We report angle-resolved second-harmonic generation (SHG) measurements from suspensions of centrosymmetric micron-size polystyrene spheres with surface-adsorbed dye (malachite green). The second-harmonic scattering profiles differ qualitatively from linear light scattering profiles of the same particles. We investigated these radiation patterns using several polarization configurations and particle diameters. We introduce a simple Rayleigh-Gans-Debye model to account for the SHG scattering anisotropy. The model compares favorably with our experimental data. Our measurements suggest scattering anisotropy may be used to isolate particle nonlinear optics from other bulk nonlinear optical effects in suspension.

Polymer materials usually present lower surface flashover strength compared with their bulk breakdown strength, particuarly in a vacuum. Flashover along the solid insulation surface may pose a significant challenge to the safety of electrical equipment. In this paper, two kinds of polymers, epoxy resin and polyethylene, have been modified with F 2 /N 2 fluorination to decrease the secondary electron yield (SEY) and further improve the surface electric withstanding strength. Experimental results have proved the effectiveness of this method in overall perspectives. The surface properties of modified samples are evaluated by comprehensive methods, i.e. surface roughness, scanning electron microscopy, charge trap characteristics and x-ray photoelectron spectroscopy. It is considered that physical factors such as roughness and chemical factors such as fluorine groups play a significant role in the modified mechanism. Reduction of SEY is linked with flashover voltage quantitively using multipactor and outgassing theory. In addition, we perform a simulation to illustrate multipactor dynamics in the presence of a surface barrier which represents specific surface roughness. The influence of trap characteristics is also analyzed theoretically to corroborate the experiment results. It is concluded that SEY is influenced by surface roughness and shallow traps. The reduction of SEY is directly linked with improvement of flashover voltage. Trap characteristics are changed by introducing fluorine groups. A comprehensive model of fluorination-based flashover mitigation is proposed considering the above features.

Gait recognition, i.e., recognizing persons from their walking postures, has found versatile applications in security check, health monitoring, and novel human-computer interaction. The millimeter-wave (mmWave) based gait recognition represents the most recent advance. Compared with traditional camera-based solutions, mmWave based gait recognition bears unique advantages of being still effective under non-line-of-sight scenarios, such as in black, weak light, or blockage conditions. Moreover, they are able to accomplish person identification while preserving privacy. Currently, there are only few works in mmWave gait recognition, since no public data set is available. In this paper, we build a first-of-its-kind mmWave gait data set, in which we collect gait of 95 volunteers 'seen' from two mmWave radars in two different scenarios, which together lasts about 30 hours. Using the data set, we propose a novel deep-learning driven mmWave gait recognition method called mmGaitNet, and compare it with five state-of-the-art algorithms. We find that mmGaitNet is able to achieve 90% accuracy for single-person scenarios, 88% accuracy for five co-existing persons, while the existing methods achieve less than 66% accuracy for both scenarios.

Lifetime is crucial to the application of advanced thermal barrier coatings (TBCs), and proper lifetime evaluation methods should be developed to predict the service lifetime of TBCs precisely and efficiently. In this study, plasma‐sprayed YSZ TBCs were subjected to gradient thermal cyclic tests under different surface temperatures, with the aim of elucidating the correlation between the coating surface temperature and the thermal cyclic lifetime. Results showed that the thermal cyclic lifetime of TBCs decreased with the increasing of surface temperatures. However, the failure modes of these TBCs subjected to thermal cyclic tests were irrespective of different surface/BC temperatures, that is, sintering‐induced delamination of the top coat. The thickness of thermally grown oxide (TGO) was significantly less than the critical TGO thickness to result in the failure of TBCs through the delamination of top coat. There was no phase transformation of the top coat after failure. In contrast, in the case concerning the top coat surface of the failure specimens, the elastic modulus and microhardness increased to a comparable level due to sintering despite of the various thermal cyclic conditions. Consequently, it is conclusive that the failure of TBCs subjected to gradient thermal cyclic test was primarily induced by sintering during high‐temperature exposure. A delamination model with multilayer splats was developed to assist in understanding the failure mechanism of TBCs through sintering‐induced delamination of the top coat. Based on the above‐described results, this study should aid in facilitating the lifetime evaluation of the TBCs, which are on active service at relatively lower temperatures, by an accelerated thermal cyclic test at higher temperatures in laboratory conditions.

We have performed ultrafast second-harmonic generation spectroscopy of GaN/Al 2 O 3 . A formalism was developed to calculate the nonlinear response of thin nonlinear films excited by an ultrashort laser source (Ti:Al 2 O 3 ), and then used to extract zxx(2) (ϭ2 o ) and xzx (2) (ϭ2 o ) from our SHG measurements over a two-photon energy range of 2.6-3.4 eV. The spectra are compared to theory ͓J. L. P. Hughes, Y. Wang, and J. E. Sipe, Phys. Rev. B 55, 13 630 ͑1997͔͒. A weak sub-band-gap enhancement of zxx(2) (ϭ2 o ) was observed at a two-photon energy of 2.80 eV; it was not present in xzx(2) (ϭ2 o ). The enhancement, which may result from a three-photon process involving a midgap defect state, was independent of the carrier concentration, intentional doping, and the presence of the ''yellow luminescence band'' defects. In addition, we determined sample miscuts by rotational SHG; the miscuts did not generate observable strain induced nonlinearities. The linear optical properties of GaN from 1.38 to 3.35 eV were also determined.

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