Laser beam intensity and wavelength dependent nonlinear absorption in pulse laser deposited WS2 thin films

Abstract

WS2 thin films were deposited using pulsed laser and characterized by UV-VIS-near IR absorption spectroscopy, atomic force microscope (AFM), scanning electron microscope/energy dispersive X-ray spectroscopy (SEM/EDX) analysis, and Z-scan measurement. The surface roughness decreases as the thickness of WS2 thin films increases from 11 nm to 30 nm. The optical bandgap energy is evaluated 2.16 and 1.98eV for the thickness of 11 and 30 nm WS2 thin films, respectively. The Z-scan method was utilized to estimate the nonlinear absorption (NLA) coefficient using continuous He–Ne (632.8 nm) and diode laser (655 nm). The WS2 thin films exhibit a saturable absorption coefficient around 104cmkW with a negative sign (β < 0) for He–Ne laser. While the nonlinear absorption coefficient is 103cmkW with positive sign (β > 0) due to reverse saturable absorption or two photon absorption for the diode laser. The nonlinear optical absorption is strongly affected by laser beam intensity and laser wavelengths depending on the thickness of WS2 thin films. Two different nonlinear optical behavior have completely been exhibited depending on laser wavelength.