Modification of the picosecond optical absorptive nonlinearity by a nanosecond irradiation in a nanostructured ZnO thin film

Nanostructured ZnO thin solid films were synthesized by electrodeposition of hydrothermal sol-gel solutions on TCO glass. A strong enhancement in the absorptive and refractive nonlinear optical response of the sample was obtained by means of an annealing process at the end of the preparation method. A simple configuration for generating an increment in the optical transmittance of the sample with different optical irradiations was proposed. The nonlinear optical measurements were performed using the Z-scan technique with picosecond pulses and a transmittance experiment with nanosecond pulses. Picosecond results indicated an important electronic refractive nonlinearity exhibited as a self-focusing effect; whereas for nanosecond pulses, a dominant nonlinear optical response was responsible for the third order nonlinearity. In both temporal regimes a saturable absorption was identified as the main physical mechanism of nonlinear optical absorption; however strong differences in the magnitude of this phenomenon were observed. A physical explanation based on a two-level system was evoked to describe the irradiance conditions whereby the optical nonlinearities were manifested for each case. Potential applications for developing all-optical switching devices were contemplated.