Browsing by Author "Wu, Jun"

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    Development and characterization of XUV xenon laser
    (1994) Wu, Jun; Young, James F.
    Short wavelength lasers have many potential scientific and engineering applications. The goal of this research is to develop and improve a practical XUV Xe laser, and to use it for XUV holographic imaging. Measurements on gain lifetime values of this laser have been performed. The laser has been found to have gain lifetime around 250 psec to 350 psec under our experimental conditions, and thus requires a sub-nanosecond pump laser. The effects of using different target materials under identical experimental conditions have been studied. We found that a gold coated target provides an 80% increase in output energy over a plain stainless steel target, and thus becomes an attractive way of improving the Xe laser energy. In order to use Xe laser as a source for XUV holographic imaging, its coherence properties need to be studied. Some of these issues are addressed in this thesis. A possible way of improving Xe laser coherence and mode quality is also discussed at the end of this thesis.
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    Observing the silent world under COVID-19 with a comprehensive impact analysis based on human mobility
    (Springer Nature, 2021) Wang, Shaobin; Tong, Yun; Fan, Yupeng; Liu, Haimeng; Wu, Jun; Wang, Zheye; Fang, Chuanglin; Kinder Institute for Urban Research
    Since spring 2020, the human world seems to be exceptionally silent due to mobility reduction caused by the COVID-19 pandemic. To better measure the real-time decline of human mobility and changes in socio-economic activities in a timely manner, we constructed a silent index (SI) based on Google’s mobility data. We systematically investigated the relations between SI, new COVID-19 cases, government policy, and the level of economic development. Results showed a drastic impact of the COVID-19 pandemic on increasing SI. The impact of COVID-19 on human mobility varied significantly by country and place. Bi-directional dynamic relationships between SI and the new COVID-19 cases were detected, with a lagging period of one to two weeks. The travel restriction and social policies could immediately affect SI in one week; however, could not effectively sustain in the long run. SI may reflect the disturbing impact of disasters or catastrophic events on the activities related to the global or national economy. Underdeveloped countries are more affected by the COVID-19 pandemic.