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Abstract
This study develops a large-area pixelated filter that can achieve colors covering the entire visible range with a fixed period under normal incidence. Vivid colors as blue, green, and yellow (peak efficiency of ~60%) are experimentally achieved based on a Fano-resonance by altering the overlay’s refractive index, which is highly sensitive to the surrounding material. Furthermore, the feasibility of using this device in large-area color printing and index sensors is discussed in detail, wherein a large-area (3 cm × 3 cm) logo and a figure of merit of 254 are achieved. Therefore, this developed structure can be regarded as an alternative to traditional periodic-dependent structure colors, which can also be performed as index sensors with high sensitivity.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Plasmonic structures have induced great interest in recent years for various applications [1–36]. Structure color, which is developed from the interaction between bionic structure and visible light [7–10], has been a promising avenue for diverse applications: color printings [16,17], index sensor [5], solar cells [19], display/image and anti-counterfeiting certifications [11–15], for example. Furthermore, these structure-based color effects have advantages in the resolution and sustainability over the selective absorption of colorant dyes [6], and their ultrathin structural thickness naturally bring an increased mechanical flexibility that is required in a variety of printing technologies.
To date, there are two main categories according to the structure composition of structure colors. The first one is multilayers-stacked films that based on the asymmetry/symmetry Fabry-Perot cavity resonance [17–20,25,31]. Most of these are stacked metal-dielectric-metal (MIM) films, where the bottom layer is a total reflective/semi-transparent layer along with the top layer is a semi-transparent mirror. Wherein colors are finely defined by the dielectric layer’s thickness, nevertheless, for such filters, lacking the flexibility in controlling colors, which limits their applications that need active variable optical appearance. The other one is based on artificial engineered structures, such as subwavelength gratings [28,29], nanoholes [6], nanopatches [3,4,26], nanoslits [32], and nanodisks [11,22]. The physical mechanism underlying these colors is the excitation of, e.g., localized surface plasmonic resonance (LSPR) [5,24,36], Wood Rayleigh anomaly (WRA) [36], standing-wave resonance (SWR) [21], and extraordinary optical transmission (EOT) [6]. Compared to the Fabry-Perot cavity resonance based structure colors, plasmonic color filters afford more flexible in controlling optical appearance. However, complicated lithographic procedures are involved in their fabrications, which limit their applications in large scale.
Alternatively, controlled optical appearance can be modified by overlay’s refractive index under the same period. For example, Luc Duempelmann et. al presents an approach for customizing the optical properties of plasmonic substrates via inkjet printing [36], wherein plasmonic substrates consisting of tilted aluminium (Al) nano-lamellas providing colors. However, the color gamut of this device is unable to cover the entire visible range under the same period at normal incidence, which will influence its applications in display or image.
Here, we present a pixelated plasmonic filter that can cover the entire visible range with a fixed period under normal incidence. And Fano-resonance is used as an idealized basis, which is ultra-sensitive to environment’s refractive index. Therefore, the overlay is varied to precisely control the transmission colors. In particular, colors as magenta (M), blue (B), cyan (C), green (G), and yellow (Y) colors, with a peak efficiency ~60%, are achieved at the same period under transverse magnetic (TM)-polarized light. A continuously variable spatial frequency photolithography is involved for large-area, cost-efficient fabrication. As an ideal candidate for the index sensors, a figure of merit (FOM*), introduced by J. Becker [37], of 254 is achieved for the developed structure.
2. Structural design and experimental details
The proposed device is schematically presented in Fig. 1, where one-dimensional (1D) 230-nm-thick photoresist grating arrays, 30-nm-thick Al nanopatch arrays and a 200-nm-thick overlay are stacked sequentially on the quartz substrate, the period of this device is 420 nm. Here, a TM-polarized light is incidence from the top surface. Simulations are performed using the finite-difference time-domain (FDTD) method [38], and the index information associated with the Al is derived from Palik [39], while the index of the photoresist grating is 1.46.
Figure 2 demonstrates the process procedure for fabricating this device. The pixelated grating is fabricated by the continuously variable spatial frequency photolithography process (laser wavelength: 355 nm, repetition rate: 1 kHz, pulse duration < 15 ns, objective lens: 50 × ), an ideal choice for up-scale and fast nano-fabrications [40–42], on a 260 nm thick photoresist layer (AZ 4620, Suzhou Zhongxinqiheng Co., Ltd) that spin-coated on the surface of a quartz substrate. Subsequently, neatly distributed 1D photoresist gratings are formed after the photoresist layer developed (3 s) in NaOH solutions (5‰), scanning electron microscopy (SEM) images with different magnifications are displayed in Fig. 2(b), where pixelated grating arrays can be clearly observed, and the inset is an enlarged SEM image of gratings. Then a plasma stripper is employed (~8 s) to achieve vertical grating profile [Fig. 2(c)]. After that, the Al layer is deposited on the pixelated grating via electron beam evaporation (custom-built), see Fig. 2(d). Finally, deposition of transparent dielectric material onto the pixelated Al-coated grating alters the plasmon resonance by the inductively coupled plasma chemical vapor deposition (Plasma lab, Oxford instruments), which give a distinct color effects in transmission, see Fig. 2(e).
Fig. 2 Fabrication scheme including: (a) the continuously variable spatial frequency photolithography process. (b) Scanning electron microscope image of pixeled gratings. (c) The plasma stripper process. (d) Evaporation of Al films. (e) Scheme of the pixelized color generation.
3. Results and discussion
This device shows a characteristic Fano-like line shape that originates from strong coupling between the surface plasmonic resonance (SPR) and guide mode resonance (GMR). Figure 3(a) displays the transmissive spectra for the device with varied overlays under normal incidence. Obviously, the physical origin underlying the refractive index induced-color appearance is originated from three parts: the coupling between GMR and SPR [Fig. 3(c)], the SPR on the Al patches [Fig. 3(d)], and the SPR on the substrate [Fig. 3(e)]. These resonances are highly sensitive to the refractive index of the surrounding, and the refractive index of the overlay (ncoat) can be changed by deposition. Besides, a spectral signature of the SPR is the WRA [43], then the quartz substrate and coating material present two different WRAs [the red and white lines in Fig. 3(a)]. Interestingly, the resonance wavelength is also depending on the ratio of ncoat to nsub. For the coupling between GMR and SPR, its resonance wavelength changes nearly 200 nm/refractive index unit (RIU) for devices filled by mainly ncoat [purple line in Fig. 3(b)], while it changes only 100 nm/RIU (green line) for devices filled by mainly nsub. Actually, there is a trade-off between resonance variation and fabrication difficulty. Here, a configuration with a 160 nm RIU (blue line) is chosen, which is easy for fabrication. The sensitive dependence on the filling ratio can be explained with a varied resonance strength, which is similar to the optical effects under changed periods.
Fig. 3 (a) Calculated transmission spectra as a function of ncoat at normal incidence. (b) Corresponding spectral position of the coupling between the SPR and GMR for different filling of the structures (see scheme), including the WRcoat and WRsub. (c)-(e) Magnetic field of the coupling between the SPR and GMR, WRcoat and WRsub.
In short, spectra respond to the variation in the refractive index of the overlay, wherein the resonant wavelength of the coupling between the SPR and GMR, and the position of the WRcoat can be altered by the filling factor as depicted in Fig. 3(b). The spectral change leads to a great color variation and then distinct perception. It is note that the Fano-like line shape is originated from the strong coupling between these resonances that with approximately equally intensity.
To further examine how the overlay influences the transmission, Fig. 4(a) shows the measured transmission for this plasmonic device with different overlays. With increased refractive index, the resonant wavelength of WRcoat (dark cross) and the coupling between the SPR and GMR (red cross) are both red-shifted. Furthermore, the strength of the coupling between the SPR and GMR decreases with increased ncoat indicted by the increased half wave height full width. These effects result in a flattening of the Fano-like line shape, leading to varied color effects, which is agree with the calculation presented in Fig. 3(a). As plotted in Fig. 4(a), photographs of the developed sample are shown inset, and it only takes ~5 minutes to fabricate pixelated gratings in such a size (1 cm × 1 cm).
Fig. 4 (a) Measured spectra under TM-polarized incident light with varied coating material. (b) CIE 1931 color plot with ncoat between 1.0 and 2.0 and the measured samples.
The color dependence on refractive index is further presented in an International Commission on Illumination (CIE) 1931 chromaticity diagram, as presented in Fig. 4(b). These transmitted colors are calculated (ncoat = 1.0~2.0; solid lines) and measured [ncoat = air, silicon oxide (SiO2), silicon nitride (SiNx); red cross] at normal incidence under TM-polarized light, wherein brilliant colors are observed. The solid line shows a wide range of colors as M, B, C, G and Y are obtained for the developed device with different overlays, which means more colors are available by depositing other different materials. Alternatively, it is possible to obtain colors exceed solid line by using other periodic gratings, which would strongly enlarge the range of transmitted colors.
To demonstrate the feasibility of this method for structural color printing, we fabricated a large-area (3 cm × 3 cm) Soochow University logo, as depicted in Fig. 5(a), where the color appearance is induced by period-dependent diffraction effects. Remarkably, it only takes ~15 min to fabricate this logo with various periods. Following on, the Al metal was deposited on the pixelated logo, see Fig. 5(b). After that, this colorful logo coating SiNx overlays is displayed in Fig. 5(c). It is noteworthy that transmitted colors are obviously different between Figs. 5(b) and 5(c) at same periodic parts, and the rendered colors cover the entire visible range, including the three primary CMY colors, revealing the feasibility of this method for large-area structural color printing and displayed index sensor.
Fig. 5 Images taken by a mobile phone under TM-polarized light. (a)-(c): Pixelated photoresist grating, Al coated pixelated photoresist grating, and SiNx—Al coated pixelated photoresist grating, respectively.
To further explore the performance of this device for index sensors, Fig. 6(a) depicts the results of a proof for a principle experiment, as displayed by the red curve, the calculated transmission reaches a minimum of 0.002 at 509 nm with SiNx overlays. If SiO2 overlays is applied onto the top surface instead of the SiNx, an obvious increase from 0.002 to 0.386 at 509 nm is visible, resulting from the changed refractive index.
Fig. 6 (a) Measured spectra under TM-polarized light with SiO2 and SiNx overlays. (b) Experimental FOM* as a function of wavelength. (c) Simulated transmission spectra of this device for glucose solutions. (d) Calculated FOM* as a function of wavelength.
Afterwards, the FOM* can be calculated as:
where dI(λ)/I(λ) is the relative intensity change at a fixed wavelength induced by a refractive index change dn [37]. I(λ) corresponds to the intensity where FOM* reaches a maximum value. The measured maximum value of FOM* [Fig. 6(b)] for SiNx (dn = 0.54) is up to 254, therefore, our experimentally measured FOM* is relative higher. For specific applications, this device can also be designed for water or glucose solution. As an example, Fig. 6(c) shows the transmission at 522 nm increases with increased refractive index of the solution, the highest value of FOM* is still above 200 [Fig. 6(d)], and the determined sensitivity, in terms of wavelength shift per refractive index unit, is around 375 nm/RIU.Therefore, this plasmonic device can work as an index sensor and make significance for developing cheap displayed sensing equipment as only simple measurement technologies are needed and varied colours are visible. Especially, the thickness of overlays used in the index sensor is all 200 nm, essentially, either other thickness or even put this device into solutions, similar conclusions can also be obtained as the Fano-resonance happened in this device is highly sensitive to the variation of environment.
4. Summary
In summary, we have developed large-area pictures by varying the overlay, wherein vivid colors as M, B, C, G, and Y (peak efficiency ~60%) are achieved with the same period under normal incidence. The scope of colors can be widened by altering the period of pixeled gratings or the refractive index of substrates. Furthermore, this proposed device with large areas can be achieved by incorporating photolithography with inkjet printing, which will be an alternative to “periodic-dependent printing.” The plasmonic device proposed here can make significant contributions to applications as large-area color printing, displayed index sensors, optical security, and other related fields.
Funding
National Natural Science Foundation of China (NSFC) (61575132, 61107016, 61575135, 61505131) and the NFSC Major Research Program on Nanomanufacturing (91323303).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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