An all-dielectric metasurface is deemed to serve a potential platform to demonstrate spectral filters. Silicon-rich silicon nitride (SRN), which contains a relatively large portion of silicon, can exhibit higher refractive indices, when compared to silicon nitride. Meanwhile, the extinction coefficient of SRN is smaller than that of hydrogenated amorphous silicon, leading to reduced absorption loss in the shorter wavelength. SRN is therefore recommended as a scattering element from the perspective of realizing all-dielectric metasurfaces. In this work, we propose and embody a suite of highly efficient structural color filters, capitalizing on a dielectric metasurface that consists of a two-dimensional array of SRN nanodisks that are embedded in a polymeric layer. The SRN nanodisks may support the electric dipole (ED) and magnetic dipole (MD) resonances via Mie scattering, thereby leading to appropriate spectral filtering characteristics. The ED and MD are identified from field profile observation with the assistance of finite-difference time-domain simulations. The manufactured color filters are observed to produce various colors in both transmission and reflection modes throughout the visible band, giving rise to a high transmission of around 90% in the off-resonance region and a reflection ranging up to 60%. A variety of colors can be realized by tuning the resonance by adjusting the structural parameters such as the period, diameter, and height of the SRN nanodisks. The spectral position of resonances might be flexibly tuned by tailoring the polymer surrounding the SRN nanodisks. It is anticipated that the proposed coloring devices will be actively used for color displays, imaging devices, and photorealistic color printing.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The metasurface, which is based on a two-dimensional (2D) arrangement of nano-scale scattering elements and thus perceived as a planar counterpart of optical metamaterials, has been researched in virtue of their preeminent capabilities for manipulating the polarization, phase, and amplitude of light in various spectral bands [1–4]. The metasurface can potentially serve as a miniaturized version of the conventional free-space optical devices, encompassing lenses, waveplates, and spectral filters [1–4]. One of the most salient applications empowered by such a metasurface lies in structural color filters, which can provide colors in the visible wavelength regime to possibly supersede dye/pigment-based coloring in display, imaging, color printing, photovoltaic, and so forth [5–9]. To date, a variety of structural color filters and color printing have been attempted, resorting to plasmonic nanoresonators [8–16] and metasurfaces [17–25]. All-dielectric metasurfaces incorporating high-index materials like crystalline silicon (c-Si), hydrogenated amorphous silicon (a-Si:H), and titanium dioxide (TiO2) are preferred to their plasmonic counterparts, in which the inherent loss resulting from metallic films prohibits the bandwidth from being controlled. The metasurface in silicon (Si) is advantageous due to its high performance, enhanced cost effectiveness, and good compatibility with the complementary metal-oxide-semiconductor (CMOS) process; while Si, being the second-most-abundant material in nature, exhibits relatively high refractive indices [17–22]. However, it is challenging to grow a high-quality c-Si on a foreign substrate like glass. Polycrystalline silicon (poly-Si), which can be deposited via chemical vapor deposition (CVD), is a viable alternative that provides refractive indices equivalent to that of c-Si. Nevertheless, at temperatures higher than about 300 °C, the deposition of poly-Si on a glass or plastic substrate is hardly permitted. For instance, process techniques including laser/thermal annealing and atomic layer deposition (ALD) were additionally required to form a poly-Si layer on a glass substrate [26,27]. Considering poly-Si is composed of a multitude of small crystallites, optical scattering loss may be caused by the grains. An a-Si:H layer can readily be deposited on a foreign substrate via CVD process at relatively low temperature. Yet, it is vulnerable to relatively high absorptions in the short wavelengths, degrading the efficiency in the blue region . Recently, TiO2 was adopted as a prominent candidate to construct an all-dielectric metasurface [28–30] and its derivatives like color filters [24,25]. In the meantime, silicon nitride (SiN) has received ample attention as a prime platform for embodying the metasurface, recognizing that its preparation is fully compatible with the CMOS process, while the large bandgap allows for improved transparency and efficiency across the visible band [31,32]. SiN based metasurfaces were taken advantage of to create metalenses, wavefront manipulation, and imaging [31–36]. A suite of structural color filters in SiN was primarily reported based on the guided-mode resonance (GMR) [37–39]. However, no SiN metasurface rendering color generation has yet been interpreted from the viewpoint of Mie scattering mediated resonances. Since SiN gives lower refractive indices compared with the cases of c-Si, a-Si:H, and TiO2, SiN nanoparticles may not be recommended as an efficient scattering element in the case of an all-dielectric metasurface. It is reported that for nanoparticles exhibiting a certain aspect ratio, the scattering efficiency can be enhanced with increasing refractive indices . Hence, for the case of low-index materials, their surface area should be enlarged to boost the scattering. It should be mentioned that when it comes to Si-rich SiN (SRN), it contains a relatively large portion of Si and thus could offer a higher refractive index compared with SiN and TiO2. Therefore, it should be reasonably emphasized that SRN is a viable candidate as a foundation for creating a high-quality metasurface. Moreover, SRN allows for a low-temperature deposition via CVD; as a result, the benefit of low absorption in the short wavelengths could be attained, thus overcoming the critical weakness of a-Si:H.
In this work, we suggest an all-dielectric metasurface tapping into an array of SRN nanodisks, enabling considerable scattering, which is crucial for both electric dipole (ED) and magnetic dipole (MD) resonances induced by the Mie scattering. We attempt to concoct a set of structural color filters counting on the proposed SRN metasurface, creating a variety of colors in transmission mode as well as reflection mode. The operation mechanism underlying the proposed metasurface has been inspected through the observation of electric and magnetic field distributions, which are presumed to be responsible for the ED and MD resonances excited by the SRN nanodisks.
2. Optical properties of dielectric materials affiliated with Si
Figure 1 plots the optical properties in the visible wavelength region for dielectric materials associated with Si, inclusive of c-Si, a-Si:H, SRN, and SiN. The refractive indices of c-Si and a-Si:H are available from Palik , while those of SRN and SiN are obtained by characterizing their films deposited on a glass substrate using an ellipsometer (J. A. Woollam M2000D). The c-Si and a-Si:H give rise to high indices (around n = 4). The a-Si:H is unfortunately susceptible to high extinction for shorter wavelengths below λ = 550 nm, causing a significant level of absorption. In contrast, SiN exhibits a relatively lower refractive index in combination with negligible extinction. SiN film is known to be readily attained with the help of CVD at a low temperature of around 250 °C. Noting the composition of SiN film is varied by adjusting the relative flow rates for SiH4 and NH4 gases, it is remarked that SiN is engineered to assume higher refractive indices by boosting the content of Si during the deposition process. Consequently, in terms of the efficiency of scattering, SRN is reckoned to be unequivocally preferred to SiN or TiO2.
3. Nano-structural coloration enabled by the proposed SRN metasurface
Figure 2 shows the configuration of the proposed all-dielectric metasurface, where a 2D lattice of SRN nanodisks is embedded in a polymeric film on top of a glass substrate. A polymeric material is spin-coated to fill in the gap between nanodisks so as to provide a flat top surface, which is useful for facilitating the planarization crucial for the practical integration of image sensors [42,43]. The polymer, which can increase the refractive index of the medium surrounding the nanodisks, helps bring the transmission dip and reflection peak close together. It is hence beneficial that the center of resonance is concretely rather than vaguely defined to better predict the resulting coloring behavior. A copolymer, ZEP520A, which has been popularly utilized as an electron-beam resist, is selected for building the polymeric layer. The height (H) of the SRN nanodisks is set to 200 nm, which is nearly the thickness of the polymeric layer. The diameter (D) is varied from 80 nm to 200 nm in steps of 20 nm to implement different metasurfaces operating in the visible band, while the period (P) is determined in accordance with the relationship given by P = D + 200 [nm]. For a metasurface resorting to cylindrical nanoparticles, the resonance condition is likely to be controlled by tailoring the structural parameters, such as the height and diameter . The detailed fabrication procedure is elaborated on in a later section. Figure 2 shows that the presence of the metasurface which relies on the SRN nanodisks embedded in a polymer causes incident light to be spectrally filtered out to exhibit flexible coloring in the visible band. The corresponding electric (E-) and magnetic (H-) fields are aligned parallel to the x- and y-axis, respectively. Scanning electron microscope (SEM) image of the fabricated metasurface, subscribing to the design parameters of P = 400 nm, D = 200 nm, and H = 200 nm, is displayed in the inset of Fig. 2. The SRN nanodisks embedded in the polymer (ZEP520A) appear to be embodied in a high fidelity to the design, as desired.
The measured and calculated transmission responses for different metasurfaces are plotted in Fig. 3(a). The spectral positions in relation to the ED and MD resonances are traced for each case and marked in dashed lines. The operation and origin of the resonances are discussed further in the next section. The transmission response has been calculated using a commercially available simulation tool, FDTD Solutions (Lumerical, Canada), which is based on the finite-difference time-domain (FDTD) method . The optical transmission spectra were practically examined by a fiber-coupled spectrometer. The observed coloring outputs in transmission mode for the proposed devices, which were fabricated in the pixel sizes of 40 μm × 40 μm, are revealed in Fig. 3(b). The related chromaticity coordinates were obtained from the measured and calculated spectra and mapped on the standard International Commission on Illumination (CIE) 1931 chromaticity diagram, as shown in Fig. 3(c).
The reflection spectra for the prepared color filters were similarly scrutinized, in conjunction with the coloring performance in reflection mode. The reflection spectra and corresponding color images as captured by a microscope are presented in Figs. 4(a) and 4(b), respectively. The observed outcome indicates that the device performance has been slightly affected by fabrication errors. The prepared metasurfaces are monitored to provide colors scanning from light blue, through yellowish green, to orange in reflection mode, when the structural parameters of the SRN nanodisks like the period and diameter are varied. The proposed SRN metasurface tends to give rise to higher efficiencies in longer wavelengths than in shorter wavelengths, which is attributed to the fact that the resonances mediated by the Mie scattering are highly promoted in the spectral regime near λ = 550 nm, in light of the dimensions of the SRN nanodisks. In contrast, the reflection efficiency is severely degraded in the shorter wavelength region, on account of nonnegligible optical extinction on top of inefficient scattering mediated resonance. The efficiency can be elevated in the shorter wavelength band below λ = 550 nm, when structural parameters including the height and diameter are properly adjusted to mediate scattering. This work was particularly concerned about the height of H = 200 nm. The related chromaticity coordinates for the reflection spectra are plotted on the CIE 1931 chromaticity diagram, as sketched in Fig. 4(c).
In order to achieve a variety of colors, we adjusted the structural parameters such as P and D at the same time. A periodic boundary condition was adopted for the calculation to address coupling between adjacent nanodisks . We also explore the spectra by changing the gap between adjacent nanodisks, which are enclosed in the polymer, while the contour map of the calculated reflection spectra is depicted in Fig. 5(a). The gap is varied from 100 nm to 300 nm in steps of 10 nm, while the height (H) and diameter (D) of the nanodisks are fixed at 200 nm. The calculated scattering cross-sections corresponding to gaps of 150 nm, 200 nm, and 250 nm, are delineated in Fig. 5(b). The scattering cross-section is defined as the ratio of the scattered power going through the closed surface to the power-per-unit area for the incident wave. It is observed that for the proposed metasurface which resorts to periodically arranged SRN nanodisks, the spectral response may be tailored by changing the gap between neighboring elements, which might be mutually coupled to a certain extent and affect the resonance characteristics. It is expected that the coupling can be taken advantage of to adjust the phase of transmitted light and manipulate the wavefront [47,48].
As depicted in Fig. 2, the proposed metasurface exploits an array of SRN nanodisks that are immersed in a polymeric layer. It is worthwhile to consider the influence of the optical medium enclosing the SRN nanodisks upon the metasurface performance. Toward that end, our metasurface, leveraging the configuration of embedded SRN nanodisks, is compared to a metasurface in which nanodisks are just exposed to air, as summarized in Fig. 6. Here, the metasurfaces are assumed to comprise an identical lattice of SRN nanodisks with P = 400 nm and D = 200 nm. For the case of the nanodisks in contact with air, separate reflection peaks are observed at λ = 582 nm and 642 nm, accounting for the ED and MD resonances, respectively, as indicated by the blue curve. For the proposed metasurface in which the SRN nanodisks are enclosed in a commercialized polymer, ZEP520A with n = ~1.55, the reflection peak in connection with the ED resonance is confirmed to red-shift as anticipated, so that the ED and MD resonances approximately coincide to establish a single peak. The polymer is deemed to increase the effective refractive index of the medium surrounding the nanodisks, so that the resonance peak could shift to longer wavelengths. The ED resonance is believed to red-shift and broaden when the cylindrical nanodisks are immersed in a homogeneous medium, whereas the MD resonance is comparatively less sensitive to the surrounding media [40,49]. Considering that color reproduction is governed by a combination of various wavelengths, a single band of spectral response might be preferable from the perspective of predicting coloring behavior, compared with a multi-band of spectral response [50,51]. The results confirm that the polymeric layer plays the role of preferentially making the reflection peaks for the ED and MD resonances closely neighbor, thereby enhancing the definition of the created colors.
4. Mechanism underlying transmission and reflection resonances for SRN nanodisks
When it comes to nanoparticles made of high-index materials like Si, the ED and MD resonances are presumed to be primarily induced via Mie scattering [44,49,52,53]. To explore the operation principle underpinning the SRN based metasurface, the proposed nanodisks were assessed in terms of the scattering cross-section along with the field profiles, with the assistance of FDTD simulations . The performance of the metasurface under dimensions of P = 400 nm and D = 200 nm is considered, dependent on the presence of the medium surrounding the SRN nanodisks. Figure 7 shows the calculated cross-sections and field profiles for the case where the nanodisks are in contact with air. In regard to the calculated cross-sections in Fig. 7(a), the transmission dip and reflection peak are identified to account for the ED and MD resonances, respectively, which are mediated by the Mie scattering. From the standpoint of the double resonances associated with the metasurface exploiting the SRN nanodisks, the field profiles for the resonance peaks at λ = 582 nm and 642 nm are depicted in Figs. 7(b) and 7(c), respectively. As per the field profiles observed in the yz-plane, as shown in Fig. 7(b), the E-field is chiefly confined in the middle of the SRN nanodisk while the H-field develops a loop in the vicinity of its center at the shorter wavelength of 582 nm, categorically indicative of the ED resonance. As shown in Fig. 7(c), the MD resonance, transpiring at a longer wavelength of 642 nm, which is evident through the enhanced H-field that develops near the center of the disk, alongside the circular E-field pattern. Consequently, the observation of the developed field profiles can distinguish the ED and MD resonances.
Likewise, we scrutinized the proposed metasurface which engages a lattice of SRN nanodisks embedded in the polymeric layer of ZEP520A. As portrayed in Fig. 8(a), the ED and MD resonances almost coincide, exhibiting a wavelength separation of as small as 18 nm. Though the nanodisks placed in the polymer may incur a wavelength shift for the ED resonance, the field profiles in charge of the ED and MD resonances, corresponding to λ = 646 nm and 662 nm, are displayed in Figs. 8(b) and 8(c), respectively. As sketched in Fig. 8(b), the E-field in conjunction with the circular H-field is reinforced near the center of the nanodisk at λ = 646 nm, underscoring the existence of the ED resonance. The reinforced H-field near the center of the nanodisk in combination with the surrounding E-field loop is believed to represent the MD resonance at λ = 662 nm, as shown in Fig. 8(c). It is noteworthy that similar phenomena may hold true for the rest of the devices adopting different design parameters.
5.1 Numerical simulations
The refractive indices of the SRN and SiN films were obtained by using an ellipsometer (J. A. Woollam M2000D) operating in the spectral range from 193 nm to 1,690 nm, while the dispersion characteristics of c-Si and a-Si:H are derived from Palik . For the proposed all-dielectric metasurfaces, the transmission/reflection spectra, extinction cross-sections, and field profiles were estimated by means of the FDTD method-based tool . A normally incident plane wave was illuminated to a unit cell, satisfying a periodic boundary condition, so that an array of periodically arranged SRN nanodisks could be emulated.
5.2 Device fabrication
The proposed color filters were manufactured to exhibit dimensions of 40 μm × 40 μm. A 200-nm thick SRN film was deposited on a glass substrate in plasma enhanced CVD system (Plasmalab 100, Oxford), where a mixture of SiH4 and NH3 diluted in helium was used as a precursor gas. By altering the SiH4 to NH3 gas ratio, we could tailor the nitrogen content in a grown film; i.e., with the help of ammonia gas, the portion of nitrogen in the film is increased, thus reducing the extinction coefficient in the visible band and the corresponding refractive index. The adopted process entails: Gas flow rates of 7.5 sccm SiH4 / 4 sccm NH3 / 142.5 sccm He, a gas pressure of 850 mTorr, a power of 30 W at 13.56 MHz (radio frequency), and a substrate temperature of 300 °C. After spin coating of an electron beam resist (ZEP520A from Zeon Chemicals), a thin layer of e-spacer 300Z (Showa Denko) was introduced to prevent charging during subsequent electron beam exposure. The nanostructure pattern was then formed using an electron beam writer (EBL, Raith150) and developed in ZED-N50. A 60-nm thick Al layer was subsequently deposited by e-beam evaporation (Temescal BJD-2000), accompanied by a lift-off process in which the sample is soaked in a resist remover (ZDMAC from ZEON Co.). An array of remaining rectangular Al patterns was used as the etch mask to transfer the designed pattern into the SRN film through inductively coupled plasma-reactive ion etching (Plasmalab System 100, Oxford). The etching conditions were optimized to achieve a highly anisotropic profile for the SRN layer, while CHF3 with a small addition of SF6 gas was used as plasma etch chemistry. The residual Al etch mask was finally removed by wet etching. The resist, serving as the polymer, was spin-coated to enclose the SRN nanodisks.
5.3 Optical characterization
The completed SRN pattern was visually evaluated by dual beam (SEM/FIB) high-resolution scanning electron microscopy (FIB II, Quanta 3D FEG, FEI). The transmission spectra were obtained by a spectrometer (Avaspec-3648, Avantes) which is tethered to a multimode fiber, while a collimated beam originating from a halogen lamp (HL-2000-FHSA, Ocean Optics) was shone via a focusing lens to the prepared filter that was mounted on a motorized rotation stage. For the proposed devices, the spectral response is stable for incident angles ranging up to ~5°. With the intention of ensuring that the optical response is governed by normally incident light, the test setup has been devised to exhibit a numerical aperture below 0.017. The reflection spectra were observed by a spectrometer equipped with a reflection probe, which is specifically devised to accept the optical beam that normally reflects from the devices. Color images originating from each pixel of the color filters were taken via a digital microscope (Leica DM4000 M) in transmission mode and a CCD camera attached to the spectrometer was exploited in reflection mode.
6. Conclusion and discussion
Structural color filters employing an all-dielectric metasurface, which exploits a 2D lattice of SRN nanodisks embedded in a polymeric layer, were developed in the visible band to produce not only subtractive colors in transmission mode but additive colors in reflection mode as well. The proposed metasurface, which incorporates 200-nm-thick SRN nanodisks, provides a high efficiency in the green and red band. The device is anticipated to potentially yield enhanced reflection even in the shorter spectral regime when the structural parameters associated with the nanodisks are appropriately selected. Thanks to the introduction of a proper polymer medium enclosing the SRN nanodisks, the overall spectral response of the filters could be effectively engineered to approximately assume a single dominant resonance, thus enabling the prediction of achievable color outputs. The ED and MD resonances mediated by the Mie scattering were keenly scrutinized through the analysis of the field profiles pertaining to the SRN nanodisk structures. Under the deliberation that from the viewpoint of the structural coloring, SiN was mostly employed to implement narrow band filters based on the GMR effect [37–39], the refractive index of SiN has been substantially elevated to become SRN, with a view to making it pertinent to the invocation of scattering in a nanostructure, as verified through the current work. It should be remarked that the resonance could be mediated by Mie type scattering, taking into account the scattering cross-section and the corresponding field profile. Recently, perfect reflection for a periodic structure was explained based on the leaky mode resonance [54,55]. It is expected that considering the proposed metasurface featuring a periodic arrangement of nanostructures provides near-perfect reflection in the spectral band around 650 nm, the leaky mode resonance can be a viable approach for the purpose of inspecting all-dielectric metasurfaces. A wide range of coloring can be made possible by tuning the resonance via the adjustment of the structural parameters of the SRN nanodisks.
National Research Foundation of Korea (NRF) funded by the Korean government (No. 2016R1A2B2010170 and 2018R1A6A1A03025242); Australian Research Council Future Fellowship (FT110100853, Dr. Duk-Yong Choi).
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