Linear and nonlinear optical response of sulfur-deficient nanocrystallite WS₂ thin films

Multilayered-to-bulk-like films were deposited by pulsed laser deposition at various argon gas deposition pressure where a significant amount of sulfur deficiency was observed. All films showed high atomic disorder and corresponding lattice distortion attributed to the large-scale sulfur deficiency. Instead of continuous film, huge sulfur deficiency created a mixed state of metallic tungsten and WS₂ nanocrystals of sizes 4–8 nm throughout the films. The as-deposited WS₂ films showed a dramatic shift in linear optical response, with the behavior resembling that of quantum dots. A significantly large reverse saturation absorption and positive nonlinear refraction response was observed in all the films, as measured by the open- and closed-aperture Z-scan experiment under He–Ne laser at 632.8 nm. In addition, third-order nonlinear optical susceptibility of the thin films was found to be of the order of 10⁻² esu as measured from Z-scan experiment. The anomalously high nonlinear optical response of the film was attributed to the continuous-wave laser-induced thermal nonlinearity dominance over optical nonlinearity. Optical limiting was also observed in the films where optical limiting thresholds were found to increase with an increase in nonlinear absorption coefficient.