Saturable absorption in NV-doped diamond studied by femtosecond Z-scan

Nonlinear optical absorption in commercially available diamond crystals containing nitrogen-vacancy (NV) centers is investigated using femtosecond open-aperture Z-scan measurements at a wavelength of 1032 nm. Experiments were performed on a high-purity electronic-grade diamond and on samples with moderate and high NV concentrations. While the undoped crystal exhibits third-order nonlinear absorption, the NV-doped samples show a pronounced saturable absorption response. To elucidate the microscopic origin of this effect, the Z-scan results are analysed in conjunction with linear transmission spectra, which reveal the presence of additional nitrogen-related defect complexes, including H2 ( \(\hbox {NVN}^{-}\) ) centers. Although the excitation wavelength is detuned from the principal zero-phonon lines of both NV and H2 defects, it overlaps with the broader near-infrared absorption band associated with vibronic transitions. A quantitative analysis based on an effective two-level model indicates that the observed saturable absorption is most consistently explained by a dominant contribution from H2 defects, while a weaker off-resonant contribution from NV centers cannot be excluded. The extracted parameters, including the linear absorption coefficient and saturation intensity, are supported by a reconstruction of the defect-related absorbance spectrum and by consistency with previously reported nonlinear refractive index data. These results highlight the importance of complex defect environments in determining the nonlinear optical response of NV-doped diamond and provide insight into defect-specific contributions relevant for diamond-based nonlinear and quantum photonic applications.