量子科学论坛

（第七十二期）

日期和时间Date & Time：2022-Aug-5(Friday) 1:30pm (Beijing time)

线上会议Zoom Meeting： ID：662 921 9901 

报告人Speaker ：Dr. Biaolong Liu，Paul Scherrer Institute

主持人Host ：Dr. Haiyun Liu，BAQIS

题目Title：Vibrational Phase Control in Solids Using Tailored Terahertz Pulses

摘要Abstract 

Many transition metal oxides show strong electronic correlations that produce functionally relevant properties like metal-insulator transitions, ferroelectricity, and superconductivity. The development of intense femtosecond laser sources has made possible to control these functionalities and explore unknown out-of-equilibrium phase states of such complex materials by light. Selective excitation of infrared-active phonon modes by intense THz pulses has been demonstrated as a powerful tool to manipulate electronic and magnetic phases. However, it is technically challenging to excite low energy vibrational modes in the so-called “Terahertz gap” frequency regime, which hold promise to manipulate novel phases and physical processes. 

In this talk, I will first discuss a novel table-top source of narrowband intense THz pulses tunable across the frequency gap via chirped-pulse difference frequency generation in nonlinear organic crystals^[1]. By integrating this source with time-resolved THz spectroscopy, we investigated the non-equilibrium behavior of light-induced superconductivity in high temperature cuprates YBa₂Cu₃O_6+x by selectively exciting different IR-active modes ^[2]. The experimental observation confirmed the strong link between the axis apical oxygen atomic motions and the signatures of nonequilibrium superconductivity, providing insights for better understanding on such phenomenon. In another compound CoF₂, non-equilibrium ferrimagnetic states was induced from its antiferromagnetic state by pumping two frequency-degenerated phonons, which was experimentally evidenced by the pump-induced Faraday rotation and circular dichroism. This study revealed that piezomagnetism could also be reproduced by ultrafast light-induced strain in the material by resonantly exciting optical phonons, paves the way for memory and spintronic applications ^[3].

[1] B. Liu et al., Opt. Lett. 42, 129 (2017)

[2] B. Liu et al., Physical Review X, 10, 011053 (2020)

[3] A. Disa et al., Nature Physics, 16, 937 (2020)

报告人简介About the speaker

Dr. Biaolong Liu received his bachelor degree in the Physics department of University of Science and Technology of China in 2011. Then he joined the Research Center for Laser Physics and Technology (Technical Institute of Physics and Chemistry, CAS) and fulfilled his master research on high-power solid-state laser and frequency conversion with Prof. Zuyan Xu. In 2014, he joined Prof. Andrea Cavalleri’s group Max-Planck-Institute for the structure and dynamics of matter in Hamburg Germany. There he completed his PhD and one-year postdoc research on controlling the phase transition of transition metal oxides using mid-infrared and THz pulses. He becomes a tenure track laser scientist of the Lab for Nonlinear Optics at Paul Scherrer Institute (https://www.psi.ch/en/lno) in Switzerland.