Ion distribution and ablation depth measurements of a fs-ps laser-irradiated solid tin target

TL;DR

This study investigates ion distribution and ablation depth of solid tin targets irradiated by femtosecond to picosecond laser pulses, revealing how pulse length and fluence affect ablation characteristics and ionization, relevant for EUV plasma sources.

Contribution

It provides detailed measurements of ion yield, energy, and ablation depth across a range of pulse lengths and fluences, expanding understanding of laser-tin interactions for EUV applications.

Findings

Ablation depth decreases slightly with increasing pulse length.

Ion yield increases slightly with pulse length.

Ablation volume remains constant across pulse lengths.

Abstract

The ablation of solid tin surfaces by an 800-nanometer-wavelength laser is studied for a pulse length range from 500 fs to 4.5 ps and a fluence range spanning 0.9 to 22 J/cm^2. The ablation depth and volume are obtained employing a high-numerical-aperture optical microscope, while the ion yield and energy distributions are obtained from a set of Faraday cups set up under various angles. We found a slight increase of the ion yield for an increasing pulse length, while the ablation depth is slightly decreasing. The ablation volume remained constant as a function of pulse length. The ablation depth follows a two-region logarithmic dependence on the fluence, in agreement with the available literature and theory. In the examined fluence range, the ion yield angular distribution is sharply peaked along the target normal at low fluences but rapidly broadens with increasing fluence. The total ionization fraction increases monotonically with fluence to a 5-6% maximum, which is substantially lower than the typical ionization fractions obtained with nanosecond-pulse ablation. The angular distribution of the ions does not depend on the laser pulse length within the measurement uncertainty. These results are of particular interest for the possible utilization of fs-ps laser systems in plasma sources of extreme ultraviolet light for nanolithography.