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

Abstract:

Competing nonlinear optical effects are involved in the interaction of femtosecond laser pulses with transparent dielectrics: supercontinuum generation and multiphoton-induced bulk damage. We measured the threshold energy for supercontinuum generation and bulk damage in fused silica using numerical apertures ranging from 0.01 to 0.65. The threshold for supercontinuum generation exhibits a minimum near 0.05 NA, and increases quickly above 0.1 NA. For numerical apertures greater than 0.25, we observe no supercontinuum generation. The extent of the blue broadening of the supercontinuum spectrum decreases significantly as the numerical aperture is increased from 0.01 to 0.08, showing that loose focusing is important for generating the broadest supercontinuum spectrum. Using a light scattering technique to detect the onset of bulk damage, we confirmed bulk damage at all numerical apertures studied. At high numerical aperture, the damage threshold is well below the critical power for self-focusing.