CAREER: Superdiffusive Heat Transfer in Nanoscale Metal Multilayers

宏观热传输的经典理论很难预测金属在纳米尺度下的热演化，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中的传输。该项目试图用一个具体的交通建模框架来取代这种猜测工作。新框架将通过实验进行广泛测试。利用短光脉冲和电脉冲的组合来加热纳米级金属系统，该项目将测量现有理论无法预测的时间和长度尺度上的传热。这项研究将与外展到初中和高中学生在主要是西班牙裔当地社区周围UCR整合。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，并鼓励他们转学到加州大学洛杉矶分校攻读科学和工程学位。这个CAREER项目的目标是识别，量化，并最终控制电子-电子和电子-声子散射过程，这些过程控制纳米金属多层膜中的热流。在比电子散射过程短的时间尺度上，热传递可以是弹道的、超扩散的或扩散的。为了确定各种热传递机制适用的时间和长度尺度，并量化管理电子散射过程，该项目使用了超快测温方法的组合，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THz频率下的温度相关电导率。 这些方法在亚皮秒时间尺度上测量金属多层膜中的热演化。在纳米金属层的热的空间和时间演化的实验数据被用来测试热电子传输的第一性原理为基础的模型。该项目的成果极大地推进了纳米级传热的基础科学，因此可以改进热管理至关重要的技术，例如纳米电子学。该奖项反映了NSF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.