Evolution of the linear and nonlinear optical properties of femtosecond laser exposed fused silica

Three fused silica samples possessing different impurity levels and exposed to a near infrared femtosecond laser are investigated. The laser-induced defects are identified from absorption, luminescence, and Raman spectroscopy. Their linear and nonlinear optical properties are measured from Kramers-Krönig calculations and third-harmonic generation microscopy experiments. No conclusive correlation between the change in the optical properties, the initial impurity levels, and the photoinduced structures could be established based on the results obtained in this study. In addition, several hypotheses (densification and color center formation) have been rejected to explain why the linear and nonlinear optical properties of the photoinduced structures follow a contradicting evolution. This phenomenon is attributed to an experimental artifact on the measurement of the third-order susceptibility due to scattering of the photoinduced structures