Scientists at the Institute of Science and Technology Austria have developed a technique using intersecting laser beams to trap and charge single aerosol particles, observing their electrical changes in real time. This method, detailed in Physical Review Letters, mimics processes inside storm clouds and could uncover how lightning initiates. The glowing particles reveal electron loss and sudden charge bursts through a two-photon process.
Researchers led by PhD student Andrea Stöllner and Assistant Professor Scott Waitukaitis at ISTA have created an optical tweezers setup with two precisely aligned laser beams. These beams converge to hold a single transparent silica sphere, a model for cloud ice crystals, in a stable trap within a sealed container. The system, built over nearly four years, now maintains particles for weeks, a vast improvement from initial three-minute captures two years ago.
The lasers charge the initially neutral particles via a two-photon process, where simultaneous absorption of two photons ejects an electron, imparting positive charge. As exposure continues, the particle's glow indicates progressive charging, allowing precise control by adjusting laser power. Stöllner noted, '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.'
Unexpectedly, highly charged particles release charge in sudden bursts, suggesting spontaneous discharges akin to those in thunderclouds. Thunderstorms feature colliding ice crystals that exchange charges, leading to electrical imbalance and lightning. Current theories debate whether ice crystals or cosmic rays provide the initial spark, but cloud electric fields seem insufficient alone. Stöllner explained, 'Our new setup allows us to explore the ice crystal theory by closely examining a particle's charging dynamics over time.'
While lab particles are smaller than natural ice crystals, the team hopes these observations illuminate larger atmospheric electrification. Stöllner recalled her first success: 'The first time I caught a particle, I was over the moon.' The anti-vibration table ensures precision, protecting the green laser beams from disturbances.
This work, co-authored with Isaac Lenton and others, appears in Physical Review Letters (2025; 135 (21)). It advances understanding of cloud particles' role in severe weather.