Scanning ion (top) and optical microscope images of the letters 'RGB' created with silicon nanostructures. Source: Takahara et.al., Nano Letters. 17, 7500-7506.In the world of metamaterials, creating tunable color from structural geometry has been based on metals. As a result, optimizing color purity is a challenge -- although metallic materials are effective for achieving high resolutions, they suffer from inherent energy losses at visible wavelengths.

But silicon metamaterials are a different story.

As published recently in Nano Letters, researchers from Osaka University were able to achieve precise color control using monocrystalline silicon. The all-dielectric material is able to produce individual color pixels with both high resolution and high saturation, and needs no color mixing.

The nanoscale patterns of metamaterial arrays, moreover, function as antennae -- converting optical radiation into localized energy. The team was able to generate vivid colors controlled completely by antennae geometry. They were also able to generate white light, which is important for full-color printing. Two-color information was inherent in each pixel, and could be revealed by changing the polarization of the incident light. This offers possibilities for the creation of overlaid images, along with the maximizing of information encoded into a particular area of the array.

Demonstrating subwavelength resolution by creating a checkerboard pattern within unit areas of 300x300 nanometers, it was determined that eventual applications would see printing resolutions around 85,000 dots per inch (dpi) – an exponential gain over standard print resolutions of 300 dpi.

The team also demonstrated their control of typography, writing the letters "RGB" – the standard additive color model of red, green and blue used for screen displays – into nanoblocks.

"Our work reveals the high degree of precision possible through etching monocrystalline silicon," said study lead author 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."

The research has potential for use in anti-counterfeiting technology and advanced display technology, such as three-dimensional displays.