Investigation of laser‐produced plasma multistructuring by floating probe measurements and optical emission spectroscopy

Abstract

With the continuous development of pulsed laser deposition as a versatile technique for the deposition of complex thin films, there is a need for a better understanding of the role and control of the deposition parameters. The understanding of the particle kinetics and plasma chemistry during the deposition process can greatly improve the properties of the synthesized films. By using the floating voltage regime of the Langmuir probe technique, we performed angular and time-resolved measurements during laser ablation of an Ag target, which evidenced the structuring of the plasma plume in ultrahigh vacuum conditions. The addition of N 2 gas in the pressure range from 5 × 10-5 to 10 Pa leads to more rapid plasma thermalization and the control of its kinetic energy. The electrical measurements were complemented by optical emission spectroscopy, which showcased the presence of neutral and multiple ionized species distributed across the laser-produced plasma plume. The plasma homogenization resulted in a decrease of the mean free path of Ag ions and atoms, which increased both their excitation temperature and electron density.

The Curious Case of Transient Plasma Generated by ns-laser Ablation of Ag in Ar Gas

“…We identi ed using the specialized databases 49 the presence of atomic and single ionized species of Ag (Ag I -520.4 nm, Ag II -531.6 nm) and atomic, single and double ionized species of Ar (Ar I -455.5 nm, Ar II -655.8 nm, Ar III -350.2 nm). The increase in Ar pressure lead to an increase in all emission lines and thus an increase in thermal energy of the electrons consistent with our ndings in 8 where we investigated the effect of N 2 gas on Ag plasma. With the mention that Ar I optical signature in the 300-700 nm spectral range is scarce, we can safely assume that the Ar background gas was partially ionized throughout the plasma volume 50 .…”

“…For 0.05 Pa and 0.5 Pa, a three-peak structure of the ionic and electronic regions can be observed, similar to the distribution seen for Ag plasma expansion in N 2 8 . The structuring of the plasma has been investigated before and it is accepted that different groups of particles can be ejected through different ablation mechanisms 29,33 .…”

“…It is worth noting that these techniques work well for pure, single-element plasmas with mostly single ionized species present, which is a well-known limitation of the technique. To overcome this, we used the oating current as a reference and implemented the approach from our previous report 8 . Selected charge distributions with velocities are presented in Fig.…”

“…The plasma investigations were performed under a residual vacuum of 5 × 10 − 5 Pa, and at 5 × 10 − 2 , 5 × 10 − 1 , 2, 5, and 10 Pa Ar pressure. Each experiment was preceded by a cleaning procedure during which the LP was shielded from the incoming transient plasma while the rotating silver target was laser irradiated with 1200 pulses (for other details on the setup geometry please see 8 ). The signal from tungsten LP (diameter -0.2 mm and exposed length -2 mm) was measured by collecting the voltage signal across a load resistor with a Tektronix DPO 4140 oscilloscope.…”

“…A particular interest has been given to laser-produced Ag plasma as Ag has shown promising results in a wide range of applications spanning from plasmonic resonance 17 , exible conductive electrodes 18 or even medical and biological applications 19 . From a diagnostic perspective several papers have reported on the behavior of LP in transient plasma generated by short or ultrashort laser ablation of Ag in various con gurations in terms of laser properties 20,21 , gas nature [22][23][24][25][26] and LP geometries 8,23,27,28 . The general consensus is that during the ablation process there are generated high energy multiple charged ions 9,14,20 (up to 100 eV), and complex nanostructures 20,22 that can affect the properties of the thin lms.…”