Glasses coated in lithium could let us see in the dark

Glasses coated in a lithium compound could one day allow wearers to see clearly in the dark.

For more than a decade, researchers have been searching for the best lightweight material that can convert infrared light, which the human eye cannot see, into visible light. The idea is to replace night vision goggles, which are often heavy and cumbersome.

Until recently, the main candidate was gallium arsenide. Now, Laura Valencia Molina at the Australian National University in Canberra and her colleagues have discovered that a film made of lithium niobate, coated with silicon dioxide gratings, performs better.

“The improvements in design and material properties enabled us to obtain a 10 times higher conversion of infrared light to the visible as compared to the gallium arsenide film,” says team member Maria del Rocio Camacho-Morales, also at the Australian National University.

Through a series of experiments, the team showed that the lithium niobate film could convert high resolution imaging from infrared light at 1550 nanometre wavelengths to visible light at 550 nanometre wavelengths. This surpassed the ability of gallium arsenide.

Night vision goggles require particles of infrared light called photons to pass through a lens and a device called a photocathode to convert them to electrons. These electrons are then converted to visible light photons by passing through a phosphor screen. This entire process requires cryogenic cooling to prevent the image being distorted.

With the lithium niobate film, it is hit by infrared light coming off an object, which is simultaneously illuminated with laser light, says Molina. The film combines the infrared and laser light, with the latter increasing the former’s frequency, transforming it into visible light.

Camacho-Morales says a film thinner than plastic wrap could one day be made of lithium niobate and silicon dioxide gratings and then coated over normal glasses to improve night vision.

While still in a research stage, the laser was set up so it could be easily directed at the film alongside the infrared light coming from an object. The team is now experimenting to create a an array of nanolasers that could lie on top of the lithium niobate film.

The work is an important next step towards lightweight night vision equipment and possibly a film that can be applied to normal glasses, says Camacho-Morales. It could also help drones navigate in the dark, as current night vision equipment is too heavy for some of these vehicles to carry, she says.