New frontiers in quantitative infra-red to ultraviolet spectroscopy using diode and quantum-cascade lasers

由电信行业需求驱动的二极管激光技术的发展导致了小型（~2厘米长或更小）的可用性，低成本激光器在狭窄的频率范围（~ 1 MHz带宽）内发出足够的功率（通常为5 - 50 mW），用于依赖分子光谱的具有挑战性的新应用。这些二极管激光器在室温下有效工作，因此不需要液氮冷却，这是更传统的中红外（IR）二极管激光器所必需的。然而，电信二极管激光器的关键限制是，它们只能在有限的波长区域工作（近红外至λ ~ 2微米的波长，以及可见光谱的有限部分），这不是对痕量气体的敏感分析测量最有用的区域。如果光学技术可以用来提供可调谐的紫外（λ <390 nm）和中红外（λ bbb30微米）激光，那么一系列令人兴奋的新测量将变得可行。因此，我们的第一个目标是证明从紧凑和低成本的电信二极管激光器产生这些紫外和中红外波长的几个新想法的有效性。紫外和中红外是分析光谱的关键区域，因为它们对应于许多普通分子的强电子和振动跃迁所在的光谱部分。因此，光谱技术可用于在一系列重要环境中对化学成分进行特定的、定量的和高度敏感的测量。新的紫外和中红外激光源，以及商用（但尚未广泛使用）量子级联激光器（其产生的中红外波长长于~4.5微米）将与光学腔技术相结合，制成超高灵敏度吸收光谱仪。通过将激光捕获在2个或更多反射率大于99.9%的镜子之间（组成光学腔），光可以通过放置在只有几十厘米长的台式仪器中的样品传播几公里。非常长的路径长度，结合其他技术技巧，为等离子体，大气，甚至人类呼吸的微量成分的定量测量提供了必要的灵敏度，从百万分之一到万亿分之一（1 / 10^12）的水平。我们还将测试用于快速光谱应用的每秒多达1000次吸收测量的方法，例如观察空气中的单个气溶胶颗粒（每个颗粒的大小小于1微米）。这种超灵敏光谱仪的应用范围很广，作为该研究计划的关键部分，我们不仅将展示新方法，而且将其应用于前沿科学课题。一个例子是对等离子体化学成分的研究。等离子体是由微波或射频电磁辐射或直流放电激发而部分电离的气体混合物。它们既用于化学气相沉积（从气体中生长现代技术材料，如金刚石或碳纳米管），也用于材料的蚀刻（如制造电子和计算机芯片的微米级半导体图案），但在生长或蚀刻表面以上的气体环境中发生的大部分化学反应尚不清楚。通过对等离子体化学的仔细研究，依赖于这种技术的工业过程将变得更加有效。应用的第二个例子是测量存在于空气中的各种重要化合物（无论是室外，还是建筑物或车辆内，人们大部分时间都在这些地方度过），例如污染或自然产生的有机分子。在不同普遍条件下测量这些微量成分对于正确了解地球大气的化学性质或室内环境对健康的影响至关重要。

Developments in diode laser technology driven by the demands of the telecoms industry have resulted in the availability of small (~2 cm long or smaller), low cost lasers that give out sufficient power (typically 5 - 50 mW) in a narrow range of frequencies (~ 1 MHz bandwidth) for challenging new applications that rely on molecular spectroscopy. These diode lasers operate efficiently at room temperature and thus do not need the liquid nitrogen cooling necessary for more traditional mid infra-red (IR) diode lasers. The critical limitation of the telecoms diode lasers, however, is that they operate only in limited wavelength regions (the near IR up to wavelengths of lambda~ 2 microns, and limited parts of the visible spectrum), which are not the most useful regions for sensitive analytical measurements of trace gases. If optical techniques can be used to provide tuneable UV (lambda<390 nm) and mid-IR (lambda>3 microns) laser light, a whole host of new and exciting measurements become feasible. Our first goal is thus to demonstrate the effectiveness of several new ideas for generating these UV and mid-IR wavelengths from the compact and low cost telecoms diode lasers. The UV and mid-IR are critical regions for analytical spectroscopy because they correspond to parts of the spectrum where strong electronic and vibrational transitions of many common molecules lie. Spectroscopic techniques can thus be used for specific, quantitative and highly sensitive measurements of chemical composition in a range of important environments.The new UV and mid-IR laser sources, as well as commercial (but not yet widely used) quantum cascade lasers (which generate mid-IR wavelengths longer than ~4.5 microns) will be combined with optical cavity techniques to make ultra-high sensitivity absorption spectrometers. By trapping laser light between 2 or more mirrors with reflectivities greater than 99.9% (making up the optical cavity), the light can be made to travel distances up to several km through a sample housed in a table-top apparatus only a few tens of cm long. The very long pathlengths, combined with other technical tricks, provide the sensitivity necessary for quantitative measurement of trace constituents of plasmas, the atmosphere, or even human breath at parts per million down to parts per trillion (1 part in 10^12) levels. We will also test methods to make as many as 1000 measurements of absorption per second for rapid spectroscopy applications such as looking at individual aerosol particles (each less than 1 micron in size) in air. The applications of such ultra-sensitive spectrometers are wide ranging, and as a key part of this program of research, we will not only demonstrate the new methods, but also apply them to cutting-edge science topics. One example is studies of the chemical composition of plasmas. Plasmas are mixtures of gases that are partially ionized by excitation with microwave or radiofrequency electromagnetic radiation, or by a dc electric discharge. They are used in both chemical vapour deposition (growth of modern technological materials such as diamond or carbon nanotubes from gases) or etching of materials (such as micron-scale patterning of semiconductors to make electronics and computer chips) but much of the chemistry occurring in the gaseous environment above the growing or etching surface is poorly understood. Industrial processes relying on such technology will be made more effective by careful study of the plasma chemistry. A second example of applications is in measurement of a variety of important compounds present in air (either outside, or within buildings or vehicles, where people spend the great majority of their time) such as polluting or naturally occurring organic molecules. Measurements of these trace constituents under different prevailing conditions are vital for a proper understanding of the chemistry of the Earth's atmosphere, or for the impact of the indoor environment on health.