Researchers have demonstrated precise color control using monocrystalline silicon. Their work exploits the resonance of silicon materials to enable high reflectance and purity of color. A research team from Osaka University developed a silicon metamaterial surface that allows for color printing with diffraction-limited resolution. The metamaterial arrays feature nanoscale patterns that function as antennae for converting optical radiation into localized energy. A bright-field optical microscope image of the Si nanostructure arrays. Si nanostructures of different sizes exhibit distinct reflection colors (scale bar is 20 μm). Courtesy of Takahara et al. The team used electron beam lithography to create masks. The purpose of the masks was to protect the silicon surface from subsequent plasma etching. Researchers demonstrated the ability to generate vivid colors controlled completely by the geometry of the antennae. The nanostructures made from monocrystalline silicon exhibited various colors depending on how the physical dimensions of each antenna were tuned. The team also demonstrated white light generation. Each pixel was characterized by two-color information and this information could be accessed by changing the polarization of the incident light. The dual-color properties of the pixels could potentially be used to create overlaid images as well as to maximize the information encoded into a particular area of the array. The pixels generated individual color even if operating as a single element, which facilitated subwavelength-resolution encoding without color mixing. To demonstrate subwavelength resolution, researchers generated a clearly discernable yellow and blue checkerboard pattern within unit areas of just 300 × 300 nm. These results indicate that an application based on the technology could print at about 85,000 dpi. Demonstration of a subwavelength pixel. Scanning ion (a) and optical microscope images (b) of a checkered pattern consisting of alternating nanoblocks of two different sizes. Scanning ion (c) and optical microscope images (d) of the letters RGB by means of Si nanostructures generating the corresponding color (scale bar is 2 μm). Courtesy of Takahara et al. “Our work reveals the high degree of precision possible through etching monocrystalline silicon,” said researcher Yusuke Nagasaki. “The agreement between the calculated and experimental reflectance values for our system also supports our confidence in the robust nature of the technique we created.” Until now, the metamaterials used to create tunable color from structural geometry have been based on metals. Although effective in achieving high resolutions, metallic materials suffer from inherent energy losses at visible wavelengths, which makes optimizing color purity with metals challenging. “The use of silicon allows us to achieve both high resolution and high saturation,” researcher Junichi Takahara said. “All-dielectric materials that can produce individual color pixels with high resolution, without color mixing, offer distinct advantages over metallic materials.” The Osaka team’s printing strategy could be used to further extend the degree of freedom in structural color design. The work also shows potential for use in anticounterfeiting technology and advanced display technology such as 3D displays. The research was published in Nano Letters (doi: 10.1021/acs.nanolett.7b03421).