Efficient femtosecond second harmonic generation in thin ZnO crystal rod

Abstract

In this work, nonlinear optical properties of needle-shaped thin ZnO crystal rod with the diameter of 0.2 mm prepared by flux method using KOH as the flux in a silver crucible was studied by using femtosecond laser as the excitation source. At room temperature, coherent second-harmonic generation (SHG) and incoherent UV band-edge emission as well as green emission were observed in the sample when the excitation wavelength is shorter than 790 nm which is almost two folds of the corresponding wavelength (i.e., 396 nm) of ZnO fundamental band gap. As the excitation wavelength exceeds 790 nm, the band-edge emission and green band become undetectable. In the meanwhile, the SHG of the crystal becomes more efficient. This phenomenon can be understood by considering the different natures of the SHG and the band-edge emissions. The SHG is a standard two-photon process (TPP), while the band-edge emission will change from a TPP to a multi-photon process (MPP) when the excitation wavelength is longer than 790 nm. Observation of efficient SHG signal in ZnO single crystal rod means strong second-order optical nonlinearity of ZnO and suggests possible applications of ZnO in nonlinear optics.