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

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New York State College of Ceramics at Alfred University. Inamori School of Engineering.
Lanthanum aluminosilicate (SAL) glasses are of interest due to their superior physical and chemical properties. The large glass forming region allows for incorporation of rare-earth elements for expanded and detailed tuning of glass properties. Lanthanum inclusions within the aluminosilicate network increase the refractive index based on the concentration. The amount of lanthanum can be tailored to develop optical materials with a corresponding third-order susceptibility applicable for non-linear optical applications. A novel method for altering the base glass refractive index utilized femtosecond laser modification, where the timescale of the pulse duration permits structural alterations in a locally defined volume for the modification of optical components. We studied femtosecond laser modifications of SAL glasses with 11-23 mol% La2O3 content. Refractive indices of these glasses in the visible and the terahertz frequencies were measured and correlated to change in glass structure. The role of lanthanum was compared to traditional glass modifiers within the glass structure in terms of polarizability and optical basicity for femtosecond laser refractive index modification.²⁷ Al Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) was used to identify Al-speciation within the SAL glasses. We used terahertz time-domain spectroscopy (THz-TDS) for THz refractive index measurements. Our results show a linear correlation between refractive index measurements in visible and terahertz frequencies. In addition, our results demonstrate that THz-TDS can be possible means of extending the detectable limits in refractive index measurements of disordered glass structures.
Thesis completed in partial fulfillment of the requirements for the degree of Master of Science in Glass Science at the Inamori School of Engineering, New York State College of Ceramics at Alfred University
Optical materials, Refractive index, Glass