Abstract: 

　　Photocathodes—materials that convert photons into electrons through the photoelectric effect (explained by Einstein)—are important for many modern technologies that rely on light detection or electron-beam generation. However, existing photocathode materials have become increasingly difficult to meet the performance requirements of related cutting-edge technology upgrades. Most of these materials, and the theory (Spicer’s three-step model) to understand their photoemission properties, were discovered and established more than 60 years ago. The long-term lack of original innovation at both the material and theory levels is the biggest bottleneck for the advancement of the photocathode field. In this talk, I will first show how this century-old field could be transformed dramatically through the introduction of a quantum material to obtain a quantum leap in photocathode performance. Underlying this performance boost is the surprising emergence of coherence in secondary photoemission we observed from the SrTiO₃ photocathode that defies explanation by existing theories. I will then show how our understanding of the photoemission phenomenon could be reshaped on a fundamental level by revising an underlying assumption Einstein and Spicer made in their original proposals. The resulting new theoretical framework for photoemission (four-step model) explains our observations in SrTiO₃ and predicts a whole class of photocathode quantum materials with novel functionalities beyond the scope of traditional photocathode materials. Our work opens new paradigms for the developments and applications of photocathodes as well as new doors into uncharted territories in the field of photoemission.

Biosketch：

　　Rui-Hua He is a tenured associate professor in the Department of Physics, Westlake University. He obtained a bachelor’s degree from Fudan University (2001) and a PhD degree from Stanford University (2010). He worked as a postdoctoral fellow in Lawrence Berkeley National Lab (2010-2012) and as an assistant professor in Boston College (2012-2017) prior to joining Westlake University. His research interest is in studying various emergent phenomena in quantum materials using angle-resolved photoemission spectroscopy and other experimental techniques based on ultra-high vacuum and advanced light sources as well as exploring their application potential. He is a recipient of the NSF CAREER Award and the founding director of Key Laboratory for Quantum Materials of Zhejiang Province and Zhejiang Institute for Advanced Light Source.