In a recent article published in the Journal of Alloys and Compounds, researchers investigated the structural, optical, and nonlinear optical properties of borotellurite glasses infused with gold nanoparticles (Au NPs), synthesized using the melt-quenching technique.
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Background
Borotellurite glasses are attracting significant interest for nonlinear-optical applications, including frequency converters, optical switches, and signal-processing components. They possess higher values of third-order nonlinear susceptibility (χ(3)) compared to typical silicate glasses, largely attributed to the high polarizability of tellurium ions. Incorporating borate helps balance strong nonlinear properties with good glass-forming ability, which is necessary for producing optical fibers and bulk samples with enhanced nonlinear characteristics. Heavy metal oxides, such as bismuth oxide (Bi2O3), are also used to enhance linear and nonlinear refractive indices due to the strong polarizable nature of Bi³? ions, making the resulting materials valuable for optoelectronic devices and IR transmission. Furthermore, glasses doped with metal nanoparticles (like Au, Ag, or Cu) display unique optical characteristics, particularly surface plasmon resonance (SPR), resulting from the collective oscillation of conduction electrons. Nonlinear optical responses in these nanoparticle-embedded glasses can be dramatically amplified by SPR, electronic transitions, and local field enhancement. Gold is specifically highlighted for its high plasmonic efficiency, greater stability, and more pronounced resonance peaks in the visible region. Although some studies have explored Au-doped borate and tellurite systems, limited reports exist on borotellurite glasses containing gold nanoparticles.
The Current Study
The prepared glasses were characterized for morphological, structural, thermal, and optical properties. High-resolution TEM (HR-TEM) was used for morphological studies, confirming the uniform distribution of nearly spherical Au nanoparticles. X-ray diffraction (XRD) confirmed the amorphous nature of the samples. Structural analysis was carried out using Fourier-transform infrared (FTIR) and Raman spectroscopy. Thermal behaviour was assessed using Differential Scanning Calorimetry (DSC). Optical absorption spectra were recorded in the 200–1100 nm range. The nonlinear optical behaviour was investigated using the Z-scan technique. Nonlinear absorption coefficient (α2), and closed aperture (CA) measurements to evaluate the nonlinear refractive index (n2) were determined.
Results and Discussion
HR-TEM confirmed uniformly distributed, spherical Au nanoparticles with sizes ranging from approximately 8 to 22 nm, showing size growth at higher gold concentrations. Elemental mapping validated the uniform distribution of these nanoparticles. The decrease in TeO4-related bands and the increase in borate-based bands suggest that Au nanoparticles encourage the formation of non-bridging oxygens (NBOs) and boost structural disorder.
Optical studies revealed systematic modifications with increasing Au concentration. Specifically, the indirect optical bandgap (Eg) decreased from 2.22 to 1.68 eV, while the Urbach energy (ΔE) increased from 0.113 to 0.347 eV. This decrease is linked to structural changes, including the conversion of TeO4 units into more distorted TeO3 units and the formation of NBOs, which introduce additional localized states within the band-tail region, thereby narrowing the bandgap. The appearance of a distinct surface plasmon resonance (SPR) band, particularly in samples with higher Au content (NaBiBTeAu-0.02 and NaBiBTeAu-0.05), confirms the presence of nanosized Au particles. The refractive index increased with Au concentration due to the greater electronic polarizability of NBOs, which are formed as the vibrational dynamics of Te-O bonds are disturbed.
Nonlinear optical measurements using the Z-scan technique demonstrated a significant enhancement in reverse saturable absorption (RSA). The nonlinear absorption coefficient (α2) increased from 1.25×10−11 to 1.90×10−11 m W?¹, while the nonlinear refractive index (n2) improved from 0.18×10−18 to 0.63×10−18 m² W?¹. This enhancement is attributed to the presence of Au NPs, as their localized surface plasmon resonance (LSPR) creates a strong local electromagnetic field when exposed to high-intensity laser radiation, which boosts the material's nonlinear polarization. The presence of RSA nonlinearity and a positive nonlinear refractive index confirm strong optical limiting behaviour. All tested samples exhibited a figure of merit (FOM) greater than 1, indicating their potential suitability for nonlinear optical applications.
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Conclusion
Borotellurite glasses containing gold nanoparticles were successfully synthesized using the melt-quenching technique. HR-TEM and elemental mapping confirmed the uniform distribution of spherical Au NPs, and XRD verified the amorphous nature of the glasses. Structural analyses (FTIR and Raman) confirmed the formation and evolution of Te–O and B–O structural units. The optical studies showed that increasing Au concentration resulted in an SPR band, a decrease in the optical bandgap, and an increase in Urbach energy, indicative of enhanced structural disorder, and the introduction of localized states. The Z-scan analysis demonstrated that the incorporation of Au NPs significantly enhanced the glasses' nonlinear absorption and nonlinear refraction. The enhanced α2 and n2 values confirm the strong nonlinear responses of these Au-doped borotellurite glasses, marking them as promising candidates for advanced optical limiting applications, offering protection for sensitive optical components and human eyes against intense laser radiation.
Journal Reference
Ganji P.V. and Pasha U.M., "Structural, optical and nonlinear optical properties of gold nanoparticle-embedded borotellurite glasses," Journal of Alloys and Compounds, vol. 1050, 185776, 2026. https://doi.org/10.1016/j.jallcom.2025.185776, https://www.sciencedirect.com/science/article/pii/S0925838825073402