Abstract
The nonlinear optical properties of thin ZnO film are studied using interferometric autocorrelation (IFRAC) microscopy. Ultrafast, below-bandgap excitation with 6-fs laser pulses at 800 nm focused to a spot size of 1 µm results in two emission bands in the blue and blue-green spectral region with distinctly different coherence properties. We show that an analysis of the wavelength-dependence of the interference fringes in the IFRAC signal allows for an unambiguous assignment of these bands as coherent second harmonic emission and incoherent, multiphoton-induced photoluminescence, respectively. More generally our analysis shows that IFRAC allows for a complete characterization of the coherence properties of the nonlinear optical emission from nanostructures in a single-beam experiment. Since this technique combines a very high temporal and spatial resolution we anticipate broad applications in nonlinear nano-optics.
Highlights
When illuminating semiconducting or metallic solid state nanostructures with intense and broadband ultrashort optical pulses, a variety of nonlinear optical processes such as second or third harmonic generation (SHG, THG), multiphoton-induced luminescence (MPL), photoemission and others are induced
The origin of the blue-green emission has strongly been debated in the literature [21] and it is generally believed that this emission involves multiple defects and/or defect complexes
We have studied the nonlinear optical properties of thin zinc oxide films using interferometric frequency-resolved autocorrelation (IFRAC) microscopy following impulsive excitation with 6-fs optical pulses focused to a spot size of 1 μm
Summary
When illuminating semiconducting or metallic solid state nanostructures with intense and broadband ultrashort optical pulses, a variety of nonlinear optical processes such as second or third harmonic generation (SHG, THG), multiphoton-induced luminescence (MPL), photoemission and others are induced. The wide band gap energy of ZnO of 3.37 eV at room temperature and its large exciton binding energy of ~60meV makes it a highly interesting material for various optoelectronics applications [3,4]. The nonlinear optical properties of a variety of different ZnO thin films [7,8,9,10] and nanostructures [11,12,13,14,15] have been studied extensively. It is found that nonlinear optical efficiencies in ZnO nanostructures can be significantly larger than those of ZnO thin films but that the relative intensities of the different harmonic generation and photoluminescence contributions depend critically on the structural and morphological characteristics of the nanostructures as well as on the nature and concentration of defects in these samples [16]
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