"Invisible grating" bends laser using just air and sound
Guiding lasers where they need to go is a key part of optics systems, and now engineers at DESY have developed a way to bend laser beams without anything touching them. The light is deflected using an invisible grating made of air shaped by acoustics.
In optics systems, lasers are usually redirected using lenses and mirrors, but at high energies, such as those used for material processing, particle accelerators or fusion energy research, these delicate components may need replacing too often.
“In this power range, the material properties of mirrors, lenses, and prisms significantly limit their use, and such optical elements are easily damaged by strong laser beams in practice,” said Christoph Heyl, lead researcher on the new project. “In addition, the quality of the laser beam suffers. In contrast, we’ve managed to deflect laser beams in a quality-preserving way without contact.”
The DESY team’s alternative is to use air sculpted by acoustics. Sound waves are essentially just variations in air pressure, so crank the volume high enough and you can generate sound waves powerful enough to levitate objects – or in this case, manipulate light itself.
The researchers used a pair of ultrasound loudspeakers facing each other, which produced pockets of air with higher or lower density, forming a striped grating pattern. When an infrared laser was beamed through this grating, the light was deflected with an efficiency of over 50%. Higher efficiencies should be possible with further work, the team says.
These tests involved quite powerful equipment – the laser packed a power of 20 gigawatts, while the speakers needed to blast at 140 decibels, which is about the volume of a jet engine from a few meters away. Thankfully though, being ultrasound it isn’t detectable by human ears.
The team says that the technique could be useful as a fast switch for lasers, and future work could experiment with forming other shapes besides gratings, including lenses and waveguides. And they don’t need to be limited to regular old air, either.
“First, we tried out our technique with ordinary air,” said Heyl. “Next, for example, we will also use other gases in order to tap into other wavelengths and other optical properties and geometries.”
The research was published in the journal Nature Photonics.