Nonlinear features of UV laser-induced excitonic luminescence in ZnO nanopowder

ZnO is one of the best photoactive wide-gap metal-oxide semiconductor materials, whose optical and electronic properties are of great interest for modern nanophotonics and quantum electronics [1,2]. In particular, photoexcited ZnO exhibits excitonic photoluminescence (PL) in the near UV spectral range even at room temperature (RT). But despite advances in ZnO nanophysics and nanotechnology, some features of RT excitonic PL of ZnO nanoparticles (NP) are not so clear, especially in disordered nanostructures with random light scattering, such as nanopowder. We tried to study this issue in more detail. Experiments were carried out with fine-disperse ZnO powders of high grade quality obtained by hydrothermal process and characterized by XRD, SEM, Raman and UV-visible spectroscopy. Measurements showed that ZnO NPs have a nodular shape with sizes from 1 μm to 100 nm and nanocrystallites of ~20 nm. In the PL spectra excited in ZnO powder layers by a pulsed N₂ laser at 337 nm, we observed a nonlinear amplification of excitonic UV PL emission with a maximum at 387 nm and a decay of the relatively weak visible PL emission with a maximum at 500 nm. A superlinear growth in the intensity of the excitonic UV PL band with its narrowing with increasing excitation intensity indicates stimulated emission with incoherent feedback in the ZnO nanopowder. This regime is realized over the entire excitation range due to non-resonant diffusion mode of optical amplification, similar to random lasing in Letokhov-type scattering photonic media. Estimated gain is ~100 cm^-1, threshold is ~1 mJ/cm². The results look promising. Research is in progress. [1] A. Tashiro, Y. Adachi, T. Uchino, J. Appl. Phys. 133, 221101 (2023). [2] H. Cao, Y. Eliezer, Appl. Phys. Rev. 9, 011309 (2022).