Enhancing Plasma Diagnostics with Structured Light and Wavelength Modulation Spectroscopy*

In both research and industry, there is a significant demand for plasma diagnostics that can be applied toreal-world devices e.g., microelectronics production reactors. These environments often present challenges such as limited optical access, high background noise, and harsh conditions. Laser-based spectroscopy, known for its high sensitivity and selectivity, is a common research tool, which typically utilizes TEM00 mode lasers.[1] However, structured light beams [2], featuring complex spatial and temporal modes, have been gaining importance in fields like optical communications and quantum information processing. Despite their potential, these beams are not yet widely used in plasma spectroscopy.Spectroscopy applications using structured light will be presented with a focus on Doppler shift-based laser-induced fluorescence (LIF) diagnostics to measure velocity distribution functions (VDFs) of ions

and neutrals in plasma.[3] The confocal LIF setup was developed utilizing an axicon generated Laguerre- Gaussian annular beam, achieving a spatial resolution of approximately 5mm at a 300mm focal distance, with the potential to reach up to 1 mm, comparable to conventional LIF.[4] Wavelength modulation (WM) LIF has also been developed to improve VDF analysis.[5] This derivative technique measures spectral line profile derivatives, delivering a background-free signal highly sensitive to true shape of VDF. Additionally, the potential of vortex beams, which carry orbital angular momentum, is being investigated for their application in plasma diagnostics. [6,7]