More red is not possible: Physicists have developed a nanostructure that produces the most intense red that can be made with structural colors. They applied a principle already described by physicist Erwin Schrödinger. Such pure red is impossible in nature because optical resonance effects always produce a bluish mixture. Only the special arrangement of the small pillars in the nanostructure prevents these disruptive effects.
Be it bird feathers, butterfly scales, or beetle shell flicker: many colors in nature do not depend on pigments, but on Chassis colors. Microstructures break up the incident light in such a way that the impression of color and often iridescence is created. However, this principle reaches its limits when it comes to a single color: an intense pure red, such a saturated brown red without a chromatic hue that does not occur in nature.
“Even the red feathers of parrots appear more purple because they have larger blue and green parts in their reflection spectrum,” Zhaogang Dong of the National University of Singapore and colleagues explain. The scales of some beetles shimmer in all the colors of the rainbow, but not pure red.
Schrödinger’s recipe for the purest red color
but why? The famous physicist Erwin Schrödinger asked himself this question. He discovered that saturated red could only be produced if the material completely reflected all light with wavelengths longer than 600 nanometers, but completely absorbed all light with shorter wavelengths. Only then are the resonance effects suppressed, which otherwise results in the emission of bluish-green light components of the so-called higher modes – which otherwise fakes pure red to purple.
But this color, nicknamed “Schrodinger’s red pixel”, is difficult to achieve. “Even with artificial and natural colors that rely on light absorption, such highly saturated red tones are rare,” the researchers explained. Until now, such a pure red color could only be achieved by direct emission of red light, for example using LEDs or lasers.
Oval silicon columns as color pickers
But that has now changed. Dong and his team have now developed a nanostructure that implements Schrödinger’s recipe and thus yields a pure red structural color. The structure consists of a quartz base plate in which several elliptical columns of amorphous silicon are arranged. The axes of these pillars are tilted against each other in a special way to enable the optimum combination of light absorption and refraction. The team previously determined the right angles using a simulation model.
In order to prevent unwanted interference from reflections from the quartz base plate, the researchers also covered the back with a jet black light-absorbing layer. Combined with the vertical structure, this results in a sharp separation of the reflection and absorption at full resolution at 600 nanometers, which is where the “red pixel” of Schrödinger is required, the team explains.
New record for red saturation
Indeed: the measurements showed that the nanostructure emits two precisely defined maximum reflectance, both of which lie exactly in the red band defined by Schrödinger. However, in all other wavelength ranges, light is completely absorbed. This also eliminates the resonance effects that normally occur in the blue-green range. “To our knowledge, the result represents the highest red saturation ever achieved with a nanostructure,” the team wrote.
The result is an intense red light whose color values are 0.654 and 0.301 in the RGB scheme. “This red surpasses all previously known in the RGB triangle and is more saturated than the cadmium pigment used to create the strong red in Art Nouveau,” Dong and colleagues report. In other words, the nanostructure produces the reddest color to date.
Thus, the researchers made the “red pixel” that Schrödinger drew a reality. “This success in red architecture shows that we can sometimes outperform evolution through model simulation and intelligent nanofabrication,” said senior author Yoel Yang from the Singapore University of Engineering and Design.
According to the researchers, this nanostructure also enables new applications in electronics, optics, and even optical coding. “Producing red color with such high intensity and saturation opens up new possibilities for applications that were previously unattainable in structural colours, such as anti-counterfeiting or new color renderings,” says Dong’s colleague Cheng-Wei Qiu. (Advances of Science, 2022; doi: 10.1126/sciadv.abm4512)
Source: Agency for Science, Technology and Research (A*STAR), Singapore
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