Using lasers as tweezers to understand cloud electrification might sound like science fiction, but at the Institute of Science and Technology Austria (ISTA), it is a groundbreaking reality. By trapping and charging micron-sized particles with lasers, researchers can now observe their charging and discharging dynamics over time. This innovative method, recently published in Physical Review Letters, could provide crucial insights into the enigmatic process that sparks lightning.
Aerosols, which are liquid or solid particles suspended in the air, are ubiquitous. Some are large and visible, such as pollen, while others, like viruses, are imperceptible to the naked eye. PhD student Andrea Stöllner, part of the Waitukaitis and Muller groups at ISTA, focuses on ice crystals within clouds. By using model aerosols—tiny, transparent silica particles—she explores how these ice crystals accumulate and interact with electrical charge.
Laser Tweezers: A Novel Approach
Stöllner, alongside former ISTA postdoc Isaac Lenton, ISTA Assistant Professor Scott Waitukaitis, and others, has developed a method to capture, hold, and electrically charge a single silica particle using two laser beams. This technique holds potential for various applications, including demystifying how clouds become electrified and what initiates lightning.
Inside the laboratory, Andrea Stöllner stands amidst an array of shiny metal gadgets, with green laser beams cutting across the space, bouncing through a series of mirrors. “It’s an anti-vibration table,” Stöllner explains, highlighting its essential role in absorbing vibrations from the room and nearby equipment—crucial for precision work with lasers.
The beams converge into two streams, creating a ‘trap’ where tiny objects are held steadily by light alone, functioning as “optical tweezers.” Inside this setup, particles drift past these tweezers until a green flash signals success: a perfectly round, vibrant green glowing aerosol particle has been captured.
“The first time I caught a particle, I was over the moon,” Stöllner recalls her Eureka moment two years ago. “Scott Waitukaitis and my colleagues rushed into the lab to witness the captured aerosol particle. It lasted exactly three minutes, then the particle was gone. Now we can hold it in that position for weeks.”
Unraveling the Mystery of Particle Charging
It took Stöllner nearly four years to refine the experiment to the point of providing reliable data, starting with a previous setup developed by her former colleague 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.”
The scientists discovered that lasers charge the particle through a “two-photon process.” Typically, aerosol particles are nearly neutrally charged, with electrons swirling around in every atom. The laser beams, composed of photons, can ‘kick out’ one electron from the particle when two photons are absorbed simultaneously, causing the particle to gain a positive charge.
“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,” Stöllner notes.
This observation also reveals that as the particle becomes positively charged, it begins to discharge, occasionally releasing charge in spontaneous bursts. This phenomenon might mirror processes occurring in thunderstorm clouds.
Implications for Understanding Lightning
Thunderstorm clouds contain ice crystals and larger ice pellets that exchange electric charges upon collision. Eventually, the cloud becomes so charged that lightning forms. One theory suggests that the initial spark of a lightning bolt could originate from charged ice crystals. However, the exact science behind lightning formation remains elusive.
Alternative theories propose that cosmic rays initiate the process as the charged particles they create accelerate from pre-existing electric fields. According to Stöllner, the current scientific consensus is that the electric field in clouds seems too low to cause lightning.
“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, we aim to decode these microscale interactions to better understand the big picture.”
The ISTA scientists are now focused on these interactions, hoping to uncover more about the mysterious phenomenon of lightning. “Our model ice crystals are showing discharges, and maybe there’s more to that. Imagine if they eventually create super tiny lightning sparks—that would be so cool,” Stöllner says with a smile.
This research not only advances our understanding of atmospheric science but also opens new avenues for exploring the fundamental processes that drive weather phenomena. As ISTA continues to push the boundaries of laser technology, the potential for uncovering further mysteries of the natural world remains vast.
