Composition-dependent femtosecond laser damage and incubation effects in Pb2P2O7–Nb2O5 phosphate glasses

Femtosecond laser micromachining was investigated in a series of phosphate glasses with systematically varied Nb 2 O 5 concentration, aiming to correlate their linear optical properties with single-pulse and multi-pulse damage behavior. UV–Vis absorption spectroscopy revealed a progressive reduction in optical bandgap energy with increasing niobium oxide content, consistent with the formation of a more polarizable glass network. The bandgap values show excellent agreement with those previously reported for glasses of identical compositions, confirming the reliability of the structural–optical trends. Single-pulse and multi-pulse ablation thresholds were determined for all samples, demonstrating a monotonic decrease in damage fluence as Nb 2 O 5 concentration increases. Incubation was observed in all glasses, with a systematic reduction of threshold fluence as the number of pulses increased, revealing a clear composition-dependent sensitivity to cumulative laser exposure. The incubation coefficients extracted from the fluence-versus-pulse-number analysis confirmed that samples with lower bandgap energies exhibit more pronounced multi-pulse modification behavior. To provide additional insight into the underlying ionization regime, the Keldysh parameter was calculated for each composition using measured bandgap energies and refractive index values obtained from the literature. All glasses exhibited γ < 1 under the experimental conditions, indicating a predominantly tunnelling ionization regime for the onset of damage. Together, these results establish a direct link between the optical properties of niobium phosphate glasses and their response to ultrafast laser irradiation, offering a consistent framework for predicting micromachining performance across this glass family. • Optical bandgap decreases with increasing Nb 2 O 5 content • Laser damage thresholds correlate directly with glass optical properties • Strong incubation effects observed under multi-pulse femtosecond irradiation • Lower bandgap enhances cumulative laser sensitivity • Damage onset dominated by tunnelling ionization