THG of ZnO nanorods for efficient third order interferometric FROG

Abstract

Efficient third harmonic generation was found in ZnO nanorod layers grown by phase transport and low temperature chemical bath deposition method. Interferometric frequencyresolved optical gating of few cycle fs pulses was demonstrated. OCIS codes: (160.4236) Nanomaterials; (190.4400) Nonlinear optics, materials; (190.7110) Ultrafast nonlinear optics.ZnO nanostructures are of increasing interest for optoelectronics, biomedicine, and other applications. Depending on the particular growth method, they appear in various nanomorphologies with specific physical and chemical properties [1]. High temperature vapour phase transport (VPT) yields material with excellent emission properties but is limited with respect to the substrate size. ZnO from chemical bath deposition (CBD) at lower temperatures can be obtained with better uniformity on larger areas yet low efficiency photoluminescence (PL). The relationship between structure and nonlinear properties for complex nanomaterials, in particular third harmonic generation (THG) [2,3], is still less understood. The enhancement of THG in structured nanolayers was demonstrated to enable the complete characterization of ultrashort laser pulses at low dispersion and without the necessity of phase matching because of a crystal thickness well below the coherence length [4].Here we report on the efficient generation of THG with different types of ZnO nanorods grown by both VPT and CBD and third order interferometric frequency-resolved optical gating (iFROG) [5]. ZnO nanorods of comparable geometry and size were grown by CBD and VPT on fused silica and (11-20) sapphire substrates, respectively [6]. The samples were characterized after deposition by field emission scanning electron microscopes (SEM) (JEOL JSM-6400F, Hitachi H-4100FE) and X-ray diffraction (XRD; Bruker AXS D8 Advance Texture Diffractometer). The SEM micrographs in Figs. 1 (a),(b) show the structure of the densely packed nanorod layers. For the frequency conversion experiments, a Ti:sapphire laser oscillator (VENTEON) was used (pulse duration 6-7 fs, center wavelength 810 nm, repetition rate 80 MHz, pulse energy > 5 nJ, spetral FWHM bandwidth > 300 nm). The beam waist in the focus was 4 µm, yielding a peak intensity of ~ 1.5 x 10 12 W/cm 2 of the un-attenuated beam. iFROG measurements were performed using a home-made Michelson interferometer (piezo: 35 nm steps). The THG signal was separated from residual pump radiation by a THG reflecting mirror and an interference filter. The light