High-resolution mapping of the radial electric field in RF plasma sheath using a single optically trapped particle

Abstract Mapping individual components of the electric field, E , with high spatial resolution around the plasma sheath remains challenging, as most in situ plasma probes are generally too intrusive for reliable plasma potential measurements. An optically trapped micron-sized particle in a plasma environment represents the smallest, nonintrusive, or minimally intrusive diagnostic tool because it causes minimal disturbance to the plasma environment in which it is suspended. These microparticles can be used to map the electric field in situ with high spatial resolution in a radio frequency (RF) plasma sheath region. In this study, we optically trapped micron-sized single particles and precisely transported a single trapped particle within the cylindrically symmetric capacitively coupled plasma (CCP), over a radial distance of ~ 0 - 15 mm, a 2400-fold displacement relative to the particle size, corresponding to a spatial resolution of tens of micrometers. We measured | E | and its spatial distributions in the examined range by analyzing the particle’s trajectory in the plasma after the optical forces were turned off. The radial component of the electric field, | E r |, was measured at multiple locations parallel to the electrode at 6.7 Pa. The | E r | was strongest near the circular electrode edges, reaching 0.478 ± 0.005 V/mm, and decreased to 0.458 ± 0.001 V/mm toward the center of the electrode. We reconstructed the radial plasma potential, yielding a center-to-edge potential decrease of 4.6 V over one centimeter. This ΔV is consistent with reported CCP trends of small mid-plane radial potential gradients and serves as the confining potential for dust crystal and dust cluster studies. These results highlight the use of optical trapping of a single particle as an in situ, nonintrusive microprobe for quantitative mapping of E r (r) and V(r) in RF plasmas.