【报告题目】使用超快激光光谱探测电子、声子传输
【英文题目】Electron/Phonon Transport Probed with Ultrafast Laser Spectroscopies
【报告人】
Dr. Yaguo Wang (yaguo.wang@austin.utexas.edu)
Department of Mechanical Engineering, University of TEXAS at Austin
德州大学Austin分校机械工程系王亚果博士
【报告时间和地点】 2016年6月7日星期二下午16:00 力学一楼504教室
【主办】工程科学学院热科学和能源工程系
【摘要】
作为基本粒子，电子和声子的动力学和输运性质决定了现代电子技术的性能。例如，在光电技术中的超快光电探测器，激发载流子的弛豫时间决定了检测器的响应时间，载流子的扩散长度控制它的旅行距离，因此决定了检测的效率和灵敏度。声子是量子化的晶格振动准粒子。声子可以与电子相互作用，声子输运决定半导体器件的热学性能。电子-声子动力学通常发生在超快时间尺度，例如飞秒（10e-15秒）到皮秒（10e-12秒）级别，不能用传统的稳态测量方法捕获。飞秒激光光谱法可以提供飞秒级别的时间分辨率，并已被用来探测电子/声子的传输的细节。
As fundamental particles, dynamics and transport properties of electrons and phonons determine the performance of modern electronics. For example, in optoelectronics, such as ultrafast photodetectors, relaxation time of excited carriers measures how fast the detector can respond, carrier diffusion length controls how far carriers can travel and hence the efficiency and sensitivity. Phonons are quasiparticles derived from quantized lattice vibrations. Phonons can interact with electrons and phonon transport determines thermal properties of semiconductor devices. Electron/phonon dynamics usually occur in ultrafast time scale, e.g. femtosecond (10-15 s) to picosecond (10-12 s) regime, which could not be captured with traditional steady-state measurements. Pump-probe spectroscopy with femtosecond lasers can provide fs time resolution and has been utilized to detect electron/phonon transport with great details.
本次讲座将展示三个电子-声子输运现象的探测光谱研究实例。首先，将光栅成像技术应用于标准的飞秒激光探测光谱仪，研究了石墨烯的光激发载流子的弛豫和扩散过程。光激发载流子的迁移率比室温下的霍尔迁移率和场效应迁移率高几个数量级，这显示出光激发载流子与平衡载流子有明显不同的行为。其次，对热电材料的声子输运问题进行研究，观察到了强的界面散射和量子限制效应。最后，讨论了纳米金属薄膜的平面和跨平面的热传导率的表征方法。
This talk will demonstrate three examples of electron/phonon transport studied with pump-probe spectroscopies. Firstly, photo-excited carrier relaxation and diffusion process in graphene are probed by incorporating a grating imaging technique into standard femtosecond laser pump-probe spectrometer. The mobility of photo-excited carriers are several orders larger than Hall mobility and Field-Effect mobility measured at room temperature, indicating the remarkably different behavior of photo-excited carriers versus equilibrium carriers, as well as the necessity to use photo-carrier transport properties for optoelecronics. Secondly, phonon transport in thermoelectric materials will be discussed, where strong interface scattering and quantum confinement effect have been observed. Lastly, characterization of both in-plane and cross-plane thermal conductivities in nano metal films with nanosecond laser spectroscopy will be demonstrated.
【个人简介】
Yaguo Wang is currently an Assistant Professor of Mechanical Engineering department at University of Texas at Austin. Dr. Wang received her Bachelor's degree in Safety Science and Engineering at the University of Science and Technology of China (USTC) in 2005 and her Ph.D. degree from the Department of Mechanical Engineering, Purdue University, Indiana, in 2011. After one-year's postdoctoral experience at Purdue University, she joined UT Austin in January 2013. Dr. Wang received CAREER award from National Science Foundation in 2014. Dr. Wang's group studies a broad spectrum of topics with laser spectroscopies, including phonon transport in nanostructures, carrier transport in 2D materials, thermoelectrics, thermal transport in extreme environments, etc. She is also interested in atomistic simulations of energy-conversion materials and thermal management in nanoelectronics.