Pump-Probe-Ellipsometrie zur Messung transienter optischer Materialeigenschaften bei der Ultrakurzpuls-Lasermaterialbearbeitung

Description

Ultrashort pulsed lasers have gained widespread use in laser material processing applications, as they enable precise ablation due to their highly efficient energy deposition and low thermal damage. Recent investigations using double laser pulses for metal ablation have shown that the laser ablation efficiency depends on the temporal delay of the laser pulses. In order to understand this process the interaction between the ultrashort laser pulse and the metal must be understood. In this work, a unique pump-probe-ellipsometry setup is developed enabling the measurement of the transient complex refractive index N = n – ik in ultrashort timescales. By means of this value early material properties and the material motion can be analysed. With these experimental data and by using them to validate a corresponding hydrodynamic simulation the ablation dynamics of aluminum (Al) are investigated in detail. A rapid increase of the refraction index n and a parallel fall in the extinction coefficient k within a few picoseconds after irradiation is measured. This behaviour is caused by ultrafast heating and melting of the Al. The lattice temperature reaches the melting temperature already during the pump-pulse impact. Additionally, the material density decreases in the irradiated volume and bulges with a velocity of about 1500 m/s caused by the high pressure build-up induced by the femtosecond pulse.

These single pulse results are discussed in terms of the impact for double pulse laser ablation of metals. It is found that the optical parameters change induced by the first pulse cannot solely cause the efficiency decrease in double pulse laser processing. A model also taking into account the material bulging, a decreased electronic penetration depth and a reduced stress confinement is being set up to explain the efficiency decrease.

Diese Einzelpulsergebnisse werden mit Hinblick auf ihre Auswirkung für die Doppelpulsablation von Metallen diskutiert. Es wird herausgefunden, dass die Änderung der optischen Parameter die Effizienzabnahme bei der Doppelpulsablation nicht alleine verursacht. Ein Modell, welches zusätzlich die Materialaufwölbung, eine verminderte elektronische Eindringtiefe und einen reduzierten Druckeinschluss berücksichtigt, wird aufgestellt, um die Effizienzabnahme zu erklären.