Spectral Dispersion of Third‐Order Nonlinearities in NbOPO ₄ ‐Modified Tellurite Glasses

ABSTRACT

Tellurite‐based glasses have been widely investigated for nonlinear photonic applications due to their high optical nonlinearity and structural flexibility. This work aims to characterize the nonlinear refractive index (n ₂ ) and two‐photon absorption coefficient () of tellurite‐phosphate niobium glasses, and to evaluate the influence of NbOPO ₄ incorporation on their nonlinear optical (NLO) response. NLO experiments were performed using the Z‐scan technique with femtosecond laser pulses (, ) across the 500–1500 nm range. In addition, x‐ray photoelectron spectroscopy (XPS) was used to investigate the chemical environment of Te, Nb, O, and P species. XPS analysis revealed predominant Nb ⁵⁺ and Te ⁴⁺ oxidation states, accompanied by a decrease in the NBO/(NBO + BO) ratio from 0.73 to 0.64 with increasing NbOPO ₄ content, which may be explained by enhanced network connectivity through Te─O─P/Nb linkages. This structural evolution justifies the n ₂ behavior with changing composition. The spectra exhibited two distinct regimes relative to the bandgap energy (): at , a strong nonlinear absorption was observed and n ₂ reached its maximum value (∼); at , became negligible, while n ₂ remained relatively high compared to silica (∼). This dispersion is consistent with the Kramers–Kronig relation and the Sheik‐Bahae model for amorphous semiconductors. Given their reduced losses in the near‐infrared (NIR) region, commonly used in telecommunications, these glasses have proven to be promising candidates for photonic devices.