Extracting a richer palette of colours from black
Image credit: Kevin Batchelor | Dreamstime
Scientists have developed a way of extracting a richer palette of colours from the available spectrum by harnessing disordered patterns inspired by nature that would typically be seen as black.
Colours that the human eye see in nature often come from nanoscale patterns that reflect light back in particular ways, experts have said. A butterfly’s wing, for example, might appear blue as tiny grooves in the surface of the wing cause only blue light to be reflected.
When surfaces appear black or white, however, it’s often because the nanoscale structures are completely disordered, causing all the light to be either absorbed or reflected.
To overcome this challenge, a team of researchers, led by the University of Birmingham, have now found a technique to control the way light passes through these disordered surfaces to produce vivid colours.
“The different ways in which nature can produce colour are really fascinating,” said lead researcher Professor Shuang Zhang. “If we can harness them effectively, we can open up a treasure trove of richer, more vivid colours than we have yet seen.”
The team, which includes scientists in Ludwig Maximilian University of Munich, Germany, and Nanjing University in China, have compared the method to techniques that artists have exploited for centuries. Among the most famous examples of this is the fourth-century Roman Lycurgus cup, made from glass that appears green when light shines on it from the front, but red when light shines through it from behind.
In a modern advance, the research team demonstrated a way of finely controlling this effect to produce extraordinarily precise colour reproduction.
The different colours in the image are represented in different thicknesses of transparent material – such as glass – on a lithographic plate. On top of this, the researchers deposited the disordered layer – in this case, made of random clusters of gold nanoparticles.
Beneath this layer, the team placed a plasmonic mirror to form a transparent cavity, which is able to trap particles of light, or photons, inside. Here, the photons behave like waves inside the cavity, resonating at different frequencies beneath the lithographic surface and releasing different colours according to the length of each wave.
By using this technique, the team was able to reproduce a Chinese watercolour painting with exquisite colour accuracy.
Study co-author Dr Changxu Liu added: “In physics, we’re used to thinking that randomness in nanofabrication is bad, but here we show that randomness can lead to being superior to an ordered structure in some specific applications. Also, the light intensity within the random structures that we produced is really strong – we can use that in other areas of physics such as new kinds of sensing technologies.”
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