Terahertz quantum-cascade laser instrumentation for high-precision gas spectroscopy

电磁波谱的太赫兹（THz）波段位于红外和微波频率之间，为气体光谱学提供了独特的能力。这些能力包括研究高层大气中气候变化现象的基础反应的能力，或者观察发生在远离我们太阳系的星星和行星形成星云内的机制的能力。电子振荡器的输出功率在> 1 THz时非常低，而传统的半导体激光器由于材料的带隙而限于红外波长。该项目将通过利用THz量子级联激光器（QCL）来克服这些限制。THz量子级联激光器（QCL）是基于多量子阱结构内的子带间跃迁的紧凑而强大的THz辐射源。迄今为止，THz-QCL气体光谱学一直基于简单的直接传输方案，无法提供研究大气反应所需的灵敏度，分辨率或速度。该项目将通过首次将高灵敏度的激光光谱技术从红外波段转换到THz波段，并展示其解决关键大气气体的能力，来应对这些挑战。这项工作将成为分析化学新领域的基础，对气候科学以及清洁燃烧、临床呼吸分析、等离子体诊断和工业过程控制等更广泛领域产生潜在影响，并将首次提供实验室研究大气反应所需的超高灵敏度、频率精度和高速。要实现的主要目标将包括：1.开发多通光学腔和检测方案，使太赫兹波能够多次穿过大气气体，将分析灵敏度提高约100.2倍。使用QCL开发光谱方案，锁定到频率梳源，允许万亿分之一的频率分辨率，并分析复杂的气体混合物。3.通过使用可广泛调谐的太赫兹光混合器对关键大气气体进行深入研究，缩小“太赫兹间隙”，首次提供1-3太赫兹波段光谱指纹的详细分析。该项目本质上是跨学科的，将高频电子和光学系统设计与灵敏的分析化学技术相结合。多程光谱学将与项目合作伙伴UKRI-STFC卢瑟福阿普尔顿实验室（RAL）合作进行，而大气化学中的应用将通过与化学学院大气与行星化学组的UAF持续合作来支持。第一年将专注于定制腔和气室的开发，以及稳定大气物种的光谱分析。第二年将开发多程光谱仪，并有可能与RAL激光光谱组合作。将在第二年和第三年制定锁频光谱方案，作为痕量大气物种分析的基础。3-4主要档案期刊中的文章（IEEE Trans. THz Sci.技术人员：选择Lett.，物理化学物理）可以预期，重点是（i）多通太赫兹光谱，（ii）频率锁定太赫兹光谱，（iii）太赫兹光谱分析的关键大气物种和（iv）太赫兹分析的大气反应产物。这项工作将支持和加强英国研究所未来领导人研究金，并利用EPSRC“超太赫兹”方案赠款的产出。

The terahertz (THz) band of the electromagnetic spectrum lies between infrared and microwave frequencies and offers unique capabilities for gas spectroscopy. These include the ability to study the reactions underpinning climate-change phenomena in the upper atmosphere, or to observe the mechanisms occurring within star and planet-forming nebulae far beyond our Solar system.Until recently, however, there has been a lack of suitable instrumentation for trace gas sensing at THz frequencies. The output power of electronic oscillators is extremely low at > 1 THz, while conventional semiconductor lasers are limited to infrared wavelengths by the bandgaps of the materials. This project will overcome these limitations by exploiting THz quantum-cascade lasers (QCLs) - compact, yet powerful sources of THz radiation, based on intersubband transitions within multiple quantum-well structures.To date though, THz-QCL gas spectroscopy has been based on simple, direct transmission schemes, which do not provide the sensitivity, resolution or speed needed for studying atmospheric reactions. This project will address these challenges by translating highly sensitive laser-spectroscopy techniques from the infrared to the THz band for the first time and demonstrating their capability to resolve key atmospheric gases. This work will underpin a new field of analytical chemistry, with potential impact in climate science, and wider fields including clean combustion, clinical breath analysis, plasma diagnostics and industrial process control.This will deliver, for the first time, the ultra-high sensitivity, frequency precision and high speed needed for studying atmospheric reactions in the laboratory. Key objectives to be delivered will include:1. Development of multi-pass optical cavities and detection schemes, enabling THz waves to pass many times through atmospheric gases, improving the sensitivity of analysis by a factor of ~100.2. Development of spectroscopy schemes using a QCL, locked to a frequency-comb source, allowing part-per-trillion frequency resolution, and analysis of complex gas mixtures.3. Closing the "THz gap" via in-depth studies of key atmospheric gases using widely tunable THz photomixers, providing the first detailed analysis of spectral fingerprints in the 1-3 THz band.This project is inherently interdisciplinary, coupling high-frequency electronic and optical system design with sensitive analytical chemistry techniques. Multi-pass spectroscopy will be undertaken in collaboration with a project partner, UKRI-STFC Rutherford Appleton Laboratory (RAL), while applications in atmospheric chemistry will be supported by ongoing engagement with UAFs in the Atmospheric & Planetary Chemistry group within the School of Chemistry.Year 1 will focus on development of custom cavities and gas cells, and spectral analysis of stable atmospheric species. Year 2 will develop multi-pass spectroscopy apparatus, with potential to collaborate with the RAL laser spectroscopy group. Frequency-locked spectroscopy schemes will be developed in Years 2 and 3, underpinning analysis of trace atmospheric species. 3-4 articles in primary archival journals (IEEE Trans. THz Sci. Technol., Opt. Lett., Phys. Chem. Chem. Phys.) can be expected, focusing on (i) multi-pass THz spectroscopy, (ii) frequency-locked THz spectroscopy, (iii) THz spectral analysis of key atmospheric species and (iv) THz analysis of atmospheric reaction products. This work will support and strengthen a UKRI Future Leaders Fellowship and leverage the outputs of the EPSRC "Hyper-THz" programme grant.