Unlike pigment-based color, plasmonic color is created when light reflects and interferes with fine arrays of metallic nanostructures, which confine electrons and force them to resonate at frequencies specific to the desired color. It can be found in the security holograms we routinely see behind our credit cards, where a chaotic pattern of tiny dots under a microscope produces a floating image with faint colors. While plasmonic holograms avoid problems such as toxicity and fading associated with physical dyes, the limited range and low vibrancy of plasmonic color have prevented widespread adoption.
But new research at A*STAR could one day make those colors more vivid and bright, thanks to the research conducted by Jinghua Teng, a Principal Scientist at the Institute of Materials Research and Engineering (IMRE), in collaboration with Cheng-Wei Qiu and Aaron Danner from the National University of Singapore, and Joel Yang from the Singapore University of Technology and Design. Together, they have created shallow silver nano-disks for plasmonic color with fewer manufacturing steps.
“Usually, there are several steps involved in producing these disks. We first cover the silver with a protective photoresist pattern, and then etch away the uncovered silver, before finally removing the photoresist to give the desired disks,” Teng explained.
However, when Menghua Jiang, a PhD student in Teng's lab inspected the photoresist-patterned silver under cross-polarized light, he found that it reflected vivid colors—even before etching had been performed. Further experiments and computer modeling verified that stopping halfway had created photoresist-on-silver surface plasmon resonances, with less than a five percent spread in the wavelengths of light reflected.
Varying the size and spacing of the photoresist ellipses allowed the researchers to produce a wide range of colors, with enough brightness to replicate the entire color output of a standard RGB monitor and 98 percent of all colors produced by the latest ultra-high-definition TVs.
“Our results have shown, for the first time, that plasmonic coloration offers a range and depth of color matching—and even outclassing—competing nanotechnology based designs,” Teng said. “Furthermore, our simple design and unique etch-free technique allow for much easier and cheaper manufacturing.”
Teng envisions that this technology could see a variety of applications, such as anti-counterfeiting tags in bnk notes and luxury goods, plasmon-based biosensors and high-resolution displays. His team will continue to develop tunable plasmonic colors for use in dynamic displays, as well as explore other design structures for color encryption.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).