Optical emission spectroscopy of low-pressure DC argon plasmas for reconfigurable metadevices

Abstract Plasma discharge tubes (PDTs) are a promising medium for dynamically reconfigurable metasurfaces, owing to their tunable electron density ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mtext>e</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> ). This work provides an empirical framework for designing low-pressure, direct current-driven argon PDTs by defining their operational limits. Optical emission spectroscopy was utilized to characterize the spatial distributions of excitation temperature ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>exc</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> ) and electron density <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mtext>e</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> , via Boltzmann plots and Stark broadening, respectively. Under stable operation, it was observed that increased gas pressure of PDTs resulted in a decrease in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>exc</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> and a corresponding increase in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mtext>e</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> . However, a critical failure mode was identified at high currents. This degradation, characterized by a white glow, quenched argon emission, and electrical instability, was attributed to ion-induced sputtering of the nickel hollow cathode of PDTs. These findings indicate that the operational current must be maintained within a well-defined range to ensure device viability and lifetime, thus offering a critical design guideline for the development of robust plasma-based reconfigurable metasurfaces.