Electro-thermal poling in bioactive sodium‑calcium phosphate-silicate glass: Anodic near-surface network connectivity changes and second harmonic generation

Abstract

We report Raman spectra in the sub-anodic region after electro-thermal poling of the 46S4 bioglass and show a correlation between structural changes and the induced second harmonic generation (SHG). The Raman peak at ca. 625 cm⁻¹, due to the symmetric stretching-bending vibration of Si-O-Si bridges, was employed to quantify the poling-induced structural changes since its frequency ν_s(Si-O-Si) varies linearly with the silicate network connectivity, NC. Key results include the increase of network connectivity towards the anode after poling, with NC varying from 2.24 before poling to 2.37 at the anode after poling. SHG measurements showed the presence of an optically-inactive layer of 1.0 μm thick below the anode, followed by an optically-active layer where the SHG signal is maximized from about 1.5 to 4.0 μm. In this layer the NC takes intermediate values and reflects modest changes with respect to the pristine bioglass. The present findings suggest that the transport of electrons near the metal film anode is faster than the Na ion transport towards the cathode.