Is that hundred-dollar bill you just received real or fake? One way to tell is to check anti-counterfeiting features such as watermarks printed in special inks or hologram stickers, which rely on reflecting visible light.
Metamaterials that not only reflect but can change other properties like the polarization of light could be used to make anti-counterfeiting methods even more secure. However, creating metamaterials that interact with higher frequency ultraviolet (UV) light is difficult.
Instead of more conventional materials like silicon and titanium, researchers led by Hong Liu at A*STAR’s Institute of Materials Research and Engineering (IMRE), turned to niobium pentoxide, a transition metal oxide with a large bandgap.
“In the near-UV spectrum (350-400 nm), we found that niobium pentoxide exhibits lower absorption loss and higher transmission efficiency than other materials like silicon and silicon nitride,” said Liu. In fact, the team achieved a transmission efficiency of approximately 80 percent in the near-UV spectrum, the highest reported to date for UV metasurfaces.
Using high aspect ratio nanoblocks of niobium pentoxide, Liu’s team developed a hologram using the polarization of UV light as an added security dimension. When the nanoblocks are illuminated with UV light, three different holographic images are formed depending on how the light is polarized. Compared to the single image generated by conventional holograms used in anti-counterfeiting features like the ones on banknotes, this UV-based approach would enable enhanced levels of security, Liu said.
In addition to anti-counterfeiting applications, the high transmission efficiency of niobium pentoxide could pave the way for miniaturized optical and photonics elements such as lenses. “Also, the higher the transmission efficiency, the lower the power consumption of the device. Efficiency, therefore, is one of the key technical specifications of all optics and photonics systems,” Liu remarked.
“We are currently trying to develop UV metalenses, which can potentially be applied to high-resolution lithography, as well as other flat optical elements for beam steering, shaping and polarization control, to tailor UV light properties. We have also received industry interest in high-performance UV diffractive optical elements to split UV light for laser processing,” he said.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).