Investigation of tunable structural color based on hexagonal boron nitride

Color is one of the most important attributes of human vision [1]. About 85% of the information that enters the human brain comes from the eyes. The most common colors are produced by pigments and dyes [2]. This color is formed by absorbing a specific range of wavelength in visible light, and has disadvantages such as small-gamut, low resolution, easy fading, and environmental pollution. To address these issues, researchers have conducted extensive research into color generation techniques, especially the color produced by structural materials. Compared with dyes and pigments, the main advantage of structural colors is the ability to produce different colors only by adjusting structural parameters [3]. This has aroused great interest among researchers, and the main research focuses on plasmonic structural colors and all-dielectric structural colors. Zhu Xiaolong et al. has used the visible  light absorption of Ge to achieve color printing with a resolution of more than 100,000 dots per inch (DPI) [4]. Xiao Shumin et al. experimentally confirmed that the color gamut area increased from 78% of sRGB in the air to about 181.8% of sRGB by using silicon metasurfaces [2]. However, most of the previous work has focused on implementing static colors, which limits many potential applications [5]. Display technologies that construct colors in a dynamically reconfigurable manner play a vital role in our daily lives [6]. The most straightforward solution for dynamic color display is to turn on or off a single element with a predetermined color. For example, Chen Shuqi et al. fabricated a polarization-sensitive color filter based on elliptical titanium oxide nanopixels on a silica substrate, and realized synchronized adjustment of hue and saturation [7]. But this solution is not conducive to the miniaturization of the display device. The ideal situation is to directly change the dielectric properties of the surrounding environment of the metasurface to obtain different colors. This is more practical than fixing the pixel color in advance. Based on hydrogenation/dehydrogenation kinetics of Mg, Duan Huigao et al. designed a Mg-based pixelated Fabry-Pérot Fabry–Pérot​ cavity to generate dynamic color displays [5]. But the hydrogenation/dehydrogenation process requires a longer response time. Gao Yisheng et al. have experimentally demonstrated an in situ dynamic color display by photon doping, triggered by the interplay of extrinsic structural colors and intrinsic photon emission of lead halide perovskite gratings [8]. However, due to the need for external pump light, the equipment is relatively large. Sun Shang et al. have proposed a microfluidic reconfigurable all-dielectric metasurface and experimentally verified real-time tunable structural colors by injecting different solvents into the microfluidic channel [9]. Although the transition time can be as small as 16 ms, the manufacturing process is complicated. Wang Shaowei et al. reviewed the typical artificial structural colors generated by multilayer films, photonic crystals, and metasurfaces, and discussed the approaches to achieve dynamically tunable structural colors, including the approach of reconfigurable liquid crystals, electrochromic and plasmonic modulation, mechanical stretching, and self-assembled colloidal microspheres [10]. Besides, some emerging two-dimensional (2D) materials can be used not only for transistor design but also for structural color research. Yin Zhigang et al. comprehensively describes the latest developments in atomic layer deposition of metal oxides and chalcogenides with tunable bandgaps, compositions, and nanostructures for the fabrication of high-performance field-effect transistors [11]. These new advances in structural color research and device fabrication give us good inspiration.

If there is a kind of 2D materials whose refractive index can be dynamically changed by means of electrical, magnetic, or mechanical forces, then the in-situ rapidly dynamic color display will be easily realized. Graphene and hexagonal boron nitride (h-BN) are exactly this 2D material. Cai Yijun et al. designed a graphene optical absorber, and enhanced the optical absorbance ratios of single and three atomic layer graphene up to 37.5% and 64.8%, respectively [12]. Zhu Jinfeng et al. used a metasurface composed of silicon substrate, silica, graphene film and Au nanoparticles to research the light absorption of graphene [13]. However, due to the lack of selectivity in response to visible light, graphene only increases the absorption without changing the color of light reflected/transmitted by the metasurface. h-BN consists of alternating boron and nitrogen atoms in a hexagonal lattice [14]. Initially, h-BN was considered as an ideal substrate for graphene, but now due to its unique properties such as natural hyperbolic behavior, it is increasingly investigated for nanophotonic applications. h-BN exhibits strong optical nonlinearity that depends on the thickness [15]. In general, the thickness of h-BN in applications is from a monolayer to hundreds of nanometers. It can offer novel material properties, such as the birefringent properties, which enable a broad range of optical, electro-optical, and quantum optics functionalities. The dielectric properties of h-BN can be altered by applying stress or changing its thickness [16], [17]. Y. Anzai et al. used the colors as a standard for the thickness identification, and theoretically determined the colors of h-BN as a function of the thickness in the standard RGB color space [18]. They obtained the colors mainly through the thin-film interference effect between h-BN flake with different thicknesses and SiO2/Si substrates. Other applications of h-BN in metasurfaces have also been studied [19], [20], but to the best of our knowledge, the application of h-BN in tunable structural color has not been reported.

In this paper, we combine h-BN 2D material with the metal metasurface to investigate the tunable structural color. h-BN is a 2D material with excellent optoelectronic properties. We can adjust the refractive index of h-BN by changing the thickness or applying pressure, thereby tuning its optical response characteristics. In our simulations, the real part of the complex refractive index of h-BN is adjusted by 0.1, the chromaticity coordinate in the x-direction is changed by about 0.025. The extent of change is up to 5%. The simulation results show that adjusting the refractive index of h-BN can cause a significant change in the structural color reflected by the metasurface. The investigation provides a theoretical reference for the experimental research and applications of tunable structural color.

留言 (0)

沒有登入
gif