, Volume 109, Issue 2, pp 409-420
Date: 31 Jul 2012

Ultra-fast movies of thin-film laser ablation

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Abstract

Ultra-short-pulse laser irradiation of thin molybdenum films from the glass substrate side initiates an intact Mo disk lift off free from thermal effects. For the investigation of the underlying physical effects, ultra-fast pump–probe microscopy is used to produce stop-motion movies of the single-pulse ablation process, initiated by a 660-fs laser pulse. The ultra-fast dynamics in the femtosecond and picosecond ranges are captured by stroboscopic illumination of the sample with an optically delayed probe pulse of 510-fs duration. The nanosecond and microsecond delay ranges of the probe pulse are covered by an electronically triggered 600-ps laser. Thus, the setup enables an observation of general laser ablation processes from the femtosecond delay range up to the final state. A comparison of time- and space-resolved observations of film and glass substrate side irradiation of a 470-nm molybdenum layer reveals the driving mechanisms of the Mo disk lift off initiated by glass-side irradiation. Observations suggest that a phase explosion generates a liquid–gas mixture in the molybdenum/glass interface about 10 ps after the impact of the pump laser pulse. Then, a shock wave and gas expansion cause the molybdenum layer to bulge, while the enclosed liquid–gas mixture cools and condenses at delay times in the 100-ps range. The bulging continues for approximately 20 ns, when an intact Mo disk shears and lifts off at a velocity of above 70 m/s. As a result, the remaining hole is free from thermal effects.