28 Nov. A laser trap for lightning research
Researchers have developed a method for capturing micrometer-sized particles, known as aerosols, with lasers and charging them in order to observe their charging and discharging dynamics. Doctoral student Andrea Stöllner is conducting research on ice crystals in clouds at the Austrian Institute of Science and Technology (ISTA) in the groups led by Scott Waitukaitis and Caroline Muller. To find out how ice crystals accumulate and interact with electrical charges, the scientist uses model aerosols – tiny, transparent silicon dioxide particles. The researcher has developed a technique that uses two laser beams to capture, fix, and electrically charge a single model aerosol. This approach could be applied in various fields, including research into the electrical charging of clouds and the formation of lightning.
Laser tweezers capture aerosol particles
Andrea Stöllner investigates the charging behavior of individual aerosols in a laser trap. Image: ISTA
In the laboratory, two laser beams converge in a chamber. Here, the two beams meet and form an optical trap in which microparticles are held stable by light. “When I captured a particle for the first time, I was overjoyed,” recalls Stöllner, thinking back to that moment two years ago. “Scott Waitukaitis and my colleagues rushed into the laboratory and took a quick look at the captured aerosol particle. At that time, it lasted exactly three minutes, then the particle disappeared again. Now we can hold it in this position for weeks.”
According to her own statement, it took the scientist almost four years to get the experiment to the point where it could deliver reliable data. The starting point was an earlier version of the setup. “Originally, our setup was designed to hold only a single particle and analyze its charge. We wanted to use it to find out how moisture changes its charges,” explains Stöllner. “But we never got that far. The lasers we use charge our aerosol particles.”
Two-photon absorption charges aerosols
The researchers discovered that lasers charge the particles through a two-photon process. When an aerosol particle absorbs two photons simultaneously, they can release an electron from the particle. This gives the particle a basic positive charge. Step by step, it becomes increasingly positively charged. Stllner explains: “We can now closely observe the development of a single aerosol particle from a neutral to a highly charged state and also adjust the laser power to control the charging rate.” The experiments also showed that when a particle is positively charged, it begins to discharge. This means that it occasionally releases charges spontaneously.
What triggers lightning?
This could also happen in thunderclouds containing ice crystals and larger ice grains, say the researchers. When these collide, they exchange electrical charges. Eventually, the cloud becomes so heavily charged that lightning forms. According to one theory, the first small spark of lightning could originate from the charged ice crystals themselves. However, exactly how lightning is formed remains a mystery. Alternative theories assume that cosmic rays trigger the process, as the charged particles they generate are accelerated by existing electric fields. According to Stöllner, however, the current consensus on all these variants is that the electric field in clouds is too weak to cause lightning.
“Our new setup gives us the opportunity to investigate the ice crystal theory by closely observing the charge dynamics of a particle over time,” explains Stöllner. Although the ice crystals in clouds are much larger than the model crystals, the interactions in the micro range can provide insights into the overall picture. “Our model ice crystals show discharges. Maybe there’s more to it than that. Imagine if the mini particles could eventually generate tiny lightning bolts – that would be really cool,” smiles Stöllner.
Project funding
This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreements No. 949120 and No. 805041).
Original publication
[Stoellner et al.: Using optical tweezers to simultaneously trap, charge and measure the charge of a microparticle in air; Physical Review Letters, 2025, DOI: 10.1103/5xd9-4tjj, https://doi.org/10.1103/5xd9-4tjj]
Source and image: ista.ac.at