Pre-ablation regime light-induced optical changes in nanometer thick metal films.

Area of Science:

• Materials Science
• Nanotechnology
• Laser Physics

Background:

• Investigating laser-matter interactions in thin films is crucial for understanding material modification processes.
• The pre-ablation regime, where subtle morphological changes occur without bulk damage, is of particular interest.

Purpose of the Study:

• To investigate the effects of single femtosecond laser pulses on nanometer-thick gold and aluminum films below the ablation threshold.
• To correlate observed morphological changes with in-situ optical reflectivity measurements.

Main Methods:

• Illumination with single, femtosecond 400 nm wavelength pump laser pulses.
• In-situ monitoring of optical reflectivity using a weak probe beam.
• Characterization using dark-field, scanning electron, and atomic force microscopy, and Electron Backscatter Diffraction.
• Theoretical analysis using Liu-analysis, transfer-matrix, and two-temperature models.

Main Results:

• Observed small, irreversible morphological changes, including subwavelength spallation in aluminum and delamination in gold.
• Measured minute increases in optical reflectivity (0.1-2%) coinciding with morphological changes.
• Inferred that aluminum layers reach melting temperature, with grains melting and resolidifying into larger ones.
• Attributed increased reflectivity and spallation in aluminum to grain resolidification; attributed optical changes in gold to the etalon effect from delamination.

Conclusions:

• Laser-induced melting and resolidification of aluminum grains in the pre-ablation regime lead to increased reflectivity and spallation.
• Delamination of gold films in the pre-ablation regime causes an etalon effect, resulting in increased optical reflectivity.