Nonlocal Terahertz Nanospectroscopy and Nanoimaging

在许多对未来器件应用具有重要意义的新兴材料系统中，材料表面电荷的传播可能是器件性能的关键决定因素。例如，基于氮化镓的器件，包括大多数蓝色LED和蓝色二极管激光器，其性能往往受到其表面或界面上的晶体缺陷的限制，这些缺陷扰乱了电子在相邻材料层之间的传输。在另一个例子中，太阳能电池中使用的聚光材料多晶膜中晶体微粒之间的边界会强烈影响吸收太阳光产生的电荷的收集速度，最终限制设备的效率。该项目寻求开发一套新的实验技术来研究这些问题，同时具有高空间和时间分辨率。这些技术依赖于物质中运动的电荷与太赫兹光谱范围内的电磁辐射之间非常强的相互作用。在某些情况下，这些移动的电荷可以辐射超短太赫兹辐射，其中包含电荷载流子动力学的重要特征。在其他情况下，从材料表面反射的短太赫兹脉冲可以用来表征移动电荷的特性，时间分辨率为皮秒级。我们的工作将把这些想法扩展到纳米领域，使我们能够使用太赫兹技术来研究电荷传输，空间分辨率只有几十纳米。这个研究计划的目的是展示革命性的太赫兹纳米显微镜测量技术，并使用它们来收集关于材料动力学过程的新信息。特别是，我们将通过发展一套非局域光泵THz探针纳米显微镜和非局域THz发射纳米显微镜等方法，建立非局域THz纳米显微镜的思想。然后，我们将使用这些新的实验技术来研究几个与当前技术相关的材料系统。我们将开展合作，利用同事在样品制备和太赫兹材料特性计算方面的专业知识。这项拟议的研究将把非线性光学的力量带入具有亚皮秒时间分辨率和纳米级空间分辨率的纳米级，从而显著推动太赫兹纳米科学领域的发展。与传统的纳米技术不同，我们的新方法将揭示关于横向电荷传输的信息，而不是垂直传输。我们将研究多层石墨烯薄膜中极化子通过单个台阶边缘的输运，并探索单个晶界对多晶钙钛矿薄膜中电荷输运的影响。我们还将把这些新想法与太赫兹振动光谱的前沿结果相结合，通过研究局部纳米级缺陷对介观相干长度振动模式的影响，以及通过观察纳米级激发对太赫兹振动介导的简单化学反应的影响。这些测量将为研究材料中的纳米级现象开辟新的可能性，揭示使用其他方法无法获得的重要信息。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

In many emerging material systems which will be important for future device applications, the propagation of charges at the material surface can be the key determining factor in device performance. For example, devices based on gallium nitride, which includes most blue LEDs and blue diode lasers, are often limited in their performance by crystalline defects at their surfaces or interfaces, which perturb the transport of electrons between adjacent layers of the materials. In another example, the boundaries between crystalline micro-grains in a polycrystalline film of light-harvesting materials used in solar cells can strongly influence the speed at which charges created by absorbed sunlight are collected, ultimately setting a limit on the device efficiency. This project seeks to develop a suite of new experimental techniques to study such issues, with both high spatial and temporal resolution. These techniques rely on the very strong interaction between the charges moving in the material and electromagnetic radiation in the terahertz spectral range. In some cases, these moving charges can radiate an ultrashort burst of terahertz radiation, which contains important signatures of the charge carrier dynamics. In other cases, a short terahertz pulse reflected from the material surface can be used to characterize the properties of the mobile charges, with temporal resolution on the scale of a picosecond. Our work will extend these ideas to the nanoscale realm, allowing us to study the charge transport using terahertz techniques with spatial resolution of only a few tens of nanometers.The aim of this research program is to demonstrate revolutionary new measurement techniques in terahertz nanoscopy, and use them to glean new information about dynamical processes in materials. In particular, we will establish the idea of non-local THz nanoscopy, by developing a suite of methods such as non-local optical-pump THz-probe nanoscopy and non-local THz emission nanoscopy. We will then use these new experimental techniques in studies of several material systems of current technological relevance. We will initiate collaborations to leverage the expertise of colleagues in sample preparation and calculations of THz material properties. The proposed research will significantly advance the field of terahertz nanoscience by bringing the power of nonlinear optics to the nanoscale with sub-picosecond temporal resolution and nanoscale spatial resolution. Unlike traditional nanoscopy techniques, our new methods will reveal information about lateral charge transport, rather than vertical transport. We will study the transport of polaritons across an individual step edge in a multi-layer graphene film, and probe the effects of an individual grain boundary on charge transport in a polycrystalline perovskite thin film. We will also couple these new ideas to cutting-edge results in terahertz vibrational spectroscopy, by studying the effects of local nanoscale defects on vibrational modes with mesoscopic coherence lengths, and by observing the influence of nanoscale excitations on simple chemical reactions mediated by terahertz vibrations. These measurements will open up new possibilities for the study of nanoscale phenomena in materials, revealing important information that cannot be obtained using other methods.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.