RUI: Acoustic Phonons in Nanostructures: Surface Waves, Thermal Transport, and Imaging

非技术摘要激光产生的高频超声波可以用来制作埋在半导体器件中的纳米结构的图像吗？人们认为，纳米科学和技术的进步有赖于我们制作纳米图像的能力的持续进步。该项目将使用超快激光，产生持续时间不到百万分之一秒的光脉冲，以产生和检测频率高于医疗或工业超声波1000倍以上的超声波。这项研究将需要使用新的二维材料来增强我们检测这些超声波的能力，并将拓宽我们对这些高频波在固体半导体材料中传播时的行为的理解。该项目将包括广泛的本科生在暑期研究和独立研究课程的学分在本学期。它还将加强一项针对新生的计划，这些新生是代表不足的少数族裔或第一代大学生。技术摘要该项目将试图证明相干声学声子可以用来成像埋藏的纳米结构。以前在这方面的工作已经显示出希望，但由于0.1到1太赫兹范围内声子的高衰减而导致的低信噪比问题被证明是令人望而生畏的。这项工作将调查过渡金属二卤化物或石墨烯等二维材料是否可以被证明是皮秒激光超声成像实验的有效传输层。第二个研究将探索通过对周期性图案化的层状纳米结构进行超快光学泵浦-探测实验来产生和检测高频表面声波。超快光学产生了有史以来探测到的最高表面声波频率，但对光学产生和检测缺乏完整的了解。这项工作将把光学数据与计算机模拟的机械位移和入射/反射电磁波进行比较。该项目的最后一个组成部分将是研究非晶态薄膜和晶体中的声子衰减和热传输。时间域热反射测量将与相干声子衰减测量相结合，以形成长波振动模对热输运贡献的完整图景。同样，对晶体材料中相干声子衰减的测量将提供重要的数据，这些数据可能有助于回答亚THz声子的行为对材料热导率的影响这一问题。

Non-Technical AbstractCan laser generated high frequency ultrasound be used to make images of nanostructures buried inside semiconductor devices? It is taken as a given that progress in nanoscale science and technology relies on continuing progress in our ability to make nanoscale images. This project will employ ultrafast lasers that generate light pulses that are less than a millionth of a millionth of a second in duration in order to generate and detect ultrasound that is over 1000 times higher in frequency than medical or industrial ultrasound. The research will entail the use of new 2-Dimensional materials to enhance our ability to detect these ultrasonic waves, and will also broaden our understanding of how such high frequency waves behave as they travel around inside solid semiconductor materials. The project will involve a wide range of undergraduate students in summer research and independent research courses for credit during the semester. It will also enhance a program for incoming freshmen who are under-represented minorities or first-generation college students.Technical AbstractThis project will seek to demonstrate that coherent acoustic phonons can be used to image buried nanostructures. Previous work on this has shown promise, but issues of low signal to noise due to high attenuation of phonons in the 0.1 to 1 THz range have proved daunting. The work will investigate whether 2-D materials such as transition metal dichalcogenides or graphene could prove to be effective transducer layers for a picosecond laser ultrasonic imaging experiment. A second investigation will explore high frequency surface acoustic wave generation and detection by ultrafast optical pump-probe experiments on periodically patterned layered nanostructures. Ultrafast optics has produced the highest surface acoustic wave frequencies ever detected, but a complete understanding of the optical generation and detection is lacking. The work will compare the optical data with computer simulations of the mechanical displacements and the incident/reflected electromagnetic waves. The final component of the project will be a study of phonon attenuation and thermal transport in thin amorphous films and crystals. Time-domain thermoreflectance measurements will be combined with coherent phonon attenuation measurements to form a complete picture of the contribution of long wavelength vibrational modes to thermal transport. Similarly, the measurement of coherent phonon attenuation in crystalline materials, where the thermal conductivity is already well known, will supply important data that may help answer the question of what the behavior of sub-THz phonons says about a material's thermal conductivity.