EAGER: High-performance Optical-phonon-based Terahertz Sources Operating at Room Temperature

非技术描述中红外(MIR)至太赫兹(THz)光谱范围具有独特的科学和技术意义，因为它拥有无数分子物种最强和指纹状的吸收线，使其成为开发具有卓越选择性和灵敏度的传感技术的理想光谱范围。量子级联激光器(QCL)结构紧凑、操作方便、输出功率高，是目前该光谱范围内许多应用的首选光源。然而，在太赫兹区域，QCL的性能仍然不足以满足各种现实世界的应用。太赫兹QCL的性能远低于MIR QCL，其最高工作温度仍限制在~200K。此外，目前没有QCL可以在5THz到11THz的范围内运行。在这个探索性项目中，我们计划开发一种新型的紧凑型、高性能和室温工作的太赫兹光源，以覆盖这一“间隙”光谱范围。所提出的装置基于一种未经测试但很有前途的新工作原理，这种装置的成功示范将给太赫兹源的研究领域带来革命性的影响，并使各种应用成为可能。因此，本文的研究结果适用于国家自然科学基金的EIGER计划。该项目将允许研究生和本科生积极参与尖端研究，并获得必要的知识、技能、经验和广阔的视野，以期在竞争激烈的全球舞台上领导未来的科学研究和技术发展。将研究与教育和推广活动相结合也将是我们工作的重点，旨在使所有年龄段和背景的学生受益，包括那些来自代表人数较少的群体的学生。该项目的目标是系统地探索如何实现一种基于根本不同的装置工作原理的新型太赫兹源。该器件的工作原理包括两个关键过程：(1)在多量子阱(MQW)中通过共振的子带间跃迁产生光学声子；(2)将产生的光学声子的能量传递到共振的THz天线，然后THz天线向自由空间发射光子。由于装置的工作原理对温度不敏感，这种太赫兹源应在室温及以上环境下正常工作。MQW和THz天线的设计将得到优化，以使这两个过程都有效，从而产生比典型THz QCL高出几个数量级的高总能量转换效率。此外，这种太赫兹光源具有表面发射配置，因此输出功率随器件面积的增大而增大。所提出的研究还可能使我们获得新的和/或更深入的见解，以了解多量子阱中的子带间跃迁、光学声子和光子结构的电磁共振之间相互作用的有趣和复杂的物理。涉及这三种激发的相互作用还没有得到系统的研究。对潜在物理的更好理解将指导我们改进设备设计，并可能激励我们追求更先进设备的新可能性。

Title: High-performance Optical-phonon-based Terahertz Sources Operating at Room TemperatureNon-technical DescriptionThe mid-infrared (MIR) to terahertz (THz) spectral range has its unique scientific and technological significance, as it hosts the strongest and fingerprint-like absorption lines of countless molecular species, making it the ideal spectral range for developing sensing technologies with superior selectivity and sensitivity for a broad range of applications. Quantum cascade lasers (QCLs) are currently the preferred light sources for many applications in this spectral range, thanks to their compactness, convenient operation and high output power. However, in the THz region the performance of QCLs is still not sufficient for various real-world applications. THz QCLs have much lower performance than MIR QCLs, and the highest operating temperature of THz QCLs is still limited to ~200K. Furthermore, currently no QCL can operate within the 5 THz to 11THz range. In this exploratory project, we plan to develop a new type of compact, high-performance and room-temperature operating THz sources to cover this "gap" spectral range. The proposed devices are based on an untested but promising new operation principle, and the successful demonstration of such devices will bring transformative impacts to the research field of THz sources and enable various applications. Therefore, the proposed research is suitable for the NSF EAGER program. This project will allow graduate and undergraduate students to actively participate in cutting-edge research, and acquire the knowledge, skills, experiences and broad perspectives necessary for their future leadership in scientific research and technology development on the competitive global stage. Combining research with education and outreach activities will also be a focus of our work, aiming at benefiting students of all age-groups and backgrounds, including those from underrepresented groups.Technical DescriptionThe objective of this project is to systematically explore how to realize a new type of THz sources based on a fundamentally different device operation principle. The device operation principle consists of two key processes: (1) generating optical phonons by resonant inter-subband transitions in multiple-quantum-wells (MQWs), and (2) transferring the energy from the generated optical phonons to resonant THz antennae which then emit photons into free space. As the device operation principle is not sensitive to temperature, such THz sources should operate well at room temperature and above. Designs of the MQWs and the THz antennae will be optimized to make both processes efficient, leading to a high overall energy conversion efficiency which is potentially orders of magnitude higher than that of typical THz QCLs. Moreover, such THz sources have a surface-emitting configuration, so the output power scales up with the device area. The proposed research may also allow us to gain new and/or deeper insights into the interesting and complex physics underlying the interplay between inter-subband transitions in MQWs, optical phonons and electromagnetic resonances of photonic structures. Interactions involving all three excitations have not been systematically studied. A better understanding of the underlying physics will guide us to improve the device design, and may inspire us to pursue new possibilities of more advanced devices.