Defect-Contributed Nonlinear Absorption Properties and Optical Limiting Potential of NaBi(WO 4 ) 1.6 (MoO 4 ) 0.4 Single Crystals

In this study, the nonlinear optical response and optical limiting performance of a NaBi(WO4)1.6(MoO4)0.4 single crystal grown by the Czochralski technique were systematically examined. The optical band gap of the synthesized crystal was evaluated as 3.33 eV, while the Urbach energy associated with structural defects, lattice disorders, and thermal effects was estimated to be 0.21 eV. Photoluminescence measurements reveal a wide emission band spanning the visible region from 400 to 700 nm, with pronounced emission peaks in the 438–486 nm range, indicating dominant near-band-edge recombination processes and confirming the direct band gap nature of the material. Nonlinear absorption characteristics were investigated using open-aperture Z-scan experiments under 532 nm excitation (photon energy of 2.32 eV). The results indicate that two-photon absorption (TPA) is the primary nonlinear mechanism governing the optical response. To elucidate the role of defect-related states, the experimental data were analyzed using two distinct theoretical approaches: a conventional TPA-based model and an extended model incorporating one-photon absorption, TPA, and free-carrier absorption. The effective nonlinear absorption coefficients obtained from the extended model were consistently higher than those derived from the TPA-only model, demonstrating that defect-assisted absorption processes significantly enhance the overall nonlinear response. Additionally, the NaBi(WO4)1.6(MoO4)0.4 single crystal exhibits a low optical limiting threshold of 1.22 mJ/cm2 at the minimum incident energy density, highlighting its strong attenuation capability under high-intensity illumination. These findings suggest that this crystal is a promising candidate for optical limiting applications in the visible spectral range.