Thickness-Dependent Optical Nonlinearities of Nanometer-Thick Sb₂Te₃ Thin Films: Implications for Mode-Locking and Super-resolved Direct Laser Writing

2D materials are currently very promising candidates for various photonic applications. Optimizing their optical nonlinearities requires a thorough adjustment of several properties including the film thickness. In this work thin Sb₂Te₃ layers with different thicknesses (ranging from 2.5 to 50 nm) are prepared by the electron beam deposition technique, and then they are properly annealed in order to achieve significant third order nonlinearities. The film structure and morphology are extensively studied by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy. The presence of nanocrystals with sizes highly depending on the film thickness has been observed. Optical studies are carried out by Vis–NIR spectrophotometric studies. Finally, a thickness dependent study of the ultrafast third order nonlinear optical properties of the Sb₂Te₃ thin films is performed. This study is carried out by means of the Z-scan technique, employing 400 fs laser pulses at 1030 nm. The observed optical nonlinearities are very high, compared with those of state-of-the-art nonlinear optical materials. Moreover they are highly dependent on the thickness of the layers. The findings demonstrate the importance of a fine adjustment of the Sb₂Te₃ thickness in order to enhance its nonlinear optical efficiency. They are expected to be of significant importance for mode-locking of laser systems and super-resolved direct laser writing.