Bending light and opening markets
It has been more than a decade since materials scientists captured headlines for their attempts to mimic Harry Potter’s celebrated invisibility cloak by creating negative-index materials (NIM), advanced materials whose refractive index for an electromagnetic wave has a negative value over a frequency range. While that possibility still intrigues many observers, others have taken a more practical attitude, looking for specific market niches that could be filled by products with extraordinary optical qualities.
Those commercial possibilities inspired George Palikaras and Themos Kallos to found Metamaterial Technologies Inc. (MTI) in 2011. The company, which is based in Dartmouth, Nova Scotia, takes its name from the nano-composite structures made of highly engineered metals or polymers. Now a metamaterial promises to solve one of the more unsettling problems being faced by the airline industry: pilots being distracted or even temporarily blinded by laser beams shining through cockpit windows. Thousands of these incidents are reported annually, raising the prospect of dire consequences in the air caused by ground-based sources such as powerful, cheap to purchase, Class IV handheld laser pointers of up to 5000mW.
MTI has developed an optical film that can filter out selected intense, coherent beams of light while allowing normal wavelengths of light to pass as they would through any window. That simple goal called for some extremely complex processing methods, including a scanning holographic technique that modulated the refractive index of a photosensitive material at the nano-scale level.
“The key to success in our field has not been scaling up our output of finished products but inventing new nanofabrication processes,” explains Palikaras, who describes scanning holography and rolling mask lithography as MTI’s two core capabilities. The former is a means of synthesizing optical nanostructures within the volume of ζphotopolymer substrates, while the latter is a system for transferring nano-scale patterns onto a wide range of larger formats, from rigid panels like glass to flexible films.
For him, this capability represents nothing less than a new frontier of optical lithography.
“On the back end we have in-house computer software that allows our engineers to design novel nanostructured patterns depending on the problem we are tackling, to absorb, block or bend light for various market applications,” he says. “We’re modelling them, optimizing them, and then transferring them onto a soft, cylindrical, seamless photomask on PDMS material.”
MTI’s lithographic process creates thin films with extraordinary optical characteristics. Photo credit: MTI
This mask incorporates features on the order of 70 nm, a fraction of the wavelength of the light used to create them at 365 nm ("i-line"). These features can then be integrated into almost any surface, including glass, semiconductors, or metal. That opens up a huge range of opportunities that go well beyond altering cockpit windows to repel lasers, such as protecting the elaborate array of sensors that are now found in a growing number of automobiles and will be critical to the safe operation of autonomous vehicles in the future.
“With vision systems comes the fact that you have to protect your sensors,” says Palikaras. “Laser protection is one thing but we’re also looking at weather protection, such as how to prevent ice and humidity from forming on your sensors.”
Thanks to a collaboration with the Bayer Material Science spin-off company Covestro, MTI was able to refine its laser protection film to meet the commercial standards of the multinational aviation giant Airbus, which is now testing this technology on working aircraft. It may not be quite as outrageous and radical as an invisibility cloak, but Palikaras sees plenty of room for innovations that are no less dazzling.
“Optics is a fascinating field where materials have unique properties, and when you start engineering them at the nano-scale, they start behaving in ways that are non-traditional,” he concludes.