Structural color filters based on an all-dielectric metasurface exploiting silicon-rich silicon nitride nanodisks.

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.

metasurfaces [17][18][19][20][21][22][23][24][25]. All-dielectric metasurfaces incorporating high-index materials like crystalline silicon (c-Si), hydrogenated amorphous silicon (a-Si:H), and titanium dioxide (TiO 2 ) 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][18][19][20][21][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 [18]. Recently, TiO 2 was adopted as a prominent candidate to construct an all-dielectric metasurface [28][29][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][32][33][34][35][36]. A suite of structural color filters in SiN was primarily reported based on the guided-mode resonance (GMR) [37][38][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 TiO 2 , 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 [40]. 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 TiO 2 . Therefore, it should be reasonably emphasized that SRN is a viable candidate as a foundation for creating a highquality 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.  The measu in Fig. 3  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 [46]. 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 depicte are immersed medium enclo our metasurfa metasurface i metasurfaces and D = 200 are observed respectively, nanodisks are peak in conne ED and MD deemed to inc that the reson red-shift and medium, whe [40,49]. Con wavelengths, predicting col results confirm peaks for the the created co 5. (a) Contour ma disks is varied fro disks are fixed as 2 0 nm, 200 nm, and ed in Fig. 2

Mechani nanodisks
When it com resonances ar the operation were assessed assistance of of P = 400 n surrounding t profiles for th cross-sections for the ED an the standpoin nanodisks, the Figs. 7(b) and in Fig. 7(b), t field develop categorically transpiring at that develops the observatio 6. Reflection spectr urrounding medium ct reflection peak rsed in a polymer hift.
sm underly mes to nanopa re presumed to n principle und d in terms of t FDTD simulat nm and D = 2 the SRN nano he case where s in Fig. 7  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 Hfield 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.

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 [41]. For the proposed alldielectric metasurfaces, the transmission/reflection spectra, extinction cross-sections, and field profiles were estimated by means of the FDTD method-based tool [45]. 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.

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 SiH 4 and NH 3 diluted in helium was used as a precursor gas. By altering the SiH 4 to NH 3 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 SiH 4 / 4 sccm NH 3 / 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 CHF 3 with a small addition of SF 6 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.

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.

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][38][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 crosssection 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.