The femtosecond laser has become an important tool in the micromachining of transparent materials. In particular, focusing at high numerical aperture enables structuring the bulk of materials. At low numerical aperture and comparable energy, focused femtosecond pulses result in white light or continuum generation. It has proven difficult to damage transparent materials in the bulk at low NA. We have measured the threshold energy for continuum generation and for bulk damage in fused silica for numerical apertures between 0.01 and 0.65. The threshold for continuum generation exhibits a minimum near 0.05 NA, and increases quickly near 0.1 NA. Greater than 0.25 NA, no continuum is observed. The extent of the anti-stokes pedestal in the continuum spectrum decreases strongly as the numerical aperture is increased to 0.1, emphasizing that slow focusing is important for the broadest white light spectrum. We use a sensitive light scattering technique to detect the onset of damage. We are able to produce bulk damage at all numerical apertures studied. At high numerical aperture, the damae threshold is well below the critical power for self-focusing, which allows the breakdown intensity to be determined. Below 0.25 NA, the numerical aperture dependence suggests a possible change in damage mechanism.

Date Published: 1 April 2002
PDF: 5 pages
Proc. SPIE 4633, Commercial and Biomedical Applications of Ultrafast and Free-Electron Lasers, (1 April 2002); doi: 10.1117/12.461369

Published in SPIE Proceedings Vol. 4633:
Commercial and Biomedical Applications of Ultrafast and Free-Electron Lasers
Joseph Neev; Glenn S. Edwards; Joseph Neev; Andreas Ostendorf; Andreas Ostendorf; John Clark Sutherland, Editor(s)