Air Deflects Lasers

Innovative concept diffracts laser beams without contact using sound waves

Using a novel process, laser beams can be deflected in the air without contact. The invisible optical grid made of air used is not only immune to damage from the laser beam, but also maintains its beam quality, as the interdisciplinary development team reports in the journal “Nature Photonics”. The researchers have applied for a patent for their method.

The innovative technology uses sound waves to modulate the air in the area that the laser beam passes through. “We create an optical grating using acoustic density waves,” explains lead author Yannick Schrödel, a doctoral student at DESY and the Helmholtz Institute Jena. Using powerful special loudspeakers, the researchers create a striped pattern of denser and less dense areas in the air. Similar to how air layers of different densities in the Earth's atmosphere deflect light, the density pattern in the air takes on the function of an optical grid that diffracts the laser beam. “However, the deflection of light using a diffraction grating allows much more precise control of the laser light compared to deflection in the Earth’s atmosphere,” emphasizes Schrödel. “The properties of the optical grating can be influenced by the frequency and intensity, i.e. the volume, of the sound waves.”

Here you can try out the deflection of the laser light by sound waves by sliding the control:

In this way, a strong infrared laser pulse could be deflected with an efficiency of 50 percent in initial laboratory tests. Significantly higher efficiencies should be possible in the future, as numerical modeling shows. For the first test, the scientists had to turn up their special loudspeakers. “We are moving at a sound level of around 140 decibels, which corresponds to a jet engine a few meters away,” explains Christoph Heyl, a scientist at DESY and the Helmholtz Institute Jena, who leads the research project. “Fortunately, we are in the ultrasonic range, which our ears cannot detect.”

The team sees great potential in the technology for high-performance optics. In their experiments, the researchers used an infrared laser pulse with a peak power of 20 gigawatts, which corresponds to the output of around two billion LED bulbs. Lasers of this and even higher performance classes are used, for example, for material processing, in fusion research or for the latest particle accelerators. “In this performance range, the material properties of mirrors, lenses and prisms significantly limit their use, and in practice such optical elements are easily damaged by strong laser beams,” explains Heyl. “They also worsen the quality of the laser beam. On the other hand, we manage to deflect laser beams without contact while maintaining quality.”

The scientists emphasize that the principle of acoustic control of laser light in gases is not limited to the creation of optical gratings. It can probably also be transferred to other optical elements such as lenses and waveguides. “We have been thinking about this method for a long time and quickly realized that extreme sound levels are necessary. At first, these did not seem technically feasible,” explains Heyl. “But we didn’t give up so quickly and finally found a solution with the support of researchers from the Technical University of Darmstadt and the company Inoson. We first tried out our technology in room air, and next we will also use other gases, for example to explore other wavelengths as well as other optical properties and geometries.”

The previously demonstrated deflection of light directly into ambient air opens up promising applications, in particular as a fast switch for high-power lasers. “The potential of non-contact control of light and its expansion to other applications can currently only be imagined,” explains Heyl. “Modern optics are based almost exclusively on the interaction of light with solid matter. Our approach opens up a completely new direction.”

Researchers from the Technical University of Darmstadt, Aalen University, the University of Hamburg, Inoson GmbH in St. Ingbert, the Helmholtz Institute Jena and DESY were involved in the work.