Structural and Femtosecond Third-Order Nonlinear Optical Properties of Sodium Borate Oxide Glasses: Effect of Antimony

Structural and optical properties of antimony-containing sodium borate glasses were studied and their ultrafast third-order nonlinear optical (NLO) properties have been evaluated using Z-scan measurements with femtosecond (fs) pulses (∼150 fs, 80 MHz) at 750, 800, and 880 nm wavelengths. Glasses in the (mol %) 20Na2O–(80 – x)­B2O3–xSb2O3 (where x = 0, 10, 20, and 30) system have been fabricated via melt quench technique. The structural modifications were analyzed using the Raman and magic angle spinning (MAS)-nuclear magnetic resonance (NMR) (11B MAS-NMR and 23Na MAS-NMR) techniques. The optical absorption spectra revealed that the absorption edge was red-shifted, suggesting the decrease in band gap energy with increase of antimony content in the glasses. Raman scattering results revealed that the boroxol rings are depressed with the incorporation of Sb2O3 for replacing B2O3. 11B MAS-NMR results showed a progressive increase of B4 units at the expense of B3 units. The Raman and 11B MAS-NMR results support the formation of Sb5+ ions due to oxidation of Sb3+ that played the role of charge compensation. 23Na MAS-NMR spectra revealed a decreasing trend in the average of bond lengths of Na–O with increasing Sb2O3 contents. This suggested that sodium changed its role from charge compensator to modifier cation. The antimony-containing glasses demonstrated a reverse saturable absorption in open-aperture Z-scan mode due to two-photon absorption, while closed-aperture Z-scan signatures depicted positive nonlinear refraction due to self-focusing effect. The NLO coefficients were found to increase with Sb2O3 due to the increased nonbridging oxygens and also due to the hyperpolarizability of Sb3+ and Sb5+ ions. The observed NLO data clearly suggest that the investigated glasses are beneficial for optical limiting applications.