The research team, led by Anatoly Zayats, designed a metamaterial structure comprising an array of gold nanorods coated in palladium. When hydrogen was present at a concentration of just 2% – half the percentage that could be explosive – the amount of light transmitted and reflected by the structures changed by up to 30% and 40%. Even looking with the naked eye, a discernible dimming could be observed in the light passing through the structure within minutes of hydrogen being present.

The response is fast as well as sensitive because the gold-palladium nanorod array has a large surface area. "With any sensing technology the higher the sensitivity the better – it means it’s faster because if you can detect lower concentrations you can detect a leak sooner," explains Wayne Dickson a member of the King’s College team. In another first, Dickson, Mazhar Nasir, also a member of the King's College team, and their colleagues found a way to speed up the reset time for the device from hours to seconds.

Collectively stronger

Previous studies have looked at using palladium for optical hydrogen detection because the small hydrogen molecules readily integrate into the palladium crystal lattice, altering its refractive index. Plasmons – collective electron oscillations that occur in certain materials in response to light – can make this shift in refractive index easier to detect. The oscillations become resonant at specific wavelengths, greatly increasing the amount of light that is transmitted or reflected.

The problem is that plasmons in palladium lose energy quickly. Efforts to improve the sensitivity of plasmonic sensors, using photonic structures and including materials with longer-lived plasmon resonances such as gold, have so far proved unsuccessful.

The team found the solution in plasmonic metamaterials – materials patterned with interacting component structures to achieve specific properties. "In an individual rod you get a certain level of response, but in a metamaterial it is not just the individual rod but the collective interaction between them," Dickson explains.

Fast reuse

The King’s College team also discovered a technique to speed up the recovery of the sensors from two hours to just 30–45 seconds. To re-use the structures the researchers had to flow nitrogen over the structure to remove the hydrogen, but the reset time was quite long. "We had the idea of how it might be possible to speed up the process since the material can absorb strongly at the resonant wavelengths," says Dickson. "We tried laser annealing, and we were kind of surprised how well it works." Nasir and Dickson explain that the heat causes the palladium to expel the hydrogen, and says that this laser-enhanced recovery could be integrated into the sensor unit.

From idea to device

The idea for the sensors was conceived about a year ago, but it proved difficult to realize in practice. The team had no problem fabricating the gold nanorod arrays. "The challenge was to increase the quality of the palladium we deposit, making sure it is uniform, the right thickness and so on," explains Dickson. "We spent a lot of time optimizing the palladium response."

The metamaterial can be fabricated over a large area quite inexpensively, as it is based on a self-assembled fabrication process. The team has also demonstrated ultrasound detection using a plasmonic metamaterial and are now working to design a plasmonic metamaterial to detect ammonia, which is used in explosives.

Full details of the research are reported in Advanced Materials.