Abstract：

　　A quasiparticle in solid state is a composite of a true particle or excitation and its screening charge. Screening is the collective response of the system to the particle potential that reduces the interaction of the subsystem with other degrees of freedom. The screening timescale is defined by the collective plasmon response to an exposed excitation field. As screening is reduced on going from 3 to 2 dimensions, the quasiparticle interactions with the remaining system increase, and the quasiparticle concept begins to loose meaning. We explore this transition to weaker screening by exploring hot electron dynamics in silver and black phosphorus. The hot electron generation increases as the photon excitation frequency in Ag approaches its plasma frequency, where the quasiparticle begins to emerge into an unscreened particle. In two dimensions as screening weakens, hot electron generation in response to optical fields becomes intense. These transitions will be described in terms of hot electron generation in response to optical excitation. This research is performed in collaboration with Dr. Shijing Tan and his students.

报告人简介：

　　Hrvoje Petek教授1980年和1985年分别在麻省理工学院与加州大学伯克利分校获得学士与博士学位，之后在日本分子科学研究所（1985 - 1993）和日立高等研究中心（1993 - 2000）工作，2000年加入匹兹堡大学物理与天文学系，并担任R. K. Mellon 讲席教授至今。他的研究兴趣涉及固体表面的超快光谱学、显微镜和飞秒化学。他开创了飞秒时间和纳米空间相干电子动力学的超快多光子光电发射谱研究。他获得的荣誉包括Alexander von Humboldt Research Award (2000), Morino Award (2014), Ahmed Zewail Award in Ultrafast Science and Technology (2019)等，并入选美国物理学会（APS）以及美国科学与艺术学会（AAAS）会士。