Nanoscale Nonlinear Terahertz Spectroscopy

随着半导体器件变得越来越小，电子如何在越来越短的时间尺度上运动的问题变得越来越重要。研究这个问题的一种方法是测量快速加速的电子发出的辐射，这通常是在快速响应的小型设备中出现的情况。在许多情况下，可以使用非常短的光脉冲（即100飞秒持续时间）来诱导这种快速充电加速；在这种情况下，发射的辐射通常落在太赫兹光谱范围内。测量这种发射的太赫兹辐射，一种被称为“激光太赫兹发射光谱”（或LTEM）的技术，已被证明是一种非常强大的方法，可以研究许多不同种类的材料，阐明影响固体中电荷运动的最早的动力学过程。然而，它有一个重要的限制：由于对可见光聚焦的紧密程度是有限制的，因此LTEM的空间分辨率被限制在几微米或更大（即样品上光学光斑的大小）。因此，不可能利用这种光谱工具来研究单个纳米结构，因为它们太小了，无法通过入射光脉冲单独处理。在本研究计划中，我们将开发一种新的方法来应用LTEM，克服这种有限的空间分辨率。我们的技术是基于将太赫兹辐射从样品表面附近的一个非常小的金属尖端散射出来，将把LTEM的空间分辨率提高三个数量级。这将为利用LTEM技术研究纳米级现象开辟一个全新的领域。例如，我们将研究新兴的光伏材料，看看晶体晶界是否会对电荷输运产生重大影响，从而限制其整体效率。我们将研究纳米级电子材料，如氮化镓，其表面缺陷和界面紊乱被认为强烈影响其性能。这些测量将揭示许多不同材料中电荷输运的基本物理，具有纳米尺度的空间分辨率和亚皮秒的时间分辨率。技术描述：提出的研究通过使用散射型近场显微镜实现尖端介导的非线性光学，以一种新颖的方式解决了将毫米波和太赫兹波耦合到纳米结构的挑战。我们的初步工作首次确定了尖端介导太赫兹产生（二阶非线性光学过程）的可行性。在这项早期工作的基础上，我们将建立纳米级激光太赫兹发射显微镜，作为纳米材料和纳米结构光谱的一种有价值的通用工具。我们将基于这一想法开发几种研究材料系统的新方法，包括局部应用电场偏置调制和用于研究埋藏界面的攻丝调幅。我们还将通过将这些技术与时间延迟泵浦脉冲相结合来扩大这些技术的范围，用于太赫兹非线性演化的时间分辨研究。这一切都将在约10纳米的尖端尺寸有限的空间分辨率下完成。我们的工作将把线性和非线性太赫兹科学的全部力量带到纳米材料领域。我们将使用这些新技术来研究几种不同的材料体系，包括有机金属卤化物钙钛矿和GaN纳米结构和异质结构。除了回答有关纳米尺度形态在这些重要材料中电荷输运中的作用的开放性问题外，这些测量还将证明这种新开发的光谱方法的强大功能和多功能性。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。

As semiconducting devices become smaller and smaller, the question of how electrons move on shorter and shorter time scales becomes increasingly important. One way to investigate this question is to measure the radiation that is emitted by electrons which have been rapidly accelerated, as would typically be the case in a small device with a fast response. In many situations, it is possible to induce this rapid charge acceleration using a very short pulse of light (i.e., of 100 femtosecond duration); in this case, the emitted radiation typically falls in the terahertz range of the spectrum. Measuring this emitted terahertz radiation, a technique known as "laser terahertz emission spectroscopy" (or LTEM), has proven to be a very powerful method for studying many different kinds of materials, elucidating the earliest dynamical processes which influence the motion of charges in solids. However, it has an important limitation: since there is a limit to how tightly one can focus visible light, the spatial resolution of LTEM is limited to a few microns or larger (that is, the size of the optical spot on the sample). As a result, it has not been possible to leverage this spectroscopic tool in the study of single nanostructures, since they are far too small to be individually addressed by the incident light pulse. In this research program, we will develop a new way to apply LTEM which overcomes this limited spatial resolution. Our technique, which is based on scattering the terahertz radiation from a very small metal tip held near the sample's surface, will improve the spatial resolution of LTEM by three orders of magnitude. This will open up an entirely new realm of nanoscale phenomena for study using LTEM techniques. For example, we will study emerging photovoltaic materials, to see if the crystalline grain boundaries have a significant influence on charge transport which could limit their overall efficiency. We will study nanosized electronic materials such as gallium nitride, where surface defects and interfacial disorder are thought to strongly influence their performance. These measurements will reveal the fundamental physics of charge transport in many different materials, with both nanoscale spatial resolution and sub-picosecond temporal resolution.Technical description: The proposed research attacks the challenge of coupling millimeter and terahertz waves to nanostructures in a novel way, by using a scattering-type near-field microscope to implement tip-mediated nonlinear optics. Our preliminary work established for the first time the feasibility of tip-mediated terahertz generation (a second-order nonlinear optical process). Building on this early work, we will establish nanoscale laser terahertz emission microscopy as a valuable and versatile tool for spectroscopy of nanomaterials and nanostructures. We will develop several new methods for studying material systems based on this idea, including locally applied electric field bias modulation, and tapping amplitude modulation for studying buried interfaces. We will also broaden the scope of these techniques by combining them with a time-delayed pump pulse, for time-resolved studies of the evolution of the terahertz nonlinearities. This will all be done with a tip-size-limited spatial resolution of ~10 nanometers. Our work will bring the full power of both linear and nonlinear terahertz science to the realm of nanomaterials. We will use these new techniques to study several different material systems, including organometallic halide perovskites and GaN nanostructures and heterostructures. As well as answering open questions about the role of nanoscale morphology in charge transport in these important materials, these measurements will also serve as demonstrations of the power and versatility of this newly developed spectroscopic approach.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.