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

Abstract

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.

Ultrafast Nonlinear Optical Response and Carrier Dynamics in Layered Gallium Sulfide (GaS) Single-Crystalline Thin Films

“…And the figure of merit (FOM) for the third-order optical nonlinearity (FOM Im |Imχ (3) /α 0 |) (Dong et al, 2016) can be determined to be −2.7 × 10 -13 (esu cm) and 8.9 × 10 -13 (esu cm) for 532 and 650 nm, respectively. The FOM of β-GaS film is higher than the reported values of WS 2 and Sb 2 Te 3 thin films (Dong et al, 2016;Verrone et al, 2020), indicating the β-GaS is a promising candidate for applications in optical limiting and ultrafast saturable absorber.…”

“…The Z-scan curves were fitted using the standard Z-scan theory (Sheik-Bahae et al, 1990), and the effective nonlinear absorption coefficients were determined to be −1.8 × 10 -8 m/W and 4.9 × 10 -8 m/W for 532 and 650 nm, respectively. Pervious results demonstrate that the film thickness has a significant influence on the NLO response of 2D material (Zhang et al, 2018;Verrone et al, 2020). However, the experimental data of β-GaS thin film with different thickness was not available due to the limitation of film deposition system and the low laser damage threshold of few-layer thin film.…”

Laser Annealing of Sb2Te3 2D Layers towards Nonlinear Optical Applications

“…Our group had previously demonstrated that Sb 2 Te 3 films thicker than 6 nm exhibit significant nonlinear absorption, while nonlinear refraction is negligible [7]. For this reason, in the case of the 10 nm thick Sb 2 Te 3 films studied in this work, the open aperture recordings were of main interest.…”

“…A quartz microbalance was used to measure the Sb 2 Te 3 thickness with a precision of ±0.2 nm. A thickness of 10 nm was chosen because we had previously demonstrated that the maximum nonlinearity for the Sb 2 Te 3 material by our deposition method was between 8 and 11 nm [7]. The layers were capped by a 2 nm thick SiO 2 layer.…”

“…Our group had previously demonstrated that Sb 2 Te 3 thin films can have unprecedented nonlinearities when irradiated with laser pulses. Our demonstrations were conducted in the nanosecond and femtosecond regimes in the visible and IR parts of the spectrum [5][6][7]. The optical nonlinearities of the Sb 2 Te 3 material were employed to perform super-resolution and mode-locking of laser systems [8,9].…”