymmetric-lattice nanohole array design enabled two different primary colors and their mixed states in one fixed design with modulating the polarization of the incident light electrically. The color-tuning shift was greater than 120 nm, depending on the design layout of the nanohole array. The electrically controlled polarization state enabled the precise analysis and prediction of the continuously tunable EOT phenomenon. The metallic layer with the nanohole array structure was used as the electrode of the electrical polarization rotator and it gave an important merit to the structure of device as it brought more simplification to structural design. Additionally, in our integrated and tunable color-filter structure, the interfacial index mismatching in the plasmonic effects was effectively minimized, thanks to an LC alignment layer formed between the Al film with the nanohole array and the LC layer. ■ RESULTS AND DISCUSSION Figure 1a shows a schematic diagram of the proposed tunable color filter device, which consists of an asymmetriclattice metal nanohole-array film combined with a TN-LC layer. The metallic nanohole-array film was utilized as the bottom electrode of the TN-LC cell and to perform the color tunable switching. An indium-tin-oxide (ITO)-coated glass substrate was used as the other electrode of the LC cell. LC alignment layers were formed on both substrates: the rubbing directions for both substrates were mutually orthogonal to achieve the TN-LC configuration. The LC layer was formed between both substrates with a thickness of 5 µm, with the use of silica spacers. In the proposed tunable colorfilter device, to minimize the chromatic aberration of the LCs’ phase retardation, a TN-LC mode was employed. The LCs’ configuration can be changed by varying the applied voltages between the two substrates. The initial state is a TN state, and the electrically saturated state is a vertical alignment (VA) state. In the TN state, linearly y-polarized incident light from the polarizer is converted to x-polarized light by an anisotropic phase delay and the rotating director axis of the TN state. In the VA state, the linearly y-polarized incident light passes through the VA LC layer along the optic axis of the LC layer, and the polarization direction of the light is not changed. Incident white light, whose polarization direction is controlled by the TN-LC layer, is coupled with the rectangular-lattice metal nanohole-array. Therefore, the polarization dependence of the surface plasmonic resonance occurs due to the effect of the asymmetric geometry of the metal nanohole-array, resulting in two primary colors. In the rectangular-lattice nanohole array, because the lattice constant in the x-direction is larger than that in the y-direction, the resonance wavelength of the x-polarized light is larger than that of the y-polarized light, as shown in Figures 1b–c. Consequently, by controlling the applied field of the TN-LC layer in the tunable color filter with an asymmetric-lattice, two primary colors can be selected and their mixed states can


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Figure 1. (a) Schematic representation of the electrically tunable color filter, where a TN-LC layer is integrated to control the polarization of the incident light on the surface of an asymmetric-lattice metal nanohole array, as an in-cell type device. For incident white light, (b) a longer and (c) a shorter wavelength beam exhibits stronger plasmonic resonance with two primary colors, when the polarization of the incident light is parallel to the longer and the shorter periodic nanohole array direction, respectively, in the rectangular-lattice plasmonic metal surface. be continuously produced, as well as intermediate field levels. Before integrating the plasmonic surface and the TN-LC layer for the in-cell structure of the tunable color filter, the optical characteristics of the plasmonic surface with the nanohole array were studied. The arrangement of the nanohole arrays is in the form of rectangular lattices with individual periods for each x-direction (Px) and y-direction (Py). The plasmonic surface with the rectangular-lattice nanohole arrays produces two different primary color states, depending on the polarization direction of the incident light. To prepare the plasmonic surface with nanohole arrays, electron beam lithography (EBL) was conducted on the Al surface on a glass substrate. The details of the EBL process were described in the methods section and Figure S2 in the SI section. Figure 2a shows one example of the asymmetric nanohole arrays structures (Px = 350 nm and Py = 200 nm) made on an Al film. The diameters of the nanoholes (D) are set to half of the period of the nanohole array along the y-direction, and the thickness of the Al film is 70 nm. To study the dependence of color selectivity on the periodicity of the nanohole array, we analyzed the optical characteristics of the symmetric square-lattice nanohole arrays with different array periods. Figure 2b shows the color images and transmission spectra of the plasmonic surfaces with the square-lattice nanohole arrays for various periods, from 200 nm to 400 nm, with an interval of 25 nm. By increasing the period of the nanohole arrays, the wavelengths of the surface plasmonic resonance were shifted from the blue to the red region. These results are consistent with previous reports11. To explain this change of spectra, localized surface plasmon (LSP) and surface plasmon polaritons (SPPs) should be considered with structural conditions such as hole size, hole shape, film thickness, or dielectric environment. In here, hybrid mode of LSP and SPPs is main working principle for various color filtering results. To investigate the effect of the asymmetric-lattice arrangement of the nanohole array on the polarizationdependent resonance, we simulated the plasmonic resonance field distributions at the glass-metal interface for two orthogonally polarized beams using finite-difference time-domain (FDTD) simulation with the rectangular-lattice nanohole array, where Px and Py are 325 nm and 225 nm, respectively, as shown in Figures 2c–d. From the results, the strong resonant fields were found both on localized near the hole edges and along the ridges of the holes corresponding to a SPP wave, depending on the polarization direction of the incident light at different wavelengths. The simulation results means that EOT spectra results are supported by hybrid mode combined with SPPs and LSP mode35. In the case of y-polarization (Figure 2d), which is coupled to the y-directional hole geometry with Py, a shorter wavelength (λ= 419 nm) produces strong resonant fields, whereas, in the case of x-polarization (Figure 2c), which is coupled to the ydirectional hole geometry with Px, a relatively longer wavelength (λ= 565 nm) produces strong resonant fields (See Figure S3-1 in SI). The resonant intensities shown in Figure 2d are larger than those (Figure 2c) obtained in the xpolarization case, because the hole diameter is more optimized to y-direction geometry. The larger hole size can also induce the broad transmission spectrum. Thus, it makes poorer color purity contrary to higher transmittance. Therefore, the hole size should be selected considering various factors of application. However, it is clear that the relation between the period of the nanohole arrays and the main wavelength of the filtered light transmission is almost linear proportional and theoretical calculation is also supporting this tendency. 11 Even though this simple relation between the structural factors of nanohole array and a main filtered color can be enough to design some applications, the discussion of the


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Figure 2. (a) Scanning electron microscope image of the plasmonic surface with the rectangular-lattice nanohole arrays. (b) Optical photographs and transmission spectra of the square-lattice nanohole array structures, measured for unpolarized incident white light. (c–d) Results of FDTD simulation of the plasmonic resonance field distributions for the asymmetric-lattice nanohole arrays (Px = 325 nm and Py = 225 nm) obtained under an air interface condition, where the incident beam is (c) the x-polarized light (λ=565 nm) and (d) the y-polarized light (λ=419 nm). (e–f) Optical photographs of the rectangular-lattice nanohole arrays prepared with different Px and Py conditions, where the incident conditions are (e) x-polarized (φ = 0o) and (f) y-polarized (φ =90o). (g) Optical photographs and transmission spectra, measured at two orthogonal incident polarization conditions for the plasmonic metal surface with the rectangular-lattice nanohole arrays (Px = 325 nm and Py = 225 nm). details in EOT spectra is also important to optimize the system. The results of the maximum transmissions at λpeak are not high, especially in the longer wavelength region due to the cut-off function of the subwavelength apertures in the nanohole array.36,37 In general SPPs mode, two sets of resonance peaks are obtained in the transmission spectra, which are supported by the surface plasmonic resonances of both the top and the bottom interface layers. To achieve a high transmission at λpeak or a high purity color, the difference in the dielectric constants of the top and the bottom interface layers needs to be minimized to be matched the different surface plasmonic (SP) modes38. In this case, the top and bottom interface layers are air and glass. In the proposed electrical tunable color filter structure of Figure 1a, the maximum transmission at λpeak was improved after coating with the LC alignment layer, which reduced the difference in the interfacial dielectric constants compared to the air condition. The details will be discussed further in the results of Figure 3. In addition, the minimum states of EOT spectrum is not deeper and the transmission spectra is a little broader than other reported results39. Main reason are Al film thickness and hole size (Figure S4 in SI). For the thin film condition, the material and the structural condition of sample greatly affects the intensity and spectral location of the transmission resonance, which can appear strongly shifted from cut-off condition depending on refractive index of substrate38,40. The resonance wavelength at metal-air and at metal-glass interface is different due to the difference of the dielectric material. In this case, the deep local minimum is shown between the resonance wavelength at metal-air and metal-glass interface. With reducing film thickness, these two modes can be interacted and substantially coupled38, and then, it can induce shallow minimum states and spectra broadening. Also, in our structure design, since LSP mode and


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SPPs mode coexist, dip local minimum is not clearly seen36. Therefore, the minimum was much clearer for 150 nm thickness samples. However, since the total level of transmission was also decreased with increasing film thickness, 70nm film condition was selected for this experiment considering applications such as display device or image sensor in which transmittance is one of the most important factors for evaluation. In addition, because of process condition for ebeam lithography and dry etching process, a nearby hole edge was thinner than that of original thickness. Therefore, the background produced by light leakage of unexpected modes induced the broadening effect of spectra with hole size effect, as well. Figures 2e–f show the experimental results of the color-filtering effect of the plasmonic surface with the various rectangular-lattice nanohole array designs. Here, the periods of the nanohole array were changed from 200 nm to 400 nm with an interval of 25 nm for the x- and y-directions. The diameters of the holes were set to half of the periods of the ydirectional geometry. In the case of the x-polarization (φ = 0°), the selected colors was affected by Px, while in the case of the y-polarization (φ = 90°), the selected colors was affected by Py. Because of the asymmetry effect of the rectangular lattice design, two different colors were obtained in each unit structure by selecting an incident polarization between 0° and 90°. Figure 2g shows the typical results for a specific nanohole array (Px = 325 nm and Py = 225 nm). It shows the two different polarization-dependent color states, blue and green color; the value of the peak shift is about 134 nm. To interpret the effect of the polarization-dependent resonances on the asymmetric-lattice nanohole array structure, the different lattice vectors of the asymmetric-lattice nanohole array in the x- and y-directions should be considered because different peak wavelengths in the rectangular-lattice nanohole array structure are obtained based on the polarization direction of the incident light. In the case of the x- and y-polarized light, the plasmonic resonance is strongly affected and decided by Px and Py respectively. Consequently, by simply changing the polarization state of the incident light, different color states can be produced by the rectangular-lattice nanohole array design. To integrate the plasmonic surface with the asymmetric-lattice nanohole array and the TN-LC layer for the in-cell type of the tunable color filter, a rubbed-polyimide (PI)-type LC alignment layer (SE-5811, Nissan chemical) was formed on the plasmonic surface using spin coating and annealing, to form the easy-axis of the surface LC directors. Because of the different interfacial conditions imposed by the LC alignment layer, whose refractive index and extinction coefficient are shown in Figure S5 in SI, some optical characteristics of the plasmonic surface with the nanohole array were changed. To conduct a comparison with the air conditions of the plasmonic surface, a plasmonic surface with a square-lattice nanohole array, the same conditions as those shown in Figure 2b, was prepared after coating with the LC alignment PI layer. As shown in Figure 3a, due to the changing optical characteristics of the surrounding material from the air to the LC alignment layer on the Al surface with a nanohole array, the plasmonic resonance wavelength is shifted to a longer wavelength, even though the geometric designs of the nanohole array are unchanged. The results of Figure 3a shows that the relationship between the nanohole array period and the surface plasmonic resonance is changed as λpeak, x = λpeak, y ≈2.27P to the surface plasmon modes of x- and y-direction for the square-lattice nanohole array coated with the LC alignment layer. The scaling constant related to the dielectric constant of the interface layer is increased, compared to the air boundary condition. Because of the higher refractive index of the PI alignment layer, all resonance peaks are shifted to a longer wavelength. Therefore, for the in-cell structure of the tunable color filter integrated with the plasmonic surface and the TN-LC layer, to get a blue color, a smaller period nanohole array is required (below 200 nm). As mentioned previously, if nanohole arrays sandwiched between two different dielectric media (air and glass) in which a SP mode can be tuned into resonance on either the one or the other interface, but not on both at the same time39. Therefore, the transmission can be enhanced when the SP mode on both side are matched.34,41 Figure 3c-e shows numerical simulation results of the transmission spectrum and the resonance field distribution depending on the thickness of the PI layer. In case of bare nanohole array structure (Px = 325 nm and Py = 225 nm) , the (±1, 0) resonance wavelength at metal-air interface (λair) is 440 nm and the (±1, 0) resonance wavelength at metal-glass interface (λglass) is 545 nm, which are similar to the experimental data. However, the intensity of λair in experimental sample is a little smaller than the intensity of λglass, whereas, these two peak intensities shows the reverse result for 150 nm film thickness as like the simulation (Fig S4 c in SI), because of the mutual coupling effect of SP modes by thin film thickness and the hole size effect (Figure S4 d-f in SI). When the PI layer is coated on the nanohole array structure with thickness of 30 nm (Figure 3d), all resonance peak was red-shifted. Additionally, as the air interface changes to the PI interface, resonance peak at metal-PI interface (λPI) was further red-shifted, resulting in a superposition effect between λglass and λPI. When the PI layer is coated on the nanohole array structure with thickness of 100 nm as shown in Figure 3e, λglass and λPI became almost similar and the transmittance increased by twice compared to that of the bare nanohole array structure40,41. In real experiment of matching state using water and SiO2, the total transmittance was much enhanced by totally matching state in SiO2 coating case (Figure S6 in SI). After coating the PI layer for LC alignment, the difference in refractive index between the PI and glass decreases to 0.11, as compared with 0.51 between glass and air. For the squarelattice nanohole array with a period of 225 nm, after coating the PI layer for LC alignment, the resonance wavelength was shifted from 425 nm to 512 nm and the transmittance at λmax was enhanced to 120.8 % (Figure 3b). In comparing


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Figure 3. (a) Optical photographs and transmission spectra of the square-lattice nanohole array structures coated with an LC alignment layer, measured for unpolarized incident white light. (b) Transmission spectra of the square-lattice nanohole array (Px = Py = 225 nm) and its optical photographs, measured before and after coating the nanohole-arrayed metal surface with the LC alignment layer. (c-e) FDTD simulation results of transmission spectra and resonance field distributions for the nanohole array (Px = 325 nm and Py = 225 nm): (c) without a PI layer, (d) with a 30 nm PI layer, and (e) with a 100 nm PI layer, when the x-polarized light is incident. (f-g) FDTD simulation results of the plasmonic resonance field distributions for the asymmetric-lattice nanohole arrays (Px = 325 nm and Py = 225 nm) obtained with the LC alignment layer as the interface, where the incident beam is (f) the x-polarized light (λ = 625 nm) and (g) the ypolarized light (λ=512 nm).


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Figure 4. (a–b) Optical photographs of the rectangular-lattice nanohole arrays prepared with different Px and Py conditions, which were measured after coating with the LC alignment layer, for the incident conditions of (a) the xpolarized (φ = 0o) and (b) y-polarized (φ =90o) light. (c-d) Polarization-dependent transmission spectra and their optical photographs, measured for the rectangular-lattice nanohole arrays after coating the LC alignment layer, where the period conditions of the nanohole arrays are (c) Px = 325 nm and Py = 225 nm and (d) Px = 250 nm and Py = 200 nm. the conditions of the P = 325 nm without the LC alignment layer and the P = 250 nm with the LC alignment layer, it was found that both conditions resulted in the same resonance peak wavelength of λmax = 565 nm. However, the maximum transmittance at λmax was greatly enhanced, to 165.2 %, after the PI layer was coated for LC alignment. Figures 3f–g show the plasmonic resonance field distributions for two orthogonally polarized beams, based on the FDTD simulation of the rectangular-lattice nanohole array with LC alignment layer. The selected period conditions of Px and Py are the same as those for the structure shown in Figures 2c–d. Because of the differences between the x- and ydirection lattice constants of the nanohole array, the resonance peak wavelengths are changed depending on the polarization direction of the incident light. The resonance peak wavelengths for the x- and y-polarized lights were 625 nm and 512 nm, respectively (See Figure S7 in SI). In case of the PI-coated nanohole array, it seems that LSP mode would be predominant than SPPs mode due to the index matching condition and thin metal thickness as shown in Figure 3f-g, where the strong resonant fields were found on localized near the hole edges and weaken ridges of the holes18,35. But, in our PI-coated asymmetric-latticed nanohole array structures of which diameters are same but periods are different along to x- and y-direction, the period-dependent resonance peak shift is clearly observed according to the x-polarization and the y-polarization. The plasmonic color palettes (Figure 4a-b), transmission spectra (Figure 4c-d), and FDTD simulation results (Figure S3-2 in SI) prove the shift. Overall, the maximum transmission at λmax in the case of the xpolarization (φ = 0°) is lower than that of the y-polarization (φ = 90°), as expected the FDTD simulation. In the asymmetric-lattice nanohole array, if an elliptical hole design is applied, the maximum transmittance at λmax for the xpolarization can be also improved. In addition, the EOT resonance of the PI-coated sample is also supported by the hybrid mode, where the Bragg resonance is affected by the resonance by cut-off wavelength. Like the results in Figures 2c-d, the field intensity of the plasmonic resonance with the y-polarized light was larger than that with the x-polarized light, because the hole diameter was optimized to y-direction geometry. . The in-cell type electrically tunable color filter was prepared by integrating the rectangular-lattice metal nanohole array and the TN-LC mode for the polarization controller. The LC used was E7 (Merck). We measured refractive index and extinction coefficient of the PI layer depending on the wavelength of incident light to check the effect of PI to change of transmission spectra (Figure S5). To analyze the change in the polarization state after the application of the TN-LC mode in terms of the applied voltage, we analyzed the Stokes parameter and the LC director profile of the TNLC mode. Figure 5a shows the normalized transmittance of the TN-LC as a function of the applied voltage, measured under the crossed polarizer conditions. In the initial TN state, the polarization direction of the incident light, which is linearly polarized along the y-axis after the polarizer, is changed to the x-axis. Consequently, the incident light is transmitted without optical loss at the crossed polarizers. In the VA state induced by the applied electric field, the polarization direction of the incident light is not changed and the incident light is absorbed by the crossed polarizers. Under the crossed polarizers, the transmittance of the TN-LC sharply changed from Va = 1.5 V to Va = 2.1 V. The values of transmittance at Va = 1.69 V, 1.80 V, and 1.93 V were 75%, 50%, and 25%, respectively, where the lights were elliptically polarized after passing through the TN-LC layer at the intermediate voltages. To analyze the polarization state of the light after its passage through the TN LC layer, we measured the Stokes parameter of the light in terms of the applied voltages. The setup for the Stokes parameter measurement is shown in Figure S10 of the SI section.42,43 The Stokes parameter was obtained by measuring the voltage-dependent transmittance of the LC layer according to the transmission axis angle of the analyzer and the additional phase retardation induced by the quarter wave plate. The polarization states of the output light in terms of the applied voltage could be represented as the Stokes vector S(S1, S2, S3), and plotted on a Poincaré sphere, as shown in Figure 5b. In the initial voltage condition (Va = 0 V), the output light


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Figure 5. (a) Normalized transmittance of the TN-LC layer as a function of the applied voltage, measured under the crossed polarizers and the voltage-dependent polarization ellipse after the application of the TN-LC layer, by the Stokes parameter analysis. (b) Plot of the Stokes parameters of the voltage-dependent polarization states after the application of the TN-LC layer, described on Poincaré sphere. (c) S1 value, among the Stokes parameters, after the application of the TN-LC layer, as a function of the applied voltage. (d–e) Simulations of the electric field distributions (d) on the flat metal surface and (e) around the nanohole-arrayed metal structure. (f–g) Depth profiles of field-dependent LC directors from the bottom to the top alignment layers: (f) the azimuthal angle (φLC) and (g) the polar angle (θLC) of the LC director. incident on the asymmetric-lattice nanohole array is linearly polarized along the x-axis with S(1, 0, 0). In the saturated voltage condition (Va = 5 V), the output light after the TN-LC layer is linearly polarized along the y-axis with S(-1, 0, 0). In the intermediate voltage conditions, the polarization state described by the Stokes vector is not on the equator of the Poincaré sphere, and exhibits elliptical polarizations. Although the ellipticity angle of the elliptically polarized light increases and then decreases as the applied voltage increases, the rotation angle of the elliptically polarized light increases monotonically in the TN-LC structure. The values of the ellipticity angle and the rotation angle as functions of the applied voltages are represented in the table in Figure 5a. Thus, as the applied voltage reaches the saturated condition, the polarization state of the output light changes to the y-axis linear polarization. On the asymmetric nanohole-array structure, the resonance wavelength is determined according to the polarization direction of the incident light; where the x-axis and the y-axis polarized lights are coupled with free electrons affected by the x-direction and y-direction geometry of the nanohole array, respectively. In the case of elliptically polarized light, which is the output polarized light under the intermediate voltage condition, the transmission spectra are determined


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Figure 6. (a) Optical photographs and transmission spectra of the square-lattice nanohole array structures integrated with the TN-LC layer as an in-cell type device, obtained for unpolarized incident white light. (b–c) Optical photographs of the rectangular-lattice nanohole arrays prepared with different Px and Py conditions, which were measured after integrating the TN-LC layer for (b) the initial voltage (Va = 0 V) and (c) the saturated voltage (Va = 5 V) conditions. (d–f) Voltagedependent transmission spectra and their optical photographs, and the CIE chromaticity diagrams measured for the TNLC-integrated rectangular-lattice nanohole arrays with different asymmetric periodicity conditions: (d) Px = 250 nm and Py = 200 nm, (e) Px = 325 nm and Py = 200 nm, and (f) Px = 325 nm and Py = 225 nm. according to the ratio of the polarization field component along the x-direction and y-direction. The level of relative intensity between the x-polarized and y-polarized ones of a certain elliptic polarized light can be expressed using the value of the Stokes parameter S1. The Stokes parameter S1 of the output polarization state as a function of the voltage is plotted in Figure 5c. At the initial voltage condition (Va = 0 V), the value of the Stokes parameter S1 is 1, and it decreases monotonically as the voltage increases. Subsequently, the value of the Stokes parameter S1 reaches −1 at the saturation voltage condition (Va = 5 V). In the case of S1 = 0, the ratio of the polarization components in the x-direction and ydirection becomes 1. To utilize the metal nanohole array as the electrode of the LC layer, the distribution of the electric field near the metal


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surface with the nanohole structure was simulated using the finite element method, with COMSOL Multiphysics. As shown in Figures 5d–e, the electric field is applied vertically between the top flat ITO electrode and the bottom metal surface of the nanohole array structures. Because of the hole structure in the metal layer, the electric field distribution inside the hole is slightly deformed, as shown in Figure 5e. However, the fringe field due to the nanohole structure is localized at a depth below about 100 nm from the metal electrode surface. Considering the PI LC alignment layer (thickness ~ 100 nm) formed on the metal surface and the extrapolation length (~ 60 nm) of the LC anchoring by the LC alignment PI layer,44,45 the LC directors are not affected by the fringe field produced by the nanohole structure. The extrapolation length represents the depth of the surface LC directors anchored by the LC alignment layer.44,45 From the results, the metal nanohole array structure can act as both an electrode and a spectral filter in the proposed in-cell structure of the tunable color filter. As a result, in addition to inducing two primary color modes, the device structure can be further simplified. Figures 4f–g show the azimuthal angle (φLC) and the polar angle (θLC) of the LC directors as functions of the LC layer depth at different voltages (Va = 0, 1.3, 1.5, 1.9, 2.3, and 5 V), obtained using an LC simulator (Techwiz LCD 2D, Sanayi). The cell-gap is 5 µm, the azimuthal and polar anchoring energy is 10−3 N/m, and the applied LC parameters are ∆n = 0.22, ∆ε = 14.3, K1 = 11.2 pN, K2 = 5.5 pN, and K3 = 17.0 pN. As the voltage increases, the LC profile changes from the TN state to the VA state due to the electric field formed between the bottom nanohole-arrayed metal electrode and the top ITO electrode. As the voltage is applied, the azimuthal and polar angles of the LC directors are predominantly rotated over the bulk region rather than the surface region. This means that the voltage-dependent interfacial refractive index change is negligible for the spectral change in the plasmonic resonance. Figure 6a shows the transmission spectra of the integrated structure, the square-lattice metal nanohole array and the TN LC cell, at the initial state (Va = 0 V). Even though the TN LC layer was added, the resonance peak wavelengths and the maximum transmittances at λmax according to the period of the nanohole array were almost the same as the results in Figure 3a. It means that the plasmonic resonance condition of metal surface is not changed by LC material due to the LC alignment layer coated on the metal surface. Figure 6b–c show the plasmonic color palettes of the integrated structure of the rectangular-lattice nanohole array and the TN-LC cell, at the initial state (Va = 0 V) and at the electrically saturated state (Va = 5 V). In the initial state of the TN-LC cell, the output light was x-polarized, and in the saturated state of the TN LC cell, the output light was y-polarized. The results in Figures 6b–c are similar to those in Figures 4a-b. Therefore, two different primary color states were selected well by the electrically switchable polarization rotator. The integrated color filter of the rectangular-lattice nanohole array and the TN LC cell can show two primary colors, and it can produce various color combinations by transitions of the LC configuration between the TN state and the VA state, as well. To produce color variations from green to blue, red to blue, and red to green, we selected three periodic conditions of the rectangular-lattice nanohole arrays, Px = 250 nm and Py = 200 nm, Px = 325 nm and Py = 200 nm, and Px = 325 nm and Py = 225 nm, as shown in Figures 6d–f. In our active color filter device as shown in Figure 1, the Al nanohole array layer is used for LC switching electrode as well as the metallic plasmonic surface. Fortunately, owing to the planate PI layer and extrapolation length, there are no electrical and optical distortion at TN LC layer by the nanohole array structure, which is regardless of the period and diameter of the nanohole array structures. Therefore, the color changing speed for all of the geometric condition of the nanohole array is same, where the field-on, field-off, and total response times are 7 ms, 16 ms, and 23 ms, respectively. As like traditional LC technology, the response time can be more improved by the modification of cell structural design or driving scheme (Figure S8, SI). From the results, any color state in the CIE chromaticity diagram between two primary colors can be produced by controlling the applied voltages. If we make isolated two geometric pixel structures with different primary color two different, full-color states can be achieved, where the two selected color states are mixed at far-field. In a conventional display, two color dot structures are required to produce the similar two primary color results. However, even though the proposed structure is one unit cell, it can easily produce multicolor states. Therefore, employing this structure makes it possible to design a much higher resolution device. The operating voltage of this combination cell structure, which is under 5 V, is also an important merit for the various applications of optoelectrical devices. For the intermediate voltage condition of TN LC layer, the spectra of the tunable color filter can be precisely predicted by superposing the transmission spectra of the initial condition (T0Vint) and the saturated condition (T5Vsat) with the relative weighting factors, which are related to the Stokes parameter S1. S1 has a value between −1 and 1. When S1 = −1, only the polarization component in the y-direction exists, and when S1 = 1, only the polarization component in the xdirection exists. In addition, when the value of S1 is between −1 and 0, the polarization component in the y-direction is more dominant than that of the x-direction, and when the value of S1 is between 0 and 1, the polarization component in the x-direction is more dominant than that of the y-direction. When S1 = 0, the polarization components in the x- and ydirections are in the ratio 1: 1. The weighting factors of the polarization component in the x-direction and y-direction obtained by converting the S1 values between −1 and 1 to values between 0 and 1 are 0.5(1+ S1(V)) and 0.5(1− S1(V)), respectively. Using the information for the Stokes parameter S1, the transmission spectra for the intermediate voltage condition can be derived as


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Figure 7. Measured (Texp) and simulated (Tsim) voltage-dependent transmission spectra of the TN-LCintegrated rectangular-latticed nanohole array device, obtained for different asymmetric nanohole lattice periods: (a) Px = 250 nm and Py = 200 nm, (b) Px = 325 nm and Py = 200 nm, and (c) Px = 325 nm and Py = 225 nm. follows.

T ( S1 (V )) =

1 int T0V ( S1 (V ) + 1) − T5sat { V ( S1 (V ) − 1)} 2

(1)

where T0Vint and T5Vsat represent the measured transmission spectra at the initial (Va = 0 V) and saturation (Va = 5 V) conditions, respectively. Figures 7a–c are graphs co-plotting the measured transmission spectrum, along with the wavelength at the intermediate voltage conditions, for different nanohole array structures, and the estimated analytical results of the transmittance spectrum given by Eq. 1. It can be seen that the analytical results at the intermediate voltage conditions fit well with the experimental results. More comparison results can see in Figure S9 in SI Figures 8a–d show the electric field intensity distributions of the rectangular-lattice metal nanohole array with Px = 325 nm and Py = 225 nm in the z-axis direction, according to the TN and VA state, as obtained by FDTD simulation. For simplicity in this analysis, the weak surface anchoring condition was assumed for the LC configuration at the surface. Figures 8a–d show that the plasmonic resonance takes place within 100 nm of the metal surface for both LC geometries. These results were obtained by considering the LC layer on the PI-coated nanohole-arrayed metal surface. They imply that the effect of the refractive index change of the interfacial LC layer caused by the applied voltage on the plasmonic resonance, can be effectively neglected in our structure. This is due to the thickness of the LC alignment PI layer (thickness ~ 100 nm) and the extrapolation anchoring length (~ 60 nm) of the LC alignment. In Figures 8a–b, the incident y-polarized light is converted into x-polarized light by the TN-LC layer, and coupled with a nanohole array which has an x-directional lattice structure with a period of 325 nm. The longer wavelength (λ = 620 nm) light is better coupled to the plasmonic surface than the shorter wavelength (λ = 480 nm) light. In Figures 8c–d, the incident ypolarized light passes through the VA-LC layer without changing its polarization state, and is coupled with the nanohole array with the y-directional lattice structure with a period of 225 nm. The shorter wavelength (λ = 480 nm) light is more strongly coupled to the plasmonic surface than the longer wavelength (λ = 620 nm) light. The resonance intensity coupled with the y-directional geometry is larger than that coupled with the x-directional geometry. This is because the resonance peak wavelength is well matched between the top and bottom metal interfaces in the case of the shorter period condition of the nanohole array structure. Near the metal nanohole array, to confirm the change in the effective refractive index of the LC layer in the z-direction, the depth profiles of the LC layer, including the azimuthal and polar angles of the LC directors, were simulated as functions of the applied voltages, as shown in Figures 8e–f. Considering the depth profiles of the LC layer, the effective refractive index in the z-direction (nz,eff(z)), through the LC profile in the depth direction, is expressed as follows.

nz ,eff ( z ) =

ne no 2 e

(2)

2

n cos θ LC ( z ) + no2 sin 2 θ LC ( z )

where ne and no are the extraordinary and ordinary refractive indices of the LC, respectively, with values of 1.74 and 1.52 for the E7 LC, respectively. Applying the depth profile of the LC layer obtained from the simulation to Eq. 2, we plotted the effective refractive index in the z-direction as a function of the depth and applied voltages, which is shown in Figure 8g. Due to the strong anchoring condition (~ 10−3 N/m) between the LC layer and alignment layer, the change in nz,eff is insignificant to a depth of about 100 nm, even though it increases far from the LC alignment surface until the voltage reaches saturation. This is because the surface LC directors are strongly anchored by the LC alignment layer.


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Figure 8. FDTD simulation results of the plasmonic resonance field distributions for the asymmetric-latticed nanohole arrays (Px = 325 nm and Py = 225 nm) obtained by considering the voltage-dependent LC reorientation : (a) the xpolarized incidence (λ=620 nm) by the field-off TN-LC state, (b) the x-polarized incidence (λ=480 nm) by the field-off TN-LC state, (c) the y-polarized incidence (λ=620 nm) by the field-on VA-LC state, (d) the y-polarized incidence (λ=480 nm) by the field-on VA-LC state. Simulated voltage-dependent depth profiles of the LC director near the nanohole-arrayed metal surface: (e) the azimuthal angle (φLC), (f) the polar angle (θLC), and (g) the effective refractive index (nz,eff) along the z-direction. Consequently, the resonance condition of the nanohole array, which depends on the LC changes in the refractive index, is not affected. Because of the PI layer and the strongly anchored surface LC directors, there is almost no change in the polar angle. Therefore, the color-switching properties of the integrated structure of the rectangular-lattice nanohole array combined with the TN-LC layer are precisely predictable, using the simple analytic model in Eq. 1, which employs the voltage-dependent S1 value of the Stokes parameters, as shown in Figure 7. In the refractive-index-switching structure using the LC at the interface, the resonance wavelength shift was less than 100 nm, even with an LC that had a high birefringence (∆n ~ 0.4).23 However, a greater shift in the resonance wavelength could be electrically achieved for the proposed tunable color filter with a commercial LC (∆n = 0.22), by controlling incident polarizations. This is accomplished by utilizing the polarization-dependent resonance effects of the asymmetric-lattice nanohole array. The value of the shift in the resonance wavelength of the nanohole array with Px = 325 nm and Py = 200 nm is 120 nm.

■ CONCLUSION This study has demonstrated an electrically tunable color filter device employing a TN-LC layer as a polarization rotator on an asymmetric-lattice nanohole-array structure. This tunable plasmonic color filter produces two different primary colors, as well as mixed states, by modulating differently polarized incident light. To induce a different polarization state electrically, a TN-LC mode was assembled on the rectangular-lattice nanohole metal structure. Then, by utilizing an electronic polarization rotator, the LC-combined plasmonic color filter was able to change the color states by modulating the level of transmission of the two primary resonance color peaks. This performance was confirmed analytically and experimentally. Additionally, the transmission spectra of the tunable color filter at the intermediate


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voltage conditions were also precisely predictable, based on a Stokes parameter analysis of the TN-LC mode. The system exhibited widely changeable color states, such as blue-to-green or green-to-red. The effect of the LC alignment layer and the LC layer on the wavelength shift was also examined to enable the design of more exact color filter systems. The transmission at the resonance peak wavelength was also enhanced by the better matching effect enabled by the interface layer of the LC alignment layer. Unlike commercialized pigment-type color filters, this plasmonic color filter fills the additional role of electrode, due to its metallic-material-based structure, which can provide additional design advantages in some devices. This type of color filter can be used to realize high-resolution color devices, because the proposed changeable color filter can replace two or three conventional unit cell structures that are fixed for only one color. ■ METHODS Nanohole Array Fabrication. To fabricate the color filter, a thin Al layer with a thickness of 70 nm was deposited on a clean glass substrate using a thermal evaporator (KVE-T2000, Korea Vacuum Tech.). Subsequently, periodic nanohole arrays were patterned using an electron-beam lithography system (JEOL JBX-9300FS) with an accelerating voltage of 100 kV. A PMMA layer was formed to act as the etching mask to create holes in the Al layer. The Al films were etched by an inductively coupled plasma and reactive ion etching (ICP-RIE) system. Finally, the remaining PMMA layer was removed to prevent an undesirable color shift in the plasmonic color filter. Sample Preparation. To control the incident polarization state on the surface of the asymmetric-lattice nanohole metal, a TN-LC layer was prepared directly on the glass substrate with the nanohole patterns. To promote a stable TNLC structure, a planar LC alignment material (SE-5811, Nissan Chemical Industry) was spin-coated on the nanohole array surface. An indium-tin-oxide (ITO)-coated glass substrate was used as the other substrate, and the planar LC alignment material was spin-coated on it. For solvent evaporation and polymerization of the planar alignment layer, the pre-baking and the post-baking processes were successively conducted at 80 °C for 3 min and at 250 °C for 30 min, respectively. The thickness of the polymerized alignment layer was about 100 nm, and its refractive index was 1.62 at a wavelength of 589 nm. For the alignment layer prepared on the nanohole pattern, rubbing was performed along the direction having the longer nanohole pitch (along the x-axis direction in our work). The other LC alignment layer was orthogonally rubbed against it to obtain the TN-LC structure. Two substrates were assembled with a 5-µm cell gap, which guaranteed the orthogonal polarization switching of the TN-LC layer for the white light incidence. Between the cavities, a nematic LC E7 (Merck) was filled by capillary action. Its extraordinary and ordinary dielectric constants (εe and εo) and dielectric anisotropy (∆ε) were 19.5, 5.2, and 14.3 at 1 kHz, respectively. The extraordinary and ordinary refractive indices (ne and no) of E7 and its birefringence (∆n) were 1.74, 1.52, and 0.22 at 589 nm, respectively. Spectral Measurements and Optical Images. The transmission spectral properties of the fabricated plasmonic color filter were measured with a CCD-based spectrometer (DALSA PRO-5200) which had an available wavelength between 400 nm and 780 nm, with a 1 nm resolution. The white light source was incident normal to the tunable color filter after passing through a linear polarizer when the spectral measurements were performed. Optical microphotographs were took by a CCD image sensor. Stokes Parameter Measurement. To characterize the field-dependent polarization states incident on the nanohole arrays, the Stokes parameters of S1, S2, and S3 were measured using the optical setup shown in Figure S10. For the Stokes parameter measurements, the non-patterned metallic surface regions without the nanohole arrays were carefully selected for probe beam irradiation. As the probe beam source, a high-power continuous-wave laser (λ = 532 nm and the optical intensity = 50 mW) was used to obtain the detectable optical intensity after the opaque metallic surface. The polarization states after the metal film were further modulated using cascaded optical films of a quarter-wave plate, and an analyzer, and the final intensities were measured using a photodetector (Large Area Silicon Photoreceiver, Newport) under four different optical-film set conditions. From the four intensity measurements, three Stokes parameters were calculated after the TN-LC layer, which was biased from 0 V to 5 V with a 0.05-V step, as shown in Figure S11. Simulation. Plasmonic resonance optical field distributions were simulated using a commercial FDTD software tool (Lumerical FDTD, Lumerical Solutions). The electric field distribution on the nanohole-arrayed metal surface was simulated by the finite element method, with COMSOL Multiphysics. The field-dependent LC distributions were simulated using the LC simulator (Techwiz LCD 2D, Sanayi). ■ ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: ■ AUTHOR INFORMATION Corresponding Author


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*E-mail (Jae Eun Jang): [email protected] *E-mail (Hak-Rin Kim): [email protected] ORCID Jae Eun Jang: 0000-0002-8523-1785 Hak-Rin Kim: 0000-0002-2369-2295 Author Contributions §

Youngjin Lee and Min-Kyu Park contributed equally.

Notes

The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the MEST (2014M3A9D7070668, 2015R1A2A2A01005043), and the Korea Evaluation Institute of Industrial Technology (KEIT) funded by the Ministry of Trade, Industry, and Energy (MOTIE) (NO. 10052980). ■ REFERENCES (1) Arsenault, A. C.; Clark, T. J.; von Freymann, G.; Cademartiri, L.; Sapienza, R.; Bertolotti, J.; Vekris, E.; Wong, S.; Kitaev, V.; Manners, I. From colour fingerprinting to the control of photoluminescence in elastic photonic crystals. Nature Mater 2006, 5, 179–184. (2) Liu, J. H.; Yang, P. C.; Wang, Y. K.; Wang, C. C. Optical behaviour of cholesteric liquid crystal cells with novel photoisomerizable chiral dopants. Liquid crystals 2006, 33, 237–248. (3) Arsenault, A. C.; Puzzo, D. P.; Manners, I.; Ozin, G. A. Photonic-crystal full-colour displays. Nat. Photonics 2007, 1, 468–472. (4) Shervin, S.; Kim, S.-H.; Asadirad, M.; Karpov, S. Y.; Zimina, D.; Ryou, J.-H. Bendable III-N visible light-emitting diodes beyond mechanical flexibility: Theoretical study on quantum efficiency improvement and color tunability by external strain. ACS Photonics 2016, 3 (3), 486–493. (5) Duempelmann, L.; Gallinet, B.; Novotny, L. Multispectral Imaging with Tunable Plasmonic Filters. ACS Photonics 2017. (6) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Plasmon Shaping by using protein nanoarrays and molecular lithography to engineer structural color. Nature 2003, 424, 824–830. (7) Clark, A. W.; Cooper, J. M. Surface plasmon subwavelength optics. Angew. Chem. 2012, 124, 3622–3626. (8) Sementa, L.; Marini, A.; Barcaro, G.; Negreiros, F. R.; Fortunelli, A. Atomistic quantum plasmonics of gold nanowire arrays. ACS Photonics 2014, 1, 315–322. (9) Tan, S. J.; Zhang, L.; Zhu, D.; Goh, X. M.; Wang, Y. M.; Kumar, K.; Qiu, C.-W.; Yang, J. K. W. Plasmonic color paleetes for photorealistic printing with aluminum nanostructures. Nano Lett. 2014, 14, 4023–4029. (10) Barwick, B.; Zewail, A. H., Photonics and plasmonics in 4D ultrafast electron microscopy. ACS Photonics 2015, 2 (10), 1391–1402. (11) Ghaemi, H. F.; Thio, T.; Grupp, D. E.; Ebbesen, T. W.; Lezec, H. J. Surface plasmons enhance optical trans mission through subwavelength holes. Phys. Rev. B 1998, 58, 6779–6782. (12) Degiron, A.; Lezec, H. J.; Barnes, W. L.; Ebbesen, T. W. Effects of hole depth on enhanced light transmission through subwavelength hole arrays. Appl. Phys. Lett. 2002, 81, 4327–4329. (13) Koerkamp, K. J. K.; Enoch, S.; Segerink, F. B.; Hulst, N. F. V.; Kuipers, L. Strong influence of hole shape on extraordinary transmission through periodic arrays of subwavelength holes. Phys. Rev. Lett. 2004, 92, 183901. (14) Gordon, R.; Brolo, A. G.; McKinnon, A.; Rajora, A.; Leathem, B.; Kavanagh, K. L. Strong polarization in the optical transmission through elliptical nanohole arrays. Phys. Rev. Lett. 2004, 92, 037401. (15) Genet, C.; Ebbesen, T. W. Light in tiny holes. Nature 2007, 445, 39–46. (16) Przybilla, F.; Degiron, A.; Genet, C.; Ebbesen, T. W.; de Leon-Perez, F.; Bravo-Abad, J.; Garcia-Vidal, F. J.; Martin-Moreno, L., Efficiency and finite size effects in enhanced transmission through subwavelength apertures. Opt. Express 2008, 16 (13), 9571-9579. (17) Rodrigo, S. G.; García-Vidal, F.; Martín-Moreno, L., Influence of material properties on extraordinary optical transmission through hole arrays. Physical Review B 2008, 77 (7), 075401. (18) Carretero-Palacios, S.; Garcia-Vidal, F. J.; Martin-Moreno, L.; Rodrigo, S. G., Effect of film thickness and dielectric environment on optical transmission through subwavelength holes. Physical Review B 2012, 85 (3). (19) Kim, S.; Shin, J. H.; Kim, S.; Yoo, S.-J.; Jun, B. O.; Moon, C.; Jang, J. E. Geometric effects of nano-hole arrays for label free bio-detection. RSC Adv. 2016, 6, 8935–8940. (20) Liu, Y. J.; Zheng, Y. B.; Liou, J.; Chiang, I.-K.; Khoo, I. C.; Huang, T. J. All-optical modulation of localized surface plasmon coupling in a hybrid system composed of photoswitchable gratings and Au nanodisk arrays. J. Phys. Chem. C 2011, 115, 7717–7722. (21) Gehan, H. l. n.; Mangeney, C.; Aubard, J.; Lévi, G.; Hohenau, A.; Krenn, J. R.; Lacaze, E.; Félidj, N. Design and optical properties of active polymer-coated plasmonic nanostructures. J. Phys. Chem. Lett. 2011, 2, 926–931.


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