Journal
APPLIED SURFACE SCIENCE
Volume 417, Issue -, Pages 2-15Publisher
ELSEVIER
DOI: 10.1016/j.apsusc.2017.02.070
Keywords
Pump-probe ellipsometry; Transport properties; Optical properties; Laser-metal interaction
Categories
Funding
- Bundesministerium fur Wirtschaft und Energie (BMWi) [0325922A]
- Deutsche Forschungsgemeinschaft (DFG) [HU 1893/2-1]
- Erlangen Graduate School in Advanced Optical Technologies (SAOT) by the DFG
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In this paper, we present ultrafast measurements of the complex refractive index for copper up to a time delay of 20 ps with an accuracy <1% at laser fluences in the vicinity of the ablation threshold. The measured refractive index n and extinction coefficient k are supported by a simulation including the two-temperature model with an accurate description of thermal and optical properties and a thermomechanical model. Comparison of the measured time resolved optical properties with results of the simulation reveals underlying physical mechanisms in three distinct time delay regimes. It is found that in the early stage (-5 ps to 0 ps) the thermally excited d-band electrons make a major contribution to the laser pulse absorption and create a steep increase in transient optical properties n and k. In the second time regime (0-10 ps) the material expansion influences the plasma frequency, which is also reflected in the transient extinction coefficient. In contrast, the refractive index n follows the total collision frequency. Additionally, the electron-ion thermalization time can be attributed to a minimum of the extinction coefficient at similar to 10 ps. In the third time regime (10-20 ps) the transient extinction coefficient k indicates the surface cooling-down process. (C) 2017 The Author(s). Published by Elsevier B.V.
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