Dynamics of plasma formation, relaxation, and topography modification induced by femtosecond laser pulses in crystalline and amorphous dielectrics

Journal Article · Sat May 15 00:00:00 EDT 2010 · Journal of the Optical Society of America. Part B, Optical Physics

DOI:https://doi.org/10.1364/JOSAB.27.001065· OSTI ID:22046843

Puerto, D; Siegel, J; Gawelda, W; Galvan-Sosa, M; Solis, J ^[1]; Ehrentraut, L ^[2]; Bonse, J ^[3]

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1. Laser Processing Group, Instituto de Optica-CSIC, Madrid (Spain)
2. Max-Born-Institut fuer Nichtlineare Optik und Kurzzeitspektroskopie, Berlin (Germany)
3. Bundesanstalt fuer Materialforschung und-pruefung (BAM), Berlin (Germany)

We have studied plasma formation and relaxation dynamics along with the corresponding topography modifications in fused silica and sapphire induced by single femtosecond laser pulses (800 nm and 120 fs). These materials, representative of high bandgap amorphous and crystalline dielectrics, respectively, require nonlinear mechanisms to absorb the laser light. The study employed a femtosecond time-resolved microscopy technique that allows obtaining reflectivity and transmission images of the material surface at well-defined temporal delays after the arrival of the pump pulse which excites the dielectric material. The transient evolution of the free-electron plasma formed can be followed by combining the time-resolved optical data with a Drude model to estimate transient electron densities and skin depths. The temporal evolution of the optical properties is very similar in both materials within the first few hundred picoseconds, including the formation of a high reflectivity ring at about 7 ps. In contrast, at longer delays (100 ps-20 ns) the behavior of both materials differs significantly, revealing a longer lasting ablation process in sapphire. Moreover, transient images of sapphire show a concentric ring pattern surrounding the ablation crater, which is not observed in fused silica. We attribute this phenomenon to optical diffraction at a transient elevation of the ejected molten material at the crater border. On the other hand, the final topography of the ablation crater is radically different for each material. While in fused silica a relatively smooth crater with two distinct regimes is observed, sapphire shows much steeper crater walls, surrounded by a weak depression along with cracks in the material surface. These differences are explained in terms of the most relevant thermal and mechanical properties of the material. Despite these differences the maximum crater depth is comparable in both material at the highest fluences used (16 J/cm{sup 2}). The evolution of the crater depth as a function of fluence can be described taking into account the individual bandgap of each material.

• OSTI ID: 22046843
• Journal Information: Journal of the Optical Society of America. Part B, Optical Physics, Vol. 27, Issue 5; Other Information: (c) 2010 Optical Society of America; Country of input: International Atomic Energy Agency (IAEA); ISSN 0740-3224
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
36 MATERIALS SCIENCE
AMORPHOUS STATE
CRYSTALS
DIELECTRIC MATERIALS
DIFFRACTION
ELECTRON DENSITY
ENERGY GAP
LASER RADIATION
MECHANICAL PROPERTIES
OPTICAL MICROSCOPY
PULSED IRRADIATION
REFLECTIVITY
RELAXATION
SAPPHIRE
SILICA
SOLID-STATE PLASMA
TIME RESOLUTION
TRANSIENTS