Phase-change metasurfaces for dynamic image display and information encryption

Optical metasurfaces enable to engineer the electromagnetic space and control light propagation at an unprecedented level, offering a powerful tool to achieve modulation of light over multiple physical dimensions. Here, we demonstrate a Sb$_{2}$S$_{3}$ phase-change metasurface platform that allows active manipulation of both amplitude and phase. In particular, we implement dynamic nanoprinting and holographic image display through tuning crystallization levels of this phase-change material. The Sb$_{2}$S$_{3}$ nanobricks tailored to function the amplitude, geometric and propagation phase modulation constitute the dynamic meta-atoms in the multiplexed metasurfaces. Using the incident polarizations as decoding keys, the encoded information can be reproduced into a naonprinting grayscale image in the near field and two holographic images in the far field. These images can be switched on and off by taking advantages of the reversible tunability of Sb$_{2}$S$_{3}$ nanostructure between amorphous and crystalline states. The proposed phase-change metasurfaces featuring manifold information and multifold encryption promise ultracompact data storage with high capacity and high security, which suggests an exciting direction for modern cryptography and security applications

Online Learning in Vocational Education of China during COVID-19: Achievements, Challenges, and Future Developments

COVID-19 has challenged education systems globally. Traditional teaching and learning activities of more than 1,300 vocational colleges and nearly 11,000 vocational high schools in China have had to be paused and transformed into an online mode. A study had been conducted to trace the unprecedented change which would provide reflections on policies and practical experience worthy of reference for the follow-up development of online vocational education in China and other countries in the world. The study used two methods to collect data: (1) delivering questionnaires to 767 schools, 17009 teachers, 270,732 students, and (2) gathering 110 institute cases from 21 provinces and 170 curriculum cases from 14 provinces. The result showed that vocational institutions coped with the pandemic’s outbreak through online learning and achieved the overall goal of “Not Going to School but Classes still Ongoing.” Further, vocational institutions have faced problems and challenges of online learning in practice training and internship, organization, and technical environment. The development of vocational education in the information era requires thinking about the system-driven reform path and online learning strategy and putting it into action