Investigation of injected laser polarization effects in quantum beat spectroscopy for magnetic field measurements in plasmas.

Abstract

Magnetic fields exert a strong influence on plasma behavior, highlighting the need for diagnostic techniques that can measure magnetic fields with high accuracy while minimizing plasma perturbation. Quantum Beat Spectroscopy (QBS), recently demonstrated by Gilbert et al. [Plasma Sources Sci. Technol. 34 025020 (2025)] in a laboratory plasma, is a laser-based optical diagnostic that provides Doppler-free measurements of magnetic field strength. In this study, a femtosecond laser is employed in an argon plasma to investigate the dependence of the QBS signal on the polarization of the incident laser. The experiments are conducted in a tabletop six-way cross vacuum chamber. A pair of Helmholtz coils is mounted on a rotational stage such that the chamber remains centered between the coils, enabling precise control over the orientation of the applied magnetic field. The present study shows that the quantum beat amplitude strongly depends on the polarization of the injected laser. It is also demonstrated that excitation from a single initial state into a coherent superposition of two excited magnetic sublevels is a necessary condition for the observation of quantum beats. Furthermore, it is shown that the magnetic field orientation can be determined by rotating the polarization of the injected laser.