Morphological manipulation of the nonlinear optical response of ZnO thin films grown by thermal evaporation

ZnO thin films with different micro-/nano-structured morphologies have been fabricated using thermal evaporation technique. The micro-/nano-structures ranged from dense grains to nanorods and nanowires. The fabricated films were characterized using x-ray diffraction and field emission scanning electron microscopy (FE-SEM) techniques for determining their crystalline behavior and evaluating their morphology, respectively. Photoluminescence (PL) studies revealed two emission peaks in these films, one in the UV region due to exciton emission and the other in the visible spectral region due to Zn or Oxygen vacancies/defects. The effect of these different micro-/nano-structures on the third-order nonlinear optical (NLO) response has been scrutinized using the Z-scan technique with femtosecond (fs), MHz and picosecond (ps), kHz pulses at a wavelength of 800 nm. Various NLO coefficients such as two-photon absorption (β), nonlinear refractive index (n₂), Re [χ ⁽³⁾], Im [χ ⁽³⁾] and χ ⁽³⁾ were evaluated. The obtained χ⁽³⁾ values were ∼10⁻⁷ e.s.u. in the fs regime and ∼10⁻¹⁰ e.s.u. in the ps regime. Optical limiting studies of these films were also performed and limiting thresholds were estimated to be 15–130 μJ cm⁻² in the fs regime while in ps regime the corresponding values were 1–3 J cm⁻². The NLO data clearly designates strong third-order nonlinearities in these ZnO thin films with possible applications in photonics. (paper)