Graphene Plasmonic Nanostructures for Terahertz Light Emission

摘要：频率在1-10太赫兹范围内的电磁辐射非常适合许多要求苛刻的成像和传感应用。与可见光或近红外光不同，太赫兹辐射可以通过许多常见的包装材料传播，因此提供了对隐藏物体或缺陷的视觉访问。此外，许多对传感应用有潜在兴趣的化学品，包括炸药和非法药物，在太赫兹频率上具有独特的吸收共振，因此可以用太赫兹光谱准确地检测到。这些功能可以发挥支持作用的特定领域包括安全检查、医疗诊断、制造质量控制和艺术品保护。然而，迄今为止，由于缺乏产生太赫兹光的合适设备，这些技术的广泛采用受到阻碍。现有的源往往体积庞大，价格昂贵，通常需要低温冷却，频率可调性有限，否则无法为大多数应用提供足够的太赫兹输出功率。该项目将利用材料科学和基础纳米光子学的最新进展，开发一种新的太赫兹光发射设备技术，可以克服这些关键限制。所提出的器件可以在室温下工作，具有典型太赫兹应用所需的输出功率水平（几毫瓦），可以以低成本制造，具有高度小型化的外形因素，并且可以在太赫兹频谱范围内广泛调谐。因此，这些设备有望对各种技术部门产生变革性影响，这些技术部门可以从太赫兹成像和传感的独特能力中受益。拟议的活动还将通过培训光电子学、纳米光子学和材料科学相关领域的研究生和本科生，以及通过相关课程开发工作，促进教育。技术摘要：所提出的器件基于最近开发的二维材料和异质结构家族-特别是单层石墨烯与二维半导体二硫化钼（MoS2）的结合。潜在的辐射机制包括通过电流注入激发太赫兹等离子激元极化子（电子气的集体振荡），并在特别设计的石墨烯纳米结构中将其解耦到自由空间辐射。提出的工作重点是通过电磁设计模拟、材料开发努力以及器件制造和表征活动的结合，最大限度地提高该辐射过程的效率。通过将石墨烯纳米结构与额外的光学元件（开腔配置的金属太赫兹天线）相结合来优化等离子体提取效率，这些光学元件旨在促进与自由空间辐射的临界耦合。同时，通过在石墨烯/MoS2肖特基结等离子体纳米结构中控制注入非平衡载流子分布，可以提高等离子体内部发射效率。后一种想法类似于固态照明中使用的发光二极管（led）的工作原理，但应用于全新的材料系统和光谱区域。除了对太赫兹成像和传感的潜在技术影响外，该项目还将在二维材料的等离子体和光-物质相互作用领域创造新的知识和新的研究机会。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。

Title: Terahertz Light Sources Based on Graphene Plasmonic NanostructuresNontechnical Abstract:Electromagnetic radiation with frequency in the 1-10 THz range is ideally suited for many demanding imaging and sensing applications. Unlike visible or near-infrared light, THz radiation can propagate through many common packaging materials and therefore provides visual access to concealed objects or defects. Furthermore, many chemicals of potential interest for sensing applications, including explosives and illicit drugs, feature distinctive absorption resonances at THz frequencies and therefore can be accurately detected by THz spectroscopy. Specific areas where these capabilities can play an enabling role include security screening, medical diagnostics, manufacturing quality control, and artwork conservation. However, the widespread adoption of these technologies has so far been hindered by the lack of suitable devices for the generation of THz light. Existing sources tend to be bulky and expensive, often requiring cryogenic cooling, have limited frequency tunability, and otherwise cannot provide sufficient THz output power for most applications.This project will develop a new device technology for THz light emission that can overcome these critical limitations, by leveraging recent advances in materials science and fundamental nanophotonics. The proposed devices can operate at room temperature with the required output power levels (several milliwatt) for typical THz applications, can be manufactured at low cost with highly miniaturized form factors, and are broadly tunable across the THz spectrum. As a result, these devices are promising for a transformative impact on a diverse set of technology sectors that can benefit from the unique capabilities of THz imaging and sensing. The proposed activities will also promote education through the training of graduate and undergraduate students in relevant areas of optoelectronics, nanophotonics, and materials science, and through related curriculum development efforts. Technical Abstract:The proposed devices are based on the recently developed family of two-dimensional materials and heterostructures – specifically single-layer graphene combined with the two-dimensional semiconductor molybdenum disulfide (MoS2). The underlying radiation mechanism involves the excitation of THz plasmon polaritons (collective oscillations of the electron gas) by current injection, and their outcoupling to free-space radiation in specially designed graphene nanostructures. The proposed work focuses on maximizing the efficiency of this radiation process, through a combination of electromagnetic design simulations, materials development efforts, and device fabrication and characterization activities. The plasmonic extraction efficiency will be optimized by combining the graphene nanostructures with additional optical elements (metallic THz antennas in an open-cavity configuration) designed to promote critical coupling to free-space radiation. At the same time, the plasmonic internal emission efficiency will be enhanced through the controlled injection of non-equilibrium carrier distributions in the plasmonic nanostructures using graphene/MoS2 Schottky junctions. The latter idea is analogous to the principle of operation of light emitting diodes (LEDs) used in solid-state lighting, but applied to an entirely new material system and spectral region. In addition to its potential technological impact on THz imaging and sensing, this project will also create new knowledge and new research opportunities in the area of plasmonics and light-matter interactions in two-dimensional materials.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.