Sign Reversal of Optical Nonlinearity by Pauli Blocking in Blue Light-Emitting Samarium-Doped ZnO Nanostructures

This study reveals a notable transition in the nonlinear optical response of zinc oxide (ZnO) and samarium-doped ZnO (ZnO:Sm), exhibiting a switch from reverse saturable absorption (RSA) to saturable absorption (SA), as evidenced by nonlinear transmission measurements using open-aperture Z-scan at 532 nm nanosecond excitation. The structural, morphological, and optical properties of ZnO and ZnO:Sm nanostructures synthesized via the chemical precipitation method were systematically evaluated using X-ray diffraction, field emission scanning electron microscopy, UV–vis absorption spectroscopy, and both steady-state and time-resolved photoluminescence measurements at room temperature. Optical absorption studies reveal significant absorption in both the UV and visible regions, with a notable reduction in the band gap from 3.29 eV (pure ZnO) to 2.56 eV (6% ZnO:Sm). When the samples were exposed to nanosecond pulsed laser, it exhibits RSA for pure ZnO, which changes to SA on increasing the doping concentration of samarium for the pulse energy of 45 μJ. The third-order nonlinear optical coefficient (β) significantly shifted from 0.65 × 10 –10 m/W for pure ZnO to 0.02 × 10 –10 m/W for doped samples, with saturation intensity ( I S ) varying from 8.0 × 10 10 W/m 2 for undoped ZnO up to 100.0 × 10 10 W/m 2 for doped samples, indicating enhanced optical transparency with samarium doping. This transition is attributed to Pauli blocking, in which the filling of the conduction band limits any further absorption, resulting in a controlled nonlinear response that makes these films highly beneficial for optical limiting and ultrafast photonic device applications.