A new approach to meta materials has been developed by researchers at the University of Cambridge and the Donostia International Physics Center in Spain. The technique allows the team to construct materials from nanoscopic building blocks that can control the path of light through them and perhaps ultimately be used to develop "cloaking devices" that render objects shielded by the meta material as invisible to the outside world. [Herrmann et al., Nature Commun (2014)]
The team led by Jeremy Baumberg used unfocused laser light to "stitch" together billions of gold nanoparticles into long strings in water using barrel-shaped molecules, curcurbiturils, as spacers. The spacers lock the nanoparticles in place prior to laser illumination. The laser light then acts like a needle passing a golden thread through the gold nanoparticles. Each thread is 12 nanometers in diameter. This allows the researchers to essentially mass produce meta materials in water by using light, something that was not possible with earlier approaches. The resulting meta materials have a wide range of possible applications as molecular sensors and potentially for military and other stealth technologies.
"It was about finding a way to control that thread, or bridge, between the nanoparticles," explains team member Ventsislav Valev. "Joining a few nanoparticles together is fine, but scaling that up is challenging." He explains the role of the curcurbiturils, which are critical to controlling the way the nanoparticles are connected. When the laser is pointed at the strings of particles with their curcurbituril spacers, plasmons are generated as ripples form in the electrons on the surfaces of the conducting metal particles; as revealed by the observation of a new optical resonance in the near-infrared. The skipping electrons concentrate the light energy on the atoms at the surface and weld the nanoparticles together via tight bridges scaffolded by the presence of the curcurbituril barrels. By using ultrafast lasers, the team can pulse weld billions of these bridges in rapid succession, threading the nanoparticles into long strings.
"We have controlled the dimensions in a way that hasn’t been possible before," adds Valev. "This level of control opens up a wide range of potential practical applications." The team adds that tailoring the light fields could also allow them to create split rings and chiral strings highlighting the potential for meta materials processed in solution.
"In terms of applications, our materials could be used as devices for enhancing molecular sensing (surface enhanced Raman spectroscopy, SERS)," Valev told Materials Today. "We are also exploring the possibility to develop our process towards creating chiral meta materials through self-assembly in water. This is likely going to be a major direction for our future research."
David Bradley blogs at http://www.sciencebase.com and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".