CAREER: Superdiffusive Heat Transfer in Nanoscale Metal Multilayers

宏观热传递的经典理论很难预测纳米尺度下金属中热的演化，特别是在超快的时间尺度上，例如万亿分之一秒。在这些领域工作的工程师目前必须依靠简单的模型和不完整的数据来估计激光激发后金属的输运。这个项目试图用一个具体的框架来代替这种猜测工作，以模拟运输。新框架将通过实验进行广泛测试。该项目结合使用短光脉冲和电脉冲来加热纳米级金属系统，将测量现有理论无法预测的时间和长度尺度上的热量传递。这项研究将与外展，以初中和高中学生在加州大学洛杉矶分校周围的西班牙裔当地社区为主。这项研究还将与一个指导计划相结合，该计划旨在让有前途的社区大学生参与研究，并鼓励他们转到UCR攻读科学和工程学位。这个CAREER项目的目标是识别、量化并最终控制控制纳米级金属多层热流的电子-电子和电子-声子散射过程。在比电子散射过程更短的时间尺度上，传热可以是弹道的、超扩散的或扩散的。为了确定各种传热制度适用的时间和长度尺度，并量化控制电子散射过程，该项目使用了超快速测温方法的组合，如波长相关的时域热反射测量，时间分辨磁光克尔效应测量，以及太赫兹频率下温度相关的电导率。这些方法在亚皮秒时间尺度上测量金属多层中的热演化。利用纳米金属层中热的时空演化实验数据，对基于第一性原理的热电子输运模型进行了验证。这个项目的结果显著地推进了纳米尺度传热的基础科学，因此使得热管理至关重要的技术得以改进，例如纳米电子学。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Classical theory for macroscopic heat transport poorly predicts the evolution of heat in metals at the nanoscale, especially on ultrafast timescales, e.g. a trillionth of a second. Engineers working in these areas must currently rely on simplistic models and incomplete data to estimate transport in metals following laser excitation. This project seeks to replace such guess work with a concrete framework for modelling transport. The new framework will be extensively tested though experimentation. Using a combination of short optical and electrical pulses to heat nanoscale metal systems, this project will measure heat transfer over the time- and length-scales that cannot be predicted by existing theory. This research will be integrated with outreach to middle and high school students in the predominantly Hispanic local community surrounding UCR. The research will also be integrated with a mentoring program designed to get promising community college students involved in research and encourage them to transfer to UCR to pursue science and engineering degrees.The goal of this CAREER project is to identify, quantify, and ultimately control the electron-electron and electron-phonon scattering processes that govern heat flow in nanoscale metal multilayers. On time-scales shorter than electronic scattering processes, heat transfer can either be ballistic, superdiffusive, or diffusive. To identify the time- and length-scales over which various heat transfer regimes apply, and to quantify the governing electronic scattering processes, this project uses a combination of ultrafast thermometry methods such as wavelength-dependent time-domain thermoreflectance measurements, time-resolved magneto-optic Kerr effect measurements, and temperature dependent electrical conductivity at THz frequencies. These methods measure the evolution of heat in metal multilayers on sub-picosecond time-scales. Experimental data on the spatial and temporal evolution of heat in nanoscale metal layers are used to test first-principles based models for hot electron transport. The results of this project significantly advance the basic science of nanoscale heat transfer, and therefore allow improvements in technologies where thermal management is of critical importance, e.g. nanoelectronics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.