Second Harmonic Generation from ZnO Nanostructured Surface by Femtosecond Laser Pulses at Near Damage Threshold Intensities

In this contribution we report on experimental investigation of SHG from ZnO nanosctructured surfaces by 30 fs, 800 nm laser pulses at the intensity level in the range 1011 – 1012 W/cm2, close to the structure damage threshold. Experiments were performed with two morphologically different types of samples produced by physical vapour deposition method. The first type, the “grass” sample, is a disordered array of crystalline nanowires with an average diameter about 350 nm, average spacing about 1 μm and an average of length several micron arranged vertically on a 500 μm thick ZnO substrate deposited on a 500 μm silicon wafer. The second type, the “spaghetti” sample, represents a pile of nanowires with the parameters as for the grass-sample, but chaotically arranged along the surface (see inset images at Fig.1). Both samples have about 1 cm2 area. The experimental setup is presented in Fig.1a. A sample was located several centimetres before the focus of a 40 cm focusing length lens were the beam size is about 1.5 mm, ensuring that the intensity is well below the damage threshold even for the highest energy 0.9 mJ used in the experiments. The second harmonic emission was collected by a 1 inch size short focusing lens and an UV optical fiber located near the focus of the collecting lens and attached to a spectrometer (Ocean Optics HR4000 UV-NIR). The collecting lens and the fiber were mounted on a rotating rail with a sample located precisely on the rotation axis to measure the angular resolved pattern of the SHG emission.