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

电磁频谱的太赫兹(THz)频段位于红外和微波频率之间，为气体光谱提供了独特的功能。这包括研究高空大气气候变化现象的基础反应的能力，或观察太阳系以外恒星和行星形成星云内发生的机制。然而，直到最近，还缺乏合适的仪器来在太赫兹频率上进行痕量气体传感。电子振荡器的输出功率在&gt；1太赫兹非常低，而传统的半导体激光器受材料带隙的限制，只能输出红外波长。这个项目将通过开发太赫兹量子级联激光器(QCL)来克服这些限制。QCL是一种紧凑但强大的太赫兹辐射源，基于多量子井结构中的子带间跃迁。然而，到目前为止，THz-QCL气体光谱学一直基于简单的直接传输方案，不能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激光光谱学技术从红外波段转换到太赫兹波段并展示其解析关键大气气体的能力来应对这些挑战。这项工作将为分析化学的一个新领域奠定基础，该领域可能对气候科学产生影响，以及更广泛的领域，包括清洁燃烧、临床呼气分析、等离子体诊断和工业过程控制。这将首次提供在实验室研究大气反应所需的超高灵敏度、频率精度和高速。将实现的主要目标包括：1.开发多通道光学腔和探测方案，使太赫兹波能够多次通过大气气体，将分析灵敏度提高约100.2倍。开发使用QCL的光谱方案，锁定到频率梳状源，允许万亿分之一的频率分辨率，并分析复杂的气体混合物。通过使用可广泛调谐的太赫兹光电探测器对关键大气气体进行深入研究，缩小“太赫兹鸿沟”，首次对1-3太赫兹波段的光谱指纹进行详细分析。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结合在一起。多通道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，而大气化学的应用将通过与化学学院大气和行星化学小组的UAFs的持续参与而得到支持。今年1将侧重于开发定制腔和气室，以及稳定大气物种的光谱分析。第二年将开发多通道光谱分析仪器，有可能与激光光谱学小组合作。锁频光谱学计划将在第二年和第三年开发，为痕量大气物种的分析奠定基础。3-4篇主要档案期刊文章(IEEE译文)太赫兹科学。技术公司，选择让我们来看看，菲斯。化学。化学。Phys。)可以预期，将重点放在(一)多通太赫兹光谱学，(二)锁频太赫兹光谱学，(三)关键大气物种的太赫兹光谱分析和(四)大气反应产物的太赫兹光谱分析。这项工作将支持和加强UKRI未来领袖奖学金，并利用EPSRC“超太赫兹”方案赠款的成果。

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.