Trapping A Single Particle To Reveal Lightning’s First Spark

A daring new laser-based technique lets researchers trap and charge a single aerosol particle, opening a window into how tiny ice crystals in clouds might store and release electrical energy.

As the team discovered, laser photons can knock electrons off these particles one by one, allowing scientists to watch them charge up, discharge, and behave in ways that echo what may be happening high above in thunderstorm clouds.

The work could illuminate one of nature’s most mysterious processes—how the very first spark of lightning begins.

Aerosols and Their Hidden Complexity

Aerosols are tiny particles of liquid or solid material that drift through the air and are constantly present in our surroundings. Some are big enough to spot, like spring pollen, while others, including flu-season viruses, are far too small to see. A few are even detectable by taste, such as the salt particles floating in seaside air.

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PhD student Andrea Stöllner, a member of the Waitukaitis and Muller groups at the Institute of Science and Technology Austria (ISTA), studies the behavior of ice crystals found in clouds. To investigate how these crystals form and build up electrical charge, she works with model aerosols made of small, transparent silica particles.

Together with former ISTA postdoc Isaac Lenton, ISTA Assistant Professor Scott Waitukaitis and colleagues, Stöllner has created a method to capture, hold, and electrically charge a single silica particle using two focused laser beams. The technique could be used to explore a range of scientific questions, including how clouds become electrically active and what initiates lightning.

Catching a micron-sized particle is challenging. To get the job done, two laser beams come in handy.

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Acting like tweezers, they can trap, secure, and charge a solitary particle. Credit: © Andrea Stöllner / ISTA

Laser Trapping Breakthroughs in Aerosol Research

In her lab, Stöllner stands beside a wide table filled with polished metal instruments. Green laser beams move across the setup, reflecting from one tiny mirror to another.

The table emits a soft, steady hiss like escaping air. “It’s an anti-vibration table,” she explains, emphasizing how it prevents small disturbances from interfering with the delicate laser work.

The lasers weave their way through a carefully arranged sequence of components before merging into two aligned beams that enter a small enclosure. At that point they form a light-based “trap,” functioning as “optical tweezers” that can keep minuscule objects suspended in place. As particles drift through the enclosure, one may suddenly light up in bright green, revealing that it has been successfully captured inside the trap.

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“The first time I caught a particle, I was over the moon,” Stöllner says as she recalls her Eureka moment two years ago, just before Christmas. “Scott Waitukaitis and my colleagues rushed into the lab and took a short glimpse at the captured aerosol particle.

It lasted exactly three minutes, then the particle was gone. Now we can hold it in that position for weeks.”

Reaching this level of stability took nearly four years of refinement, building on an earlier setup designed by Lenton. “Originally, our setup was built to just hold a single particle, analyze its charge, and figure out how humidity changes its charges,” explains Stöllner.

“But we never came this far. We found out that the laser we are using is itself charging our aerosol particles.”

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How Light Kicks Out Electrons

The scientist and her colleagues discovered that lasers charge the particle through a “two-photon process.”

Typically, aerosol particles are close to neutrally charged, with electrons (negatively charged entities) swirling around in everyatomof the particle. The laser beams consist of photons (particles of light traveling at the speed of light), and when two of these photons are absorbed simultaneously, they can ‘kick out’ one electron from the particle.

In this way, the particle gains one elemental positive charge. Step by step, it becomes increasingly positively charged.

For Stöllner, uncovering this mechanism is an exciting discovery that she can leverage in her research. “We can now precisely observe the evolution of one aerosol particle as it charges up from neutral to highly charged and adjust the laser power to control the rate.”

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This observation also reveals that, as the particle becomes positively charged, it begins to discharge, meaning that it occasionally releases charge in spontaneous bursts.

Way above our heads, something similar might also be happening in clouds.

Electrified Clouds and the Origins of Lightning

Thunderstorm clouds contain ice crystals and larger ice pellets. When these collide, they exchange electric charges. Eventually, the cloud becomes so charged that lightning forms. One theory suggests that the first little spark of a lightning bolt could be initiated at the charged ice crystals themselves.

However, the exact science behind the phenomenon of lightning formation remains a mystery. Alternative theories meanwhile suggest cosmic rays initiate the process as the charged particles they create accelerate from pre-existing electric fields. According to Stöllner, however, the current understanding in the scientific community is that – in either case – the electric field in clouds seems too low to cause lightning.

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“Our new setup allows us to explore the ice crystal theory by closely examining a particle’s charging dynamics over time,” Stöllner explains. While ice crystals in clouds are much larger than the model ones, the ISTA scientists are now aiming to decode these microscale interactions to better understand the big picture.

“Our model ice crystals are showing discharges and maybe there’s to that. Imagine if they eventually create super tiny lightning sparks—that would be so cool,” Stöllner says with a smile.

Reference: “Using optical tweezers to simultaneously trap, charge and measure the charge of a microparticle in air” 19 November 2025,Physical Review Letters.
DOI: 10.1103/5xd9-4tjj

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