Beam collapse and refractive index changes inside fused silica induced by loosely focused femtosecond laser

Femtosecond laser-induced optical and structural changes inside transparent materials are crucial to ultrafast laser micromachining. In this work, on-axis beam collapse has been predicted by theoretical simulations and further demonstrated by the experimental results where a femtosecond pulse was loosely focused into a fused silica with the energy far above the self-focusing threshold. A detailed discussion of the energy thresholds and the starting locations on the optical axis of beam collapse for different beam waist radiuses is given. In addition, the spatial distribution of the refractive index has been determined using a spectroscopic ellipsometer and a roughly uniform decrease of 0.1 has been observed in the laser-modified region. Further, the relationship between the spatial distribution of the refractive index and the pulse energy density has been discussed quantitatively. The observed modulation of the beam collapse and the refractive index distribution can be expected to be a useful tool for femtosecond laser micromachining. (paper)