Numerical aperture dependence of damage and white light generation from femtosecond laser pulses in bulk fused silica

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 hi-h numerical aperture, the damage 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.