This article digs into a new way to precisely control electrical charge on microscopic particles held in optical tweezers—focused laser beams that trap objects without touching them. Building on a little-known observation from the 1970s by Nobel laureate Arthur Ashkin, an international team led by Scott Waitukaitis at the Institute of Science and Technology Austria has shown how to measure and tweak the charge of trapped particles in real time.
Their work confirms Ashkin’s early idea about laser-induced charging. It also opens fresh directions for research in aerosols, cloud physics, and the origins of lightning.
From Ashkin’s Early Insight to Modern Charge Control
Optical tweezers have become a staple in modern physics and biophysics. They let researchers grab and move objects as tiny as 100 nanometers using nothing but light pressure.
But one of the first things noticed about this technique—laser-induced charging—got mostly ignored for years. In the 1970s, Arthur Ashkin saw that small objects trapped in strong laser beams sometimes picked up an electric charge.
He guessed that electrons inside the material could absorb two photons at once, get enough energy to escape, and leave the object with a net positive charge. At the time, this was a side note; most people were excited about the trapping itself.
Rediscovering a Neglected Phenomenon
Decades later, Scott Waitukaitis and his team stumbled onto the same effect from a totally different angle. They wanted to study how charge builds up on particles acting as stand-ins for ice crystals in the atmosphere—work that matters for understanding storm clouds and lightning.
While using optical tweezers to trap these particles, they noticed the trapped objects were gaining charge under the laser. This sent them back to Ashkin’s old idea: maybe the light was kicking electrons off the particles.
Instead of seeing this as an annoyance, the team realized they could turn laser-induced charging into a tool they could control and measure.
How the New Charge Control Technique Works
The real breakthrough here is turning a vague effect into a precise, adjustable way to control charge. The researchers tuned their optical tweezers system so they could both trap particles and measure their electric charge as it changed.
They made a surprisingly simple tweak: the metal parts holding the optics became part of an electrode setup that could apply a known electric field to the trapped particle.
Using Copper Lens Holders as Electrodes
The team used copper lens holders as electrodes to generate a controlled electric field around the particle in the laser trap. This field affected the particle’s motion and the way it scattered light, which they could detect very precisely.
Small changes in how the trapped particle scattered light told them its charge state at any moment. So, the optical tweezers setup worked as both a trap and a charge sensor.
As they changed the laser’s intensity, electrons kept escaping through the two-photon absorption process Ashkin described, shifting the net charge on the particle. They could measure and use the relationship between laser power and particle charge.
Fine-Tuning Particle Charge with Light
The researchers showed they could finely adjust the particle’s electric charge just by changing the trapping laser’s intensity. Higher intensities pushed out more electrons, making the charge more positive. Lower intensities slowed things down.
This lets scientists set a specific charge value on a single nano- or microparticle—without ever touching it. Their confirmation of Ashkin’s mechanism, published in Physical Review Letters, roots this effect in solid physics and finally brings a long-overlooked phenomenon into the spotlight for precision measurement science.
Scientific Opportunities: From Aerosols to Lightning
Controlling charge on such tiny scales could have wide-ranging effects. Tons of natural and technological processes hinge on how little particles pick up, hold, and lose electric charge, especially in gases like air.
With this new approach, researchers can build experiments to probe specific electrostatic behaviors that used to be too subtle or complicated to study.
Probing Microdischarges and Atmospheric Physics
One immediate application is the study of microdischarges. These are tiny electrical breakdowns in air that start when highly charged particles show up.
Scientists think these small events might shape the early electrical activity inside clouds. They could even help set the stage for lightning, though there’s still a lot to figure out.
Now that researchers can control the charge on single particles, they’re able to:
This sharper control and measurement of charge might change how we understand a few things:
Here is the source article for this story: Electrical charge on objects in optical tweezers can be controlled precisely