Femtosecond Laser Pulse-Induced Structural Modification of Lanthanum Aluminosilicate Glasses: Refractive Indices in Visible Vs. Terahertz Frequency Regions

We studied femtosecond laser modifications of lanthanum aluminosilicate (SAL) glasses with 11-23 mol% La2O3 content. Refractive indices of these glasses were measured in the visible and terahertz frequencies and correlated to the change in the glass structure. The role of lanthanum provided a comparison to traditional glass modifiers within the glass network in terms of polarizability and optical basicity for femtosecond laser irradiation modification of the refractive index. Al-27 magic angle spinning nuclear magnetic resonance (MAS-NMR) was used to identify aluminum speciation within the SAL glasses. We used prism-coupling and terahertz time-domain spectroscopy (THz-TDS) for visible and THz refractive index measurements, respectively. Our results show a linear correlation between refractive index measurements in visible and terahertz frequencies with systematic rare-earth ion inclusions. In addition, our results demonstrate that THz-TDS can be utilized as a means of extending the detectable limits in refractive index measurements of disordered glass structures.