1. Introduction

Metasurface-based colors originating from the resonant interaction of light with nanostructures are widely explored for their great potential in applications for 3D photographs, paintings, holograms, and anti-counterfeiting devices.

By incorporating various nanostructures with different structural geometries and dimensions [1–6], three primary physical phenomena are involved in the development of reflective colors. Based on surface plasmon resonance [7–17], metallic particles, nanoprotrusions, and nanoholes are arranged as microscale arrays in the wavelength range, while the fast fabrication of metal antennas or metal gratings is a great challenge for their large-area production. Even for large-area aluminum nanostructures [12], E-beam lithography is involved to produce nano-structures with area of 1 mm², which is cost- and time-consuming. With the Fabry–Pérot cavity [18–24], types of metal-insulator-metal structures are employed, where an optical cavity is sandwiched between an etalon consisting of two parallel metal layers. The thickness of the cavity needs to be varied to generate different colors, which limits their area since E-beam technology is required to obtain different cavity depths. Incorporating guided mode resonance [25–33], cross-shaped silicon nanoantennas/one-dimensional amorphous silicon gratings, partially etched silicon-nitride films and asymmetric titanium oxide arrays have been developed. However, etching is necessary in structure fabrication due to the dielectric layer with a high refractive index, which is not suitable for large-area color displays. Furthermore, metalized dielectric nanowires and metal/metal-dielectric composite gratings [34–36] have been designed, which show great potential for large-area color displays [34,35]. However, its reflections [36] are transparent or show limited colors for s-polarization, owing to the intrinsic characteristics of surface plasmons.

In this paper, we develop a reflective color filter incorporating a sandwiched metasurface, where the photoresist grating is chosen to be the bottom layer, and the Ag gratings are sandwiched between the photoresist grating and the silicon nitride grating. Since the optical characteristics of photoresist film is similar to the UV film, which can be easily substituted by the UV film utilizing roll-to-roll nano-imprinting for large area flexible display. The analytical results show that by only changing structural periods, three primary colors, red, green and blue, can be obtained due to guided mode resonance as well as cavity resonance, and its measured peak efficiency is ∼60% for s-polarized incidence. With only photolithography and deposition processes involved, a stripe sample in an area of ∼ 300 mm² is fabricated with assembled pixelated metasurfaces, and it displays fully reflective colors with structural periods varying from 250 nm to 550 nm in steps of ∼1 nm. With metasurfaces of three structural periods, Soochow University logo is fabricated in a larger area of ∼ 900 mm². Therefore, the proposed structure performs good reflectance for full spectrum colors and shows great potential for large-area flexible display.

2. Proposed structure and its reflectance

The schematic of the proposed structure is shown in Fig. 1, where the photoresist grating, the complementary silver grating and the SiN_x grating stack on the quartz substrate in sequence. Here, P represents the structural period, while h₁, h₂ and h₃ represent the height of the photoresist grating, the silver grating and the SiN_x grating, respectively. The refractive indices for the photoresist and SiN_x are 1.46 and 2, respectively. The refractive index of Ag is derived from Palik [37]. Its reflection structure is simulated by the finite-difference time-domain (FDTD) method [38].

 

Fig. 1. Schematic of the proposed structure. The sandwiched structure is composed of a dielectric grating, a complementary silver grating and a SiN_x grating. P represents the structural period, while h₁, h₂ and h₃ represent the height of the dielectric grating, the silver grating and the SiN_x grating, respectively.

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The optimal filtering spectra and their reflections with varied parameters are analyzed, as shown in Fig. 2. Figure 2(a) shows that there are resonant peaks, as line B originates from guided-mode resonance, and lines A₁, A₂ and A₃ are excited by cavity resonances. With the increase of the photoresist grating depth h₁, the cavity depth of the photoresist grating increases, and the resonances depicted with lines A₂ and A₃ redshift accordingly. When h₁ is larger than 100 nm, the cavity of the SiN_x grating is formed, and its cavity resonates as line A₁ redshifts. To avoid multiple resonant peaks in the reflective spectrum, the photoresist depth is optimized as 100 nm, which is demonstrated with white dashed lines in Fig. 2(a). Figure 2(b) shows that the band width of the guided mode resonance increases greatly with the Ag depth over its skin depth, and side peaks emerge from the Mie resonance with the Ag depth greater than 20 nm. Therefore, the silver depth is chosen to be 20 nm considering high reflective efficiency and fewer resonant peaks. It also indicates that the resonant bandwidth can be further optimized by utilizing proper metals. Figure 2(c) demonstrates that cavity resonances denoted by line A₁ appear with SiN_x grating depths larger than 135 nm, in addition to those depicted with line A₂ and line B, which are excited by guided-mode resonance and cavity resonance in the photoresist grating. Thus, the SiN_x grating depth h₃ is chosen to be 100 nm considering the balance between the efficiency strength and less resonant peaks. Therefore, its reflection is shown as the spectrum in Fig. 2(d) with optimal structural parameters of h₁ = 120 nm, h₂ = 20 nm and h₃ = 100 nm. It can be observed that a reflective efficiency of approximately 80% is achieved at a resonant wavelength of 560 nm, which is represented by the middle triangle both in Fig. 2(a) and Fig. 2(c).

 

Fig. 2. Reflection and electric-field distributions for the proposed structure. Simulated reflective spectra for s-polarized light when (a) h₁ changes from 50 nm to 200 nm, (b) h₂ increases from 0 nm to 100 nm, and (c) h₃ varies from 0 nm to 200 nm. The inset is the reflective spectrum for the structure with optimal parameters. (d) Reflective spectrum of the metasurface with optimal structural parameters for s-polarized light.

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To insight the physical mechanism of the resonance, the electrical field distributions as well as Poynting vectors at a wavelength of 560 nm within one period are analyzed. Figure 3(a) shows that for the metasurface with optimal structural parameters, the electrical field are mainly distributed inside the ridge of the SiN_x gratings and the cavity between the ridges. It is obviously that guided-mode resonance as well as cavity resonance is excited, and the Poynting vectors indicates that guided-mode resonance plays predominant role. To further explore the role of these resonances, the electrical field distributions and Poynting vectors at wavelength of 560 nm are discussed with h₁ of 60 nm and 150 nm, which are depicted as top and bottom triangles in Fig. 2 (a). When h₁ is 60 nm, Fig. 3(b) displays that most electrical field are bounded inside the SiN_x film as well as the valley of the metasurface, and the electric field inside the SiN_x film is partially excited as the guide-mode resonance, just separating away with the electrical filed inside the valley of the metasurface. The flow of Poynting vectors verify that the excitation of the guided-mode causes the reflection. When h₁ is 150 nm, Fig. 3(c) shows that the electrical field and the Poynting vectors perform little difference compared to the ones in Fig. 3(a), where the field inside the cavity contributes less to the output field and much more Poynting vectors flow out from the ridge of the grating. Figure 3(a)-(c) indicates that the performance of these two kinds of resonance is changed with different value of h₁.

 

Fig. 3. Electric-field distributions and their pointing vectors at wavelength of 560 nm for the proposed structure with (a) h₁ of 120 nm, (b) h₁ of 60 nm and (c) h₁ of 150 nm, depicted as triangles in Fig. 2(a) as well as (d) h₃ of 80 nm and (e) h₃ of 150 nm, denoted by triangles in Fig. 2(c).

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The electric field distributions and Poynting vectors at wavelength of 560 nm are analyzed with h₃ of 80 nm, 100 nm and 150 nm, depicted as triangles in Fig. 2(c). When h₃ is 80 nm, Fig. 3(d) demonstrates that only guided-mode resonance is excited, and little Poynting vectors flow out of the ridge of the SiN_x grating. When h₃ is 150 nm, Fig. 3(e) shows that most of cavity resonance is excited while the guide mode field is mainly bounded inside the grating ridges. Figure 3(a), (d) and (e) indicate that the excitation intensity of guided-mode resonance and cavity resonance is different with the change of the SiN_x grating height h₃. Therefore, both h₁ and h₃ can be utilized to manipulate guided-mode resonance and cavity resonance to get different reflections.

Figure 4 demonstrates its reflections with the structural period changes in the range of 250 nm-550 nm. As shown in Fig. 4(a), its resonant wavelength redshifts across the entire visible range with an efficiency larger than 60%. Furthermore, the reflections for grating periods from 250 nm to 550 nm in steps of 10 nm are presented with points in an International Commission on Illumination (CIE) 1931 chromaticity diagram, as shown in Fig. 4(b). It can be observed from Fig. 4(b) that a large color gamut is achieved with full reflective spectrum colors, and red, green, and blue colors are obtained with the black-crossed marks. That is, the proposed structure can be utilized to realize three primary colors, red, green and blue, with structural periods of 320 nm, 400 nm and 500 nm.

 

Fig. 4. Simulated reflections for the proposed structure with varying periods of 250 nm - 550 nm. (a) Simulated spectra of the structure with different periods for s-polarizations. (b) The simulated colors dot in CIE 1931 with structural periods in the range of 250 nm - 550 nm in steps of 10 nm.

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3. Experimental samples

As demonstrated in Fig. 5, the proposed structure can be produced with the following steps. First, a 120-nm photoresist layer (AZ 4620, Suzhou Zhongxinqiheng Co., Ltd) is coated on a quartz substrate by spinning. Second, a one-dimensional grating is fabricated in photoresist with interference photolithography incorporating a scalable Fourier transform system [39]. Then, it is immersed in a 6‰ NaOH solution for 5 seconds to develop pixelated photoresist gratings. Third, 20 nm thick Ag is selectively deposited on the ridge and valley of the dielectric grating with an E-beam evaporation tool (EBE-07, operating at 300 °C). Finally, the SiN_x is deposited to form a 100 nm thick layer with inductively coupled plasma chemical vapour deposition (ICPCVD 380). Therefore, sample with pixelated one-dimensional gratings is produced, as shown in Fig. 5(b), and the profile of its comprising structure is demonstrated in Fig. 5(c).

 

Fig. 5. The fabrication schematic and its related SEM images. (a) The fabrication process, comprising spin-coating of the photoresist layer, fabrication of the photoresist gratings, and deposition of a 20 nm thick Ag film and a 100 nm thick SiN_x layer. (b) The SEM images for the pixelated gratings. (c) The SEM image of the sample filled in the pixel.

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The reflective spectra of these fabricated structures are measured by a LAMBDA 750 spectrophotometer, which works at an incident angle of 8°. Figure 6 illustrates the corresponding reflection spectra for the structures with periods of 320 nm, 400 nm and 540 nm. Figure 6(a) shows the simulated results with an incident angle of 8°. Compared to the reflection at normal incidence, the reflective spectra split into two peaks and broadens the bandwidth due to guided mode resonance. Figure 6(b) is the measured result for the structures with the same period. It is obvious that the reflections perform as broadband filters, their peaks are coincident with the simulated ones, and the peak efficiency is approximately 60% with a period of 320 nm. Furthermore, the reflections for both simulations and experiments are presented as points in the CIE-1931 chromaticity diagram in Fig. 6(c), and it can be found that their corresponding colors are almost the same. Although the bandwidth is broaden with the inclined incidence, it can be observed that the principle color of red, green and blue can be achieved by the proposed structures. In addition, the developed metasurfaces have measured maximum and minimum peak efficiencies of ∼60% and ∼50%, respectively. While for the large-area aluminum nanostructures [12], its maximum and minimum peak efficiencies are ∼60% and ∼40%. Therefore, they have similar performance in energy efficiency.

 

Fig. 6. Simulated and measured reflections with incident angles of 8° for samples with periods of 320 nm, 400 nm and 540 nm. (a) Simulated spectra. (a) Measured spectra. (c) Colours dot in CIE 1931 for theoretical (black points) and experimental (red points).

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To demonstrate the feasibility of this meta-surface for structural color printing, we printed large-area images, as shown in Fig. 7. Figure 7(a) is a 10 mm × 30 mm rainbow stripe taken by a mobile phone with s-polarized light, which consists of the metasurfaces with structural period varies from 250 nm to 550 nm in step of ∼1 nm. It can be observed that the stripe displays colors covering the entire visible range, including the three primary RGB colors. Figure 7(b) shows Soochow university logo taken by a mobile phone, which consist of metasurfaces with three periods, and it has a much larger area of 30 mm × 30 mm.

 

Fig. 7. The colors taken by mobile phone with s-polarizer for samples as (a) a metasurface stripe with periods of 250-550 nm in steps of ∼ 1 nm, and (b) a Soochow university logo realized with developed structures of three periods. The scar bar is 10 mm.

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4. Conclusion

In summary, we present a sandwiched structure incorporating a dielectric grating, a complementary silver grating and a SiNx grating. With guided mode resonance and cavity resonance, full-spectrum colors can be generated by merely varying the period of the meta-surface. The related experimental samples incorporating pixelated meta-surfaces are fabricated in an area of ∼ 10 mm × 30 mm, where the structural period varies from 250 nm to 550 nm in steps of ∼1 nm. These samples demonstrate full-color reflection with red, green and blue colors included, which has a peak efficiency of ∼ 60%. Finally, the pixelated Soochow University logo with metasurfaces of three structural periods is fabricated in a larger area of ∼ 30 mm × 30 mm, which shows great potential for a large-area color display.