Geometrical and energy scaling in a laser-generated multi-species low temperature plasma

The chemical diversity, variety of gas backgrounds, and shock wave characteristics of laser-generated plasmas provide many unexplored opportunities for processing of novel materials. Determining how experimental conditions impact the plasma parameters of these transient plasma flows is critical for harnessing their technical potential. In this study, we carry out Langmuir probe measurements of the multi-species plasma formed by nanosecond UV ablation of FeSe. This plasma is involved in the synthesis of β-FeSe/SrTiO₃ heterointerfaces, that may enable electric-field controlled high-T_c superconducting devices. Measurements of electron temperature, electron density, Debye length, and Mach number of the plasma expansion as a function of laser fluence and spot area show two distinct plasma regimes in the 20-630 mJ pulse energy range. The dependence of the Mach number of the plasma front is particularly revealing. It increases with pulse energy up to 180 mJ, but exhibits a reversed, monotonically decreasing trend for greater energies. This reversal suggests a switch in energy apportionment between the directed and thermal motion components of the expansion. The uniform scaling of the plasma parameters with the number of available photons per absorber in the laser-plasma interaction volume, indicates that the regime transition is governed by laser-plasma interaction events. We will discuss how the interplay of laser absorption processes by the plasma influences this transition.