Abstract
The second harmonic (SH) generation from the highly epitaxial Al-doped ZnO film on sapphire was measured, using the femtosecond Ti:Sapphire laser at the near-resonant SH wavelength, in reflection geometry to avoid the sapphire's contribution in the conventional Maker fringes technique. By investigating SH intensities as a function of the azimuthal angle along the film's normal, we found that the sapphire substrate had a negligible contribution to the reflective SH signal and the film had a pure and well-aligned c-domain. We also developed a new method to calculate the component's ratios of the nonlinear susceptibility tensor by analyzing the polarization diagrams of SH intensities under the incidence with two different angles. The ratios indicate that Kleinman's symmetry is broken due to the absorption at SH wavelength and the dominant component of the nonlinear susceptibility tensor is d(33). Calibration using the Z-cut quartz shows a possible overestimate of the nonlinear response by Maker fringes technique.
Highlights
Zinc oxide (ZnO), as a transparent conducting material [1,2], a piezoelectric oxide [3] and a wide band-gap semiconductor, has been widely used as transparent electrodes in flat panels and solar cells, transducers in acoustoelectrical devices [4], and attracted much interest in the development of various optical devices working in the near ultraviolet region due to the strong commercial desire
In the previous second harmonic generation (SHG) measurements on ZnO film grown on sapphire [9], second harmonic (SH) signal from the substrate was not carefully investigated and excluded
We report our SHG measurements at a near-resonant SH wavelength for the epitaxial Al:ZnO thin film in reflection geometry to reduce SH signal from the sapphire substrate
Summary
Zinc oxide (ZnO), as a transparent conducting material [1,2], a piezoelectric oxide [3] and a wide band-gap semiconductor, has been widely used as transparent electrodes in flat panels and solar cells, transducers in acoustoelectrical devices [4], and attracted much interest in the development of various optical devices working in the near ultraviolet region due to the strong commercial desire. Most measurements of second harmonic generation (SHG) were performed on the polycrystalline films on non-polar substrates such as glass or textured ZnO film on sapphire using Maker fringes technique at the fundamental wavelength of 1064 nm by assuming Kleinman’s symmetry [9,10]. We report our SHG measurements at a near-resonant SH wavelength for the epitaxial Al:ZnO thin film in reflection geometry to reduce SH signal from the sapphire substrate (coherent length is about 20 nm in reflection geometry). We developed a method, which is different from the Maker fringes technique, to calculate the component’s ratios of the nonlinear susceptibility tensor by analyzing the polarization diagrams of the reflected SH radiation without assuming Kleinman’s symmetry under the incidence of two different angles. The absolute values of the susceptibility were calibrated by using a Z-cut quartz crystal
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