Non-perturbative 2D spatial measurements of electric fields within a plasma sheath

Abstract

We introduce an all-optical quantum-enhanced diagnostic for electric fields in low-temperature plasmas. Trace amounts of rubidium vapor, added to argon plasma, allow us to produce spectrally narrow electric field-sensitive optical resonances via quantum optical effect of Rydberg electromagnetically induced transparency, and to non-invasively measure electric field in plasma with sensitivity exceeding 1 V/cm. By collecting fluorescence from the illuminated region of interest, we reconstruct a 2D spatial profile of the electric field magnitude with $30~μ$m resolution. As a proof-of-principle demonstration, we measured the changes in electric field within the plasma sheath surrounding a biased Langmuir probe tip. This method holds significant potential for studying sheath structures in low-temperature plasmas.

Figures (3)

• Figure 1: (a) Interaction scheme for Rb Rydberg EIT, showing Stark splitting of the excited Rydberg state $nD_{5/2}$ in electric field. $\Delta_c$ is the detuning of the coupling laser frequency from the unperturbed $5P_{3/2}\rightarrow nD_{5/2}$ transition. (b) Conceptual experimental schematic: counter-propagating red probe (780 nm) and blue coupling (480 nm) lasers are weakly focused just beneath the Langmuir probe tip within the argon plasma column. The thin arrows depict electrostatic field lines originating from the biased probe tip. Thick red and blue arrows indicating the 780 nm and 480 nm laser propagation directions. (c) The fluorescence image with the probe tip marked as a red circle. The $xz$ coordinate system orientation is shown with green arrows. The blue rectangle indicates the area shown in Fig. \ref{['fig:shrink_sheath']}(a,b). (d) Rydberg EIT ($25D$) resonances in plasma with electron density $n_e=6\times 10^{14}~\text{m}^{-3}$ and electron temperature $T_e = 0.4~\mathrm{eV}$ at 14 mTorr Ar pressure, the probe is biased at +6.0 V relative to the grounded chamber.
• Figure 2: (a)Maps (arbitrary colormap scale) showing Stark split EIT resonances evolution at increasing vertical distances ($z$-axis) from the Langmuir probe surface in plasma. The resonances correspond to the splitting of the $25D_{5/2}$ Rydberg level. The smearing of the resonances is attributed to the electric field gradient across the cross-section of the laser beam. (b) Magnitude profiles of the E-field, reconstructed from the Stark maps using Eq. (\ref{['eq:ry-eit-spectrum']}).
• Figure 3: (a) 2D reconstruction of the E-field magnitude surrounding the Langmuir probe tip, which is biased to +6.0 V in the absence of plasma. (b) 2D reconstruction with a plasma density of $n_e = 2\times 10^{16}~\text{m}^{-3}$, an electron temperature of $T_e = 0.7$ eV, and an Ar pressure of $p=14$ mTorr. Note that the aspect ratio is to scale. (c) The radial E-field profile within the plasma sheath around the Langmuir probe at a biasing voltage of +6.0 V. The evolution of the curve is caused by the increasing plasma density $n_e$ via enhanced screening. The shaded area marks the probe tip wire. The profiles were derived from angular integration of the 2D reconstructions.