All-dielectric metasurface for high-performance structural color

Structural color, arising from light-matter interaction, offers advantages over pigment-based colors in terms of gamut, saturation, brightness, and resolution. While plasmonic and dielectric nanostructures have been explored for structural color generation, existing approaches struggle to simultaneously achieve all these desirable characteristics. Pigments and dyes, commonly used for color production, suffer from limitations in brightness, gamut, and resolution. Plasmonic structural color, while achieving subwavelength resolution, has limitations in intensity and gamut. All-dielectric nanostructures like TiO2 metasurfaces produce vibrant colors but lack high spatial resolution and have limited gamuts. This research aims to address these limitations by proposing and demonstrating an all-in-one solution for structural color using a silicon metasurface and a refractive index matching layer, targeting high brightness, distinct colors, diffraction-limited resolution, and a wide color gamut.

The authors reviewed existing technologies for structural color generation, highlighting the advantages and disadvantages of plasmonic and all-dielectric approaches. Plasmonic methods, utilizing the interaction of light with metallic nanostructures, offer high resolution but often suffer from low intensity and limited color gamut (around 45% of sRGB). All-dielectric methods, employing dielectric materials, provide brighter colors and wider gamuts but typically compromise on spatial resolution. The limitations of both approaches are summarized using five key parameters: manufacturability, reflectance, full width at half maximum (FWHM) ratio, spatial resolution, and gamut area. Existing research demonstrates that no single method simultaneously optimizes all five parameters, motivating the present work to improve structural color.

The researchers utilized silicon metasurfaces, composed of silicon nanodisks on a sapphire substrate, as the foundation for their structural color. The size and spacing of the nanodisks were varied to control the reflected wavelengths and thus the color. Numerical simulations, using Lumerical FDTD Solutions and COMSOL Multiphysics software, were conducted to predict the reflection spectra for different nanodisk designs. The simulations explored the electric dipole (ED) and magnetic dipole (MD) resonances of the nanodisks and their impact on the reflected light. To improve color purity and brightness, a refractive index matching layer (n=1.48) of dimethyl sulfoxide (DMSO) was added. The simulations predicted a substantial improvement in the color gamut with the index matching layer. The metasurfaces were fabricated using electron beam lithography and reactive ion etching on a silicon-on-sapphire (SOS) wafer. The fabricated structures were characterized using scanning electron microscopy (SEM) and bright-field optical microscopy. Reflection spectra were measured using a spectrometer with a ×50 objective lens. The impact of the refractive index matching layer on the color gamut and resolution was experimentally investigated by infiltrating the metasurfaces with DMSO. Experiments were also done to demonstrate the resolution of the approach by creating patterns of “Phoenix” and “Rainbow”, with varying pixel sizes, down to the diffraction limit.

The key findings of this research are as follows: 1. **Improved Color Gamut:** The introduction of the refractive index matching layer dramatically increased the color gamut. Numerical simulations predicted a gamut of approximately 186% of sRGB, 138.7% of Adobe RGB, and 99.5% of Rec.2020. Experimental results confirmed a significant improvement, reaching approximately 181.8% of sRGB, 135.6% of Adobe RGB, and 97.2% of Rec.2020, a substantial enhancement over previous all-dielectric approaches. 2. **High Reflectance and Narrow FWHM:** The silicon metasurfaces with the index matching layer exhibited high reflectance (76% at 600 nm) and narrow full width at half maximum (FWHM) values (~34 nm), contributing to the vibrant and saturated colors. 3. **Diffraction-Limited Resolution:** The researchers demonstrated the ability to produce high-resolution images with diffraction-limited resolution. This capability was confirmed by fabricating and imaging patterns with varying pixel sizes composed of 2x2 to 9x9 arrays of nanodisks, without noticeable color degradation. 4. **Dynamic Color Switching:** The use of a liquid refractive index matching layer (DMSO) demonstrated the potential for dynamic color switching. The color of the metasurfaces changed upon infiltration with DMSO, indicating potential for use in dynamic display applications. The use of PMMA as a solid-state index matching layer further proved the versatility of the approach. 5. **CMOS Compatibility:** The silicon-based metasurfaces are inherently compatible with complementary metal-oxide-semiconductor (CMOS) fabrication technologies, offering advantages for mass production and cost-effectiveness. This is a significant step toward commercialization.

This research successfully demonstrates a high-performance structural color solution that overcomes many limitations of previous approaches. The combination of a silicon metasurface with a refractive index matching layer significantly enhanced color gamut, brightness, and resolution. The achievement of a gamut approaching Rec.2020 is a significant advancement in the field, surpassing previous all-dielectric metasurface approaches. The diffraction-limited resolution opens up new possibilities for high-density color displays and other applications. The CMOS compatibility ensures scalability and cost-effectiveness, bringing commercialization within reach. This technology could have significant implications for applications requiring high-quality structural color, such as dynamic displays, optical security features, and high-resolution imaging.

The study successfully demonstrated a high-performance structural color using silicon metasurfaces and a refractive index matching layer. The combination achieved a record-high color gamut, high reflectance, narrow spectral linewidth, and diffraction-limited resolution. The CMOS compatibility of the silicon material makes this approach promising for mass production and diverse applications. Future work could explore alternative refractive index matching materials and further optimization of the nanostructure design for even better performance.

While the study achieved impressive results, some limitations exist. The experimental results slightly underperformed the simulation results, which could be due to minor imperfections in the fabrication process or variations in the refractive index of the DMSO. Further research could investigate the long-term stability of the color under different environmental conditions. The current study focuses on reflection; future exploration could investigate transmission-mode structural colors using similar designs.