High-resolution electric field mapping in RF plasma using an optically trapped single particle.*

The electric field is a key plasma parameter affecting charged particle behavior in RF plasma and sheath kinetics. It is often measured with a Langmuir probe, which can disturb plasma parameters by depleting energetic electrons and altering the global discharge. An alternative involves using micron-sized particles, which become negatively charged and can be levitated by the plasma's electric force. By observing their motion, the electric field E can be determined if the charge Q is known, or vice versa. However, this method is limited to fixed particle positions unless plasma conditions are changed.

An optically trapped particle provides a non-invasive probe for three-dimensional plasma measurements. It can be moved freely without altering plasma parameters. In this study, an optically trapped particle was used to map the electric field at the plasma sheath edge with high spatial resolution. Once the optical force is removed, the particle moves under gravity and the plasma's electric force. By analyzing its trajectory, the electric field is measured, assuming a constant charge Q. While accuracy depends on the Q approximation, precision is high due to careful particle positioning. This method measured electric fields affected by different electrode configurations, effectively comparing modified and unmodified fields. The optically trapped particle proves to be a reliable, high-precision, non-intrusive tool for mapping electric fields in RF plasma.