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被快速加速的电子所发射的辐射，这通常是在具有快速响应的小型设备中的情况。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)来诱导这种快速电荷加速；在这种情况下，发射的辐射通常落在光谱的太赫兹范围内。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，已被证明是研究许多不同类型材料的一种非常有效的方法，阐明了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：由于人们对可见光的聚焦程度是有限的，LTEM的空间分辨率被限制在几微米或更大(即样品上光斑的大小)。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，不能通过入射光脉冲单独处理。在这个研究项目中，我们将开发一种新的方法来应用LTEM来克服这种有限的空间分辨率。我们的技术是基于样品表面附近一个非常小的金属尖端对太赫兹辐射的散射，将使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，看看晶界是否对电荷传输有重大影响，从而限制其整体效率。我们将研究纳米电子材料，如氮化镓，表面缺陷和界面无序被认为对其性能有很大影响。这些测量将揭示许多不同材料中电荷传输的基本物理，具有纳米级的空间分辨率和亚皮秒的时间分辨率。技术描述：拟议的研究以一种新颖的方式解决了毫米波和太赫兹波耦合到纳米结构的挑战，使用散射型近场显微镜来实现尖端介导的非线性光学。我们的初步工作首次证实了尖端介导的太赫兹产生(一种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。基于这一思想，我们将发展几种研究材料系统的新方法，包括局部外加电场偏置调制，以及用于研究埋藏界面的敲击幅度调制。我们还将通过将这些技术与延时泵浦脉冲相结合来扩大这些技术的范围，以用于太赫兹非线性演化的时间分辨研究。这一切都将在针尖大小受限的空间分辨率约10纳米的情况下完成。我们的工作将把线性和非线性太赫兹科学的全部力量带入纳米材料领域。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。除了回答有关纳米级形态在这些重要材料中电荷传输中的作用的公开问题外，这些测量还将作为这种新开发的光谱方法的力量和多功能性的展示。这一奖项反映了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。

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.