UNS: Collaborative Research: Ultrafast Phonon Spectroscopy for Lifetime Measurements of Phonons in 2-D Transitional Metal Dichalcogenides

1512776 /1511199 Yang，Rongui/ Siemens，Mark在非常薄的材料（只有几个原子）中，其光电特性可能与宏观的相同材料有很大的不同，这可能会影响热传导。这种材料的一种类型是过渡金属二硫属化物（TMDC）。将这些超薄TMDC集成到设备中的进一步进展需要充分了解其传热能力，但使其具有新颖物理特性的薄度也阻止了传统表征技术的使用。 来自科罗拉多大学和丹佛大学的研究人员提议开发一种新技术，用于测量TMDC中携带热量的声子的动力学。该项目的结果预计将产生关键的洞察力，设计具有所需热传导特性的新型材料和器件。主要研究人员将实施一项实践计划，鼓励学生探索有关激光和传热的科学问题。 在最近的纳米热输运研究中，声子平均自由程（与声子寿命成比例）是一个核心概念，它支配着过去二十年来观察和探索的纳米结构材料的超低和超高热导率的非凡现象。虽然载流子的平均自由程和寿命是简单的动力学概念，但声子的平均自由程和寿命的第一性原理计算只是最近才开始的。 目前还需要一种可靠的测量技术来测量频率依赖的声子寿命。 该合作研究项目的目标是使二维相干光谱学（一种用于研究电子相干性的高影响力技术）适用于声子寿命的测量。为了证明这种新技术的有效性，在2-D TMDC的频率依赖的声子寿命将被测量，并与第一性原理计算进行比较。声子寿命谱的单层和少层TMDC将产生关键信息的层依赖性的热导率和声子在少层TMDC，这可以有直接的影响，设计的二维TMDC启用的新设备。

1512776 / 1511199Yang, Ronggui / Siemens, MarkIn very thin materials (of a few atoms), the opto-electronic properties can be dramatically different from a macroscopic piece of the same material that has implications in conducting heat. A type of this material is transition metal dichalcogenides (TMDCs). Further progress in integrating these ultra-thin TMDCs into devices requires full understanding of their heat transfer capability, but the thinness that enables their novel physical properties also prevents the use of traditional characterization techniques. Researchers from the University of Colorado and the University of Denver propose to develop a new technique for measuring the dynamics of phonons which carry heat in TMDCs. Results from this project are expected to yield critical insight into the design of novel materials and devices with desired thermal conduction characteristics. The principal investigators will implement a hands-on program encouraging students to explore scientific questions about lasers and heat transfer. Graduate students will be involved in the research for their graduate degrees.Throughout the recent work on nanoscale thermal transport, phonon mean free path (proportional to phonon lifetime) is a central concept that governs the extraordinary phenomena observed and explored over the past two decades for ultra-low and ultra-high thermal conductivity of nanostructured materials. Although the mean free path and lifetime of carriers are simple kinetic concepts, the first-principle calculations of mean free path and lifetime of phonons are only recently being undertaken. A convincing measurement technique is yet needed to measure frequency-dependent phonon lifetime. The objective of this collaborative research project is to adapt two-dimensional coherent spectroscopy, a high-impact technique used to study electronic coherence, to the measurement of phonon lifetime. To demonstrate the efficacy of this novel technique, the frequency-dependent phonon lifetime in 2-D TMDCs will be measured and compared with the first-principles calculations. Phonon lifetime spectroscopy on monolayer and few-layer TMDCs will yield critical information about the layer-dependence of thermal conductivity and phonon in few-layer TMDCs, which can have immediate impact on the design of 2D TMDC-enabled novel devices.