Floating Groningen nanoparticle attracts attention

20 September 2023

Nikhef physicists at the University of Groningen have developed a technique to capture nanoparticles and characterize their shape and mass. This may be useful for the hunt for dark matter.

A paper on the trapping technique has been published as a cover story in the journal Applied Physics Letters. The article by the team of Nikhef researcher Steven Hoekstra shows how the mass and shape of small silica particles can be determined simultaneously. Until now, only techniques that could measure only one of those properties were used.

The Groningen researchers let a silica particle float in a crossfire of laser light to study it. Such particles of about 140 nanometers in size (a nanometer is one billionth of a meter) are by no means always spherical, and moreover sometimes clump together to form, for example, a dumbbell shape. As a result, they also do not always have the same mass.

It has recently been proposed to use these optically floating nanoparticles as a new kind of sensor to measure dark matter from the universe. Some 85 percent of all mass in the universe is not visible and is believed to consist of as yet unknown particles. There are numerous experiments worldwide to capture such particles, for example with the XENON detector in Gran Sasso, also a Nikhef experiment.

When such unknown particles strike a floating nanoparticle, its position in the laser trap changes slightly. How much depends on the dark-matter particle, but also on the mass and shape of the nanoparticles. So for such measurements, the properties of the nanoparticles must be known precisely. Hoekstra and his team determine these by manipulations with the electric fields and the confinement laser.

The Nikhef group in Groningen specializes in precision measurements to test fundamental physics via trapping, cooling and manipulating small quantum systems such as molecules and ions. Work on optically trapped nano-spheres is a recent addition to this.

The main project of the Nikhef group at the Van Swinderen Institute in Groningen is a study of the electric field of electrons, which could be somewhat asymmetric. In standard theory, electrons are perfectly symmetrical point particles.