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.
The authors acknowledge CIF, IIT Guwahati for micro-Raman, FETEM, AFM, FESEM and spectroscopic ellipsometer facilities. The authors also acknowledge department of Physics, IIT Guwahati, for XRD and UV–Vis facilities.
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Conflict of interest
The author(s) declares no competing interests.
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