Phase-locked arrays of high-power terahertz lasers with ultra-narrow beams

摘要标题基于一种新的用于传感和光谱应用的锁相方案的超窄光束中的高功率太赫兹激光辐射非技术描述由于缺乏高功率辐射源，电磁频谱的太赫兹区域严重不发达。现有的单色光源具有低输出功率和/或高发散光束，这使得它们不适合在太赫兹传感、成像和光谱学中的重要应用。该项目旨在开发太赫兹半导体激光器，该激光器可以在角度发散小于5度的窄光束中发射高达数百毫瓦的平均光功率。这样的性能预计将用于在紧凑型制冷机中运行的激光器，这将是对目前最先进水平的重大改进。提出了一种新的分布式反馈方案，允许对多个太赫兹金属腔进行锁相，从而显著提高了辐射效率。这些激光器将采用半导体量子级联技术进行开发，该技术将允许在单个半导体芯片上集成数十种不同的激光器，每个激光器都以一定的离散频率发射，以实现广泛的光谱覆盖。这种高功率太赫兹激光集成阵列的可获得性可能会对太赫兹科学和技术领域产生革命性的影响，因为它能够开发具有成本效益的科学仪器，用于快速非侵入性/遥测、检测和分析各种化学品和生物分子物种，如包装药物和爆炸物、药物化合物和生物样品。如果该项目的目标能够成功实现，很有可能在光电子行业建立新的业务方向。在教育层面，这项研究将向本科生开放，特别是来自不同背景的本科生，他们将接受激光设计和表征、等离子体、太赫兹科学以及低温电学和光学测量方面的高级概念的实践培训。技术说明低温冷却半导体量子级联激光器(QCL)是最强大的相干太赫兹辐射源；然而，目前在实际可行的温度下工作的单模太赫兹激光器只能辐射平均光功率约1毫瓦，不足以满足大多数目标应用。这项提议旨在将此类QCL的功率输出提高两个数量级，范围在数百毫瓦范围内，并显著改善光束质量。便携式电动斯特林制冷机将为半导体激光芯片提供所需的冷却，这些芯片可能具有数十个以离散太赫兹频率发射的锁相QCL阵列。这样的发展可能会导致太赫兹QCL的商业化，类似于非常成功的中红外QCL，它们最近在全球数十家公司催生了经济活动。太赫兹准共振腔利用具有强模式限制的平行板金属腔，提出了一种独特的通过周围介质(真空或空气)穿过表面的多个腔的模式耦合方案。由于在太赫兹频率处有很长的波长，用常规光刻方法制作的周期性光子结构可以激发耦合腔的同相表面等离子体激元-偏振子模式。由于锁相光学模式的空间扩展的辐射波前，超窄光束中的辐射有望实现。高功率太赫兹激光器的发展有可能使太赫兹光谱、传感和成像领域的商业活动成为可能。基于激光的太赫兹仪器可以在多种领域带来新的见解，包括细胞生物学，用于研究生物分子结构和动力学，对药物、药品、爆炸物的无损评估和检测，对生物样品的非侵入性成像，以及对地球大气层和外层空间的遥感。该项目的成功结果可能会导致与业界(特别是半导体激光器公司)的合作，其中一些公司最近对太赫兹仪器的开发表现出了兴趣。在大学层面，将特别努力让本科生，特别是来自不同背景的本科生参与每年的研究活动。

Abstract titleHigh-power terahertz lasers radiating in ultra-narrow beams based on a novel phase-locking scheme for applications in sensing and spectroscopyNon-technical descriptionThe terahertz region of electromagnetic spectrum is significantly underdeveloped due to lack of high-power sources of radiation. Existing monochromatic sources have low output power and/or highly divergent beams, which makes them unsuitable for important applications in terahertz sensing, imaging, and spectroscopy. This project aims to develop terahertz semiconductor lasers that could emit up to hundred milliwatts of average optical power in a narrow beam with less than five degrees of angular divergence. Such a performance is predicted for lasers operated in a compact cryocooler, and will be a significant improvement over present state-of-the-art. A new distributed-feedback scheme that allows phase-locking of multiple terahertz metallic cavities is proposed, which leads to significantly improved radiative efficiencies. The lasers will be developed with the semiconductor quantum-cascade technology that will allow integration of tens of different lasers, each emitting at a range of discrete frequencies, on a single semiconductor chip for broad spectral coverage. The availability of such integrated arrays of high-power terahertz lasers could lead to a transformative impact in the field of terahertz science and technology, by enabling development of cost-effective scientific-instruments for rapid non-invasive/remote sensing, detection, and analysis of a variety of chemicals and biomolecular species such as packaged drugs and explosives, pharmaceutical compounds, and biological samples. There is a strong probability that new business directions will be established in photonics industry if the goals of this project are met successfully. At educational level, the research will be made accessible to undergraduate students especially from diverse backgrounds, who will get hands-on training toward advanced concepts in laser design and characterization, plasmonics, terahertz science, and cryogenic electrical and optical measurements.Technical descriptionCryogenically cooled semiconductor quantum-cascade lasers (QCLs) are the most powerful solid-state sources of coherent terahertz radiation; however, presently, single-mode terahertz lasers operating at practically viable temperatures could only radiate average optical power in the order of a milliwatt, which is insufficient for most targeted applications. This proposal seeks to improve power output from such QCLs by two-orders of magnitude in the range of hundred milliwatts with significantly improved beam quality. A portable electrically operated Stirling cryocooler will provide the required cooling for the semiconductor laser chips that could have tens of such phase-locked QCL arrays emitting at a range of discrete terahertz frequencies. Such a development could lead to commercialization of terahertz QCLs similar to the highly successful mid-infrared QCLs that have spawned recent economic activity in tens of companies worldwide. Terahertz QCLs utilize parallel-plate metallic cavities with strong mode confinement, for which a unique modal coupling scheme for multiple cavities through surface via the surrounding medium (vacuum or air) is proposed. Owing to the very long wavelengths at terahertz frequencies, it is shown that in-phase surface-plasmon-polariton modes for coupled cavities can be excited with periodic photonic structures fabricated using conventional lithography. Radiation in an ultra-narrow beam is expected owing to the spatially extended radiated wavefront of the phased-locked optical mode.Development of high-power terahertz lasers has the potential to enable commercial activity in terahertz spectroscopy, sensing, and imaging. Laser based terahertz instruments could lead to new insights in fields as diverse as cell-biology for study of biomolecular structure and dynamics, non-destructive evaluation and detection of drugs, pharmaceutical products, explosives, non-invasive imaging of biological samples, and remote-sensing of Earth's atmosphere and outer space. A successful outcome of this project will potentially lead to collaborations with industry (specifically, semiconductor laser companies), some of which have shown recent interest for development of terahertz instruments. At the University level, special efforts will be made to involve undergraduate students especially from diverse backgrounds in the research activities during each year.