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

【非技术内容】激光产生的高频超声能否用于半导体器件内部纳米结构的成像？人们想当然地认为，纳米科学技术的进步依赖于我们制造纳米级图像的能力的持续进步。该项目将采用超快激光器，产生持续时间不到百万分之一秒的光脉冲，以产生和检测频率比医疗或工业超声波高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探测这些超声波的能力，也将扩大我们对这种高频波在固体半导体材料中传播时的行为的理解。该项目将涉及广泛的本科生在夏季研究和独立研究课程的学分在学期。它还将加强一项针对未被充分代表的少数民族新生或第一代大学生的计划。技术摘要：本项目将试图证明相干声子可以用于对埋藏的纳米结构成像。先前的研究已经显示出了希望，但是由于声子在0.1到1太赫兹范围内的高衰减而导致的低信噪比问题已经被证明是令人生畏的。这项工作将调查二维材料，如过渡金属二硫族化合物或石墨烯，是否可以证明是皮秒激光超声成像实验的有效换能器层。第二项研究将利用超快光泵浦-探针实验在周期性图案层状纳米结构上探索高频表面声波的产生和探测。超快光学已经产生了最高的表面声波频率被检测到，但一个完整的理解光的产生和检测是缺乏的。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薄膜和晶体中的声子衰减和热输运。时域热反射测量将与相干声子衰减测量相结合，以形成长波长振动模式对热输运贡献的完整图像。同样，测量晶体材料中的相干声子衰减，其中导热性已经众所周知，将提供重要的数据，可能有助于回答亚太赫兹声子的行为如何说明材料的导热性的问题。

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.