Metasurfaces have the capability to boost the generation of distinct colors by improving the interaction between surface materials and photons. We present a straightforward and polarization-tunable aluminum nitride metasurface. This metasurface enables the display and concealment of the composition pattern and exhibits color switching by modifying the polarization state of the incident light. We further explore the impact of different substrates on metasurface performance. The results indicate that the full width at half maximum of the reflectance curves increases with the increase of the refractive index between the nanostructures and the substrate material, which leads to a broader dispersion of the structural colors in the spectrum. Moreover, the sizes of the nanostructures can be gradually reduced for the substrate with a high refractive index. These findings not only offer polarization-tunable structural color metasurfaces but also provide essential insights in selecting nanostructure and substrate materials, which will help in the design of nanostructures for such metasurfaces.
I. INTRODUCTION
Color plays a crucial role in human perception of the world and the communication of information. In the age of rapid information progress, there is an increasing need for a wide range of vivid and dynamic colors. Conventional dyes and pigments commonly used for color production often exhibit limitations, including poor stability and uneven color distribution. In contrast, metasurfaces, which are nano-optical devices made of artificial nanostructures, can modify the subwavelength spectrum and produce vivid structural colors by enhancing the interaction between materials and photons. The devices not only overcome these drawbacks of the conventional dyes and also provide advantages such as environmental friendliness, non-toxicity, precise color rendering, and superior color quality.1–6 These benefits open up new opportunities for color fabrication and application. Therefore, metasurfaces have been extensively studied.
Initially, the high-resolution color rendering can be generated through the arrayed metal nanostructures metasurfaces within the wide subwavelength spectrum. However, due to the inherent ohmic losses in metals, the plasmon resonance effect7–9 that determines the colors is relatively inefficient, resulting in diminished color saturation and narrow color purity of the generated structural colors. To address this issue, researchers have proposed using dielectric materials with low loss and high refractive index, such as Si,10–12 TiO2,13–15 and Si3N4.16,17 These metasurfaces utilize the Mie lattice resonance effect18–20 to produce higher and purity saturation structure colors.
With nanomanufacturing technology advances, more high-performance metasurfaces with varied capabilities are being designed and fabricated.21–28 Among these investigations, the metasurfaces that have the capability of dynamic display with polarization switching have emerged as a new research focus due to the wide-ranging applications in anti-counterfeiting and encryption. For example, Chen et al.29 achieved dynamic display metasurfaces capable of polarization switching by meticulously designing Si nanoblocks on a quartz substrate, which can encode color information into the wavelength dependent polarization profile of a light beam. Similarly, Wang et al.30 developed dynamic display metasurfaces by crafting Si3N4 nanoblocks on a SiO2 substrate, which has the polarization-switching capabilities to turn on and off full-color images by changing the light polarization.
In these studies, researchers paired the higher indices materials with the low refractive indices substrates to achieve the desired functionalities. The selection of surface and substrate materials often relies more on the researchers’ experience and the current processing technology. However, the choice of substrate can significantly restrict the properties of the metasurface. To the best of our knowledge, detailed studies on substrates for the polarization-switching metasurfaces are still relatively scarce. Moreover, tunable permittivity control31 is critical in some metasurface applications, such as thermal metasurfaces.32 Recently, Nolen et al.33 have developed a thermal metasurface that adeptly modulates the phase and polarization of thermal radiation. Their findings indicate that variations in the substrate's refractive index significantly affect thermal emissivity intensity and operational range. It can be seen that the refractive index of substrate, has an important influence on the realization of the metasurface functionality. Therefore, our work focuses not only on the achievement of polarization-switching metasurfaces but also on the effects of substrate materials with different refractive indices.
For the surface, aluminum nitride (AlN), a new generation semiconductor material known for its excellent transparency in the ultraviolet and visible spectrum. High-aspect-ratio subwavelength nanostructures of AlN have been fabricated by either a dedicated reactive ion etch (RIE) process34 or a resist-based Damascene process incorporating low-temperature atomic layer deposition (ALD).35 Moreover, numerical studies36 and experimental37 demonstrations show the use of AlN metasurfaces for ultraviolet applications, as well as AlN nanofilms that display coloration within the visible light spectrum.38
Specifically speaking, we first propose polarization-sensitive metasurfaces by laying AlN nanostructures on CeF3 substrates to generate full-color structural colors. The simulations show that the nano-rectangular AlN-structured metasurface will reflect a color when the polarization of the light coming in is parallel to the long axis. However, when the polarization of the light coming in is parallel to the short axis, the metasurface will not reflect any color because its reflection is almost zero. The nano-square AlN-structured metasurface shows color-switching effects with the change of polarization states and the sharp and highly efficient reflectance peaks that are completely spectrally separated under different polarized incident lights. We further investigate the impact of different substrate materials. The simulation results reveal that with increasing refractive index disparity between the substrate and nanostructural materials, the color gamut displayed broadens, and it is possible to fabricate nanostructures on the metasurface at progressively smaller scales. Our study introduced a metasurface that demonstrated remarkable sensitivity to linear polarization, making it highly valuable for applications such as color displays, data storage, and anti-counterfeiting technologies. It also provides valuable insights that can significantly aid researchers in selecting appropriate materials for substrates and nanostructures during the design of dielectric metasurfaces.
II. DESIGN AND SIMULATION OF METASURFACES
A schematic of the proposed nano-rectangular AlN metasurface, composed of numerous basic units, is shown in Fig. 1(a). Each basic unit contained two parts: a CeF3 substrate at the bottom and an AlN nanoparticle at the top, as shown in Fig. 1(b). The dimensions of the AlN nanoparticle are specified as follows: length (L), width (W), and height (H), and the separation distance between two nanostructures is denoted as P. The structure was illuminated from the top by x- and y-polarized light, as shown in Fig. 1(c). The schematic of the nanosquare metasurfaces and the single cell of the nanosquare metasurfaces are shown in Fig. 1(d). The dimensions of the AlN nanoparticle are specified as follows: length (L) and height (H), and Px and Py are the distances between two nanostructures in the x and y directions, respectively.
The CeF3 is a scintillator with excellent magneto-optical properties and can be used as a new generation of ultraviolet laser working media. Its refractive index is much lower than that of AlN, which is between SiO239 and Al2O3.40 Therefore, based on the current experience, the combination of these two materials forms a metasurface that is expected to have polarization sensitivity and the ability to selectively reflect colors. We performed simulations of metasurfaces using the commercial software COMSOL Solutions. We obtained the refractive index of AlN from Pastrňák41 and the refractive index of CeF3 from Rodríguez-de Marcos.42 For normal incidence, periodic boundary conditions be used in the x- and y-directions to model infinite arrays and a perfect match layer in the z-direction to mimic free space. The environment is air. An incident plane wave port is provided at the top of the model. The remainder of the analysis was conducted using MATLAB.
III. RESULTS AND THEORETICAL ANALYSIS
The reflectance for the polarization of the light parallel to the long-axis is shown in Fig. 2(a). A super narrow peak was observed. By contrast, it is evident that the reflectance exhibits a stark contrast when the light’s polarization aligns with the short axis. The near-zero reflection can be observed. To illustrate the potential of metasurfaces for selective color rendering in different polarization states, we created a simple submarine image as shown in Fig. 2(b). The vivid blue color is calculated from spectral data of metasurface and color-matching functions, as defined by CIE.43,44
In Fig. 2(B-a), the polarization of x-polarized incident light is paralleled to the metasurface’s long axis; the submarine pattern exhibits a vivid and highly saturated blue color. The pattern disappears when the metasurface is rotated 90° since the light polarization is parallel to the metasurface’s short axis, as shown in Fig. 2(B-b). It is noted that we can re-emerge the pattern when converting the incident x-polarized light to y-polarized light, as shown in Fig. 2(B-c). Furthermore, the submarine pattern can disappear when the metasurface is rotated 90° because its short axis is parallel to the polarization state once more, as shown in Fig. 2(B-d).
Figure 3(a) illustrates the reflectance of light incident from the top onto the nano-rectangular AlN-structured metasurface, with its polarization parallel to the long axis. A super-narrow reflectance peak is observed at a wavelength of 578 nm. In Fig. 3(b), we present the resonance curve obtained through the multi-polarization unfolding of the scattering interface. Among these resonances, it is evident that the MD resonance is stronger than other resonances. Therefore, this narrow reflection peak is dependent on the coupling interaction between the strong MD resonance and other resonances. Moreover, the characteristics of this peak are heavily reliant on the size and shape of the subwavelength nanostructure. Since local modes are functions of certain locations, highlighting the independence of optical responses at a single meta-atom or metamolecule from neighboring unit cells, including LSPRs, Mie resonances, and defect modes with a subwavelength mode volume and negligible neighboring interactions.47 Therefore, this confirms that the peak from a Mie resonance is caused by local lattice resonances rather than a non-local high-Q Fano resonance effect.48–50 In addition, we also show the distribution of electric and magnetic fields at the peak position. In Figs. 3(c) and 3(d). The distinctive ring-shaped electric field distribution extending from the nanoparticle to the substrate is characteristic of the ED resonance. The magnetic field distribution of MD resonance is shown in Fig. 3(d). For comparison, Fig. 3(e) shows the near-zero reflection spectrum when the incident light’s polarization direction aligns with the short axis. It indicates that the reason for this situation is that these resonances are not effectively excited, as shown in Fig. 3(f).
These findings suggest that the metasurfaces exhibit extremely high polarization sensitivity, which can selectively color by changing the light polarization state. They can be used for image hiding and display and offer superior performance in optical encryption. However, its storage capacity of encrypted information for the nano-rectangular AlN metasurface is limited due to the monochromatic tuning capability. To improve the information content and achieve multi-color tunability, we additionally propose a method to achieve the goal.
We present the reflectance of the nanosquare AlN-structured metasurface (Px = 330 nm, Py = 270 nm, and L = 200 nm) for the x and y-polarized incident light, as shown in Fig. 4(a). It is evident that the two independent reflection peaks with different colors are generated by the metasurface. This character can enhance the information storage capacity of the metasurface. As shown in Fig. 4(b), the simulation barque pattern appears blue and cyan under the two polarized incident lights, respectively. In solving crosstalk problems due to low color saturation and low brightness, this structure is less complicated than the traditional nanocross structure,51 which decreases the demands on fabrication precision. Therefore, there is significant potential in a variety of applications like optical filters, wavelength division multiplexing devices, and multi-channel devices.
In addition, the multi-color tunable performance of the nanosquare AlN-structured metasurface was demonstrated by simulating the reflection spectra of the metasurface under x-polarization and y-polarization. Figure 5(a) shows that when the edge length Px in the x-direction increases from 270 to 350 nm, there is almost a small change in the reflection peak under y-polarization. In contrast, the reflection peak under x-polarization shifts by 100 nm with the Px length increase, as shown in Fig. 5(b). It can also achieve the same effect of reflection peak shifting under y-polarization and barely shifting under x-polarization by adjusting Py in the y-direction. The nanosquare AlN-structured metasurface exhibits different reflection colors under x and y polarization with high independence and displays high-quality and highly saturated colors.
To further investigate the substrate’s impact on metasurfaces, we replaced the CeF3 substrate with SiO2 and Al2O3 substrates and redesigned the polarization-sensitive structural color metasurfaces. The basic units of the three metasurfaces are shown in Figs. 6(a)–6(c). Here, the average refractive indices of SiO2, CeF3, and Al2O3 in the wavelength range of 380–730 nm are 1.46, 1.62, and 1.78, respectively, with average refractive index differences compared to AlN in this range of 0.72, 0.56, and 0.4. P is the spacing between the nanostructures. It can be observed that the volume of the nanostructures in the three metasurfaces decreases with the increasing of the difference in refractive index between the substrate material and the nanostructure material. Figures 6(d)–6(f) show the dynamic color display capabilities of the three metasurfaces, with the spacing between the nanostructures ranging from 265 to 400 nm. Figures 6(g)–6(i) Chromatic diagrams showing simulated colors in ClE 1931 color space for panels (d), (e), and (f), respectively. It is evident that the color gamut of the metasurface with the Al2O3 substrate is smaller than that with the CeF3 substrate, and the color gamut of the metasurface with the CeF3 substrate is smaller than that with the SiO2 substrate.
The reflectance and multipole expansion were analyzed to demonstrate the mechanism of the metasurfaces on three different substrates when subjected to incident light with polarization parallel to the long axis of the nanoblock. The reflectance is shown in Figs. 7(a), 7(d), and 7(g), respectively.
The multipole decomposition of the scattering cross-section for ED, MD, EQ, MQ, and TD is illustrated in Figs. 7(b), 7(e), and 7(h), respectively. Notably, the observed reflection peak corresponds to a significantly stronger MD resonance compared to other resonances, and the full width at half maximum (FWHM) of the MD resonance for the SiO2 substrate is larger than that for the other two substrates. The corresponding phases of MD and ED are depicted in Figs. 7(c), 7(f), and 7(i). At the MD resonance peak, the phase difference between the MD and ED resonances is 1.5π, and their phase magnitudes are not equal, which does not strictly conform to the second Kerker’s condition52 (where MD and ED have equal amplitude and a phase difference of π). Therefore, the reflection peak primarily results from the combined resonance effects dominated by the MD resonance. In addition, the FWHM of the MD resonance in the SiO2 substrate is higher than that in the CeF3 substrate, which in turn is higher than that in the Al2O3 substrate. This results in the reflectance FWHM following the same trend and is reflected in the color gamut: the SiO2 substrate’s metasurface exhibits a broader color gamut compared to the CeF3 substrate, which in turn has a broader gamut than the Al2O3 substrate.
Most notably, the trend in the FWHM of the reflectance peaks correlates with the refractive index difference between the substrate and its surface nanostructures. The FWHM of the reflectance peaks increases as the refractive index difference enlarges. When the refractive index difference between the substrate and its nanostructures is substantial, the metasurface can more readily produce a broader color gamut.
To demonstrate that this phenomenon is not due to variations in the dimensions of the nanostructures but rather differences in the substrates, Fig. 8 presents the reflectance curves, multipole decomposition curves of the scattering cross-section, and the corresponding phase curves of MD and ED resonances for three metasurfaces using nanoparticles of identical dimensions. For the same-size nanoparticles, it is evident that the FWHM of the MD resonances and the reflectance peaks of the three metasurfaces still exhibit a positive correlation with the refractive index difference between the substrate and its surface nanostructures. Furthermore, the primary cause of the reflection peak remains to be the combined resonance effects, mostly driven by the MD resonance.
IV. CONCLUSIONS
In conclusion, we first presented a metasurface with high-efficiency and polarization sensitivity comprising AlN nanoparticles on a CeF3 substrate. This metasurface yields high-efficiency structural color through the MD resonance from the Mie lattice resonance and achieves color with its inherent polarization-sensitive capabilities. Second, we also investigated the impact of various substrate refractive indices on metasurface performance. The simulation results show that as the difference in refractive index between the substrate and nanostructural materials grows, the strong MD resonance excited by the nanoparticles on different substrates gradually widens, ultimately broadening the color gamut. Additionally, it is possible to fabricate nanostructures on the metasurface with dynamic polarization responsiveness at progressively smaller scales. The optical properties of these metasurfaces suggest promising applications in fields such as full-color imaging, information encoding, high-density optical data storage, and optical encryption, and these findings also provide valuable insights that can significantly aid researchers in selecting appropriate materials for substrates and nanostructures during the design of dielectric metasurfaces.
ACKNOWLEDGMENTS
This work was partly supported by the National Natural Science Foundation of China (Grant No. 11604205).
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
Jiujiang Wang: Data curation (equal); Software (equal). Chenhui Lu: Funding acquisition (equal); Investigation (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal). Jiao Geng: Data curation (equal); Investigation (equal); Validation (equal); Visualization (equal). Liping Shi: Conceptualization (equal); Data curation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal).
DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author upon reasonable request.