(Nanowerk Spotlight) The screens we use every day – from smartphones to televisions – work by either filtering white light or creating their own illumination. These approaches have served well but face inherent limitations in displaying rich, vivid colors while keeping power consumption low. Nature offers an intriguing alternative: structural color, which creates vibrant hues through tiny structures that selectively reflect light, similar to how butterfly wings shimmer without pigments.
Think of structural color like a microscopic hall of mirrors that catches and reflects only certain colors of light. This approach is appealing because it can create pure, bright colors using only ambient light, potentially reducing power consumption compared to current displays that must constantly generate their own light.
Scientists have tried combining liquid crystals – materials that can change their optical properties when electricity is applied – with tiny metallic structures to create switchable structural colors. However, these attempts faced a fundamental problem: when liquid crystals come into direct contact with metal surfaces, they become disorganized and don’t function properly. It’s similar to trying to project a crisp image onto a bumpy surface – the roughness distorts the result.
Now researchers at Sun Yat-sen University have developed an innovative solution that overcomes these limitations. Their system, which they call an LC-SNA (Liquid Crystal-Silver Nanogroove Array), keeps the liquid crystals separate from the metal structures that create color, allowing each component to perform its function without interference.
As they report in Advanced Functional Materials (“Liquid-Crystal-Powered Metasurfaces for Electrically and Thermally Switchable Photorealistic Nanoprinting and Optical Security Platform”), the LC-SNA device consists of two main parts. The first is a layer of liquid crystals sandwiched between glass plates – similar to a conventional LCD but specially designed to rotate light polarization with high precision. The second component is an array of tiny silver grooves, each about one-thousandth the width of a human hair. These grooves are engineered to reflect specific colors of light based on their size and spacing. The researchers combined these components in what they call a “cascaded configuration,” where the liquid crystal layer sits atop the silver nanogroove array rather than being in direct contact with it.
Liquid Crystal-Silver Nanogroove Arrays (LC-SNAs) for electrically and thermally switchable structural colorations. a) Operating principle of switchable structural color based on an LC-SNA. b) In a twisted LC cell, the LC director will twist from the top to the bottom at a twisted angle ϕLC to match the boundary conditions (marked as easy axis 1 and easy axis 2). c) Electrically or thermally controlled polar angle 𝜃 as a switch for azimuth angle ϕLC generation. d) Schematics of an SNA. Inset shows a unit cell of the SNA whose geometry can be completely determined by its lattice a, groove width w, and groove depth h. e) Simulated cross-polarization reflection spectra showing the transitions between the bright color states (remarkable reflection peaks) and dark black states (flat curves with ≈0% reflectance) for the red (a = 350 nm, w = 105 nm, and h = 35 nm), green (a = 440 nm, w = 132 nm, and h = 35 nm) and blue (a = 560 nm, w = 168 nm, and h = 35 nm) pixels under electrical or thermal actuation. The structural colors of the pixels can be adjusted by their geometries (a, w, h). f) Schematic diagrams showing electrically or thermally switchable photorealistic nanoprinting of a strawberry image. (Image: Reprinted with permission by Wiely-VCH Verlag) (click on image to enlarge)
When electricity is applied to the liquid crystal layer, it changes how light passes through before reaching the silver grooves. This acts like a master switch controlling whether the grooves reflect their designated colors or not. The separation between layers allows each component to perform its job perfectly without interference.
The results are impressive. The system achieves a switching contrast of approximately 21, meaning the bright state is 21 times more intense than the dark state. While lower than commercial display technologies, this performance exceeds most existing liquid crystal/metasurface-based nanoprinting devices and reflects advancements in metasurface technology. The colors produced are extremely pure because the silver grooves reflect very narrow bands of light – imagine a radio that can tune to exact stations without picking up static from neighboring frequencies.
To create full-color images, the researchers built tiny pixels containing three sub-sections: one that reflects red light, one green, and one blue. By controlling how much each sub-section is switched on or off, they can mix these primary colors to create any desired color. The system packs these pixels so densely that it can fit over 4,000 of them in one inch – for comparison, most current smartphone screens have around 400-500 pixels per inch.
This precise color control enables interesting security applications. The researchers created a QR code that looks normal but won’t scan until activated by electricity or heat. The code contains two types of pixels: some that maintain their appearance when activated and others that change, revealing the complete, scannable pattern only when triggered.
The system can switch states in less than a second and works reliably through hundreds of on-off cycles. However, some technical hurdles remain before it could be used in commercial products. The current version requires higher voltages than existing displays – 30 volts compared to the 5-10 volts used in most portable devices. This means it would need more complex and expensive power management circuits.
Manufacturing the silver groove arrays with consistent quality over large areas presents another challenge. Like making millions of identical tiny prisms, any imperfections can affect color quality. The researchers are working to refine their fabrication process and optimize the liquid crystal design to address these limitations.
Despite these challenges, this work represents an important advance in display technology. By solving the fundamental problem of combining liquid crystals with metallic structures, it opens new possibilities for displays that offer better color reproduction while using less power. The same technology could enable new types of security features and optical sensors.
The approach shows that sometimes the best solution isn’t forcing incompatible components to work together but rather finding clever ways to let them cooperate while staying separate. This principle could guide the development of other hybrid technologies where different materials need to work in harmony without direct contact.
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