Thermo-optic effects mediated self focusing mechanism and optical power limiting studies of ZnO thin films deposited on ITO coated PET substrates by RF magnetron sputtering under continuous wave laser regime

Abstract

In this work, ZnO thin films are deposited on ITO coated PET substrates employing RF magnetron sputtering technique. The evolution of crystallites orientation on (002) plane with increasing thickness of the films is unveiled from XRD patterns. Further, the appearance of (103) plane in the highest thickness film (183 nm) is attributed to the formation of surface layer during sputtering process. The smooth and homogenous natures of the films are displayed by the interference pattern in transmittance spectra. The presence of defect level emission in the luminescence spectra is ascribed to compressive stress in the films. The exhibition of orange-red emission confirms the excess oxygen in the sputtered films (89 nm and 183 nm). The dominance of thickness on the third-order nonlinear optical parameters is revealed by single beam Z-scan measurements. The lower thickness ZnO film (42 nm) shows an interesting self focusing behavior under closed aperture Z-scan measurements. A transition from self focusing to self defocusing behavior is noticed with increasing thickness values from 42 nm to 183 nm. The self focusing property is attributed to the combination of presence of large dipole moments at the substrate-film interface as result of large energy gap gradients and thermo-optic effects.