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

在电信业需求的推动下，半导体激光技术的发展导致了小型(~2厘米长或更小)、低成本激光器的出现，这些激光器在狭窄的频率范围(~1 MHz带宽)内提供足够的功率(通常为5-50 mW)，以满足依赖分子光谱的新应用的挑战。这些二极管激光器在室温下高效工作，因此不需要更传统的中红外(IR)二极管激光器所需的液氮冷却。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(波长高达2微米的近红外线和有限的可见光光谱)工作，而这些区域不是对痕量气体进行灵敏分析测量的最有用的区域。如果光学技术可以用来提供可调谐的紫外光(波长390 nm)和中红外(波长3微米)激光，那么一系列新的令人兴奋的测量就变得可行了。因此，我们的第一个目标是证明几个新想法的有效性，这些新想法可以从紧凑和低成本的电信二极管激光器中产生这些紫外光和中红外波长。紫外光和中红外光谱是分析光谱学的关键区域，因为它们对应于光谱中许多常见分子的强电子和振动跃迁所在的部分。因此，光谱技术可以在一系列重要环境中用于特定的、定量的和高灵敏度的化学成分测量。新的紫外光和中红外激光光源，以及商业(但尚未广泛使用)量子级联激光器(产生长于~4.5微米的中红外波长)将与光学腔技术相结合，以制造超高灵敏度吸收光谱仪。通过在两个或更多反射率大于99.9%的反射镜之间捕获激光(构成光腔)，光可以通过装在只有几十厘米长的桌面设备中的样品传播几公里。超长的路径长度与其他技术技巧相结合，为定量测量等离子体、大气甚至人类呼吸中的微量成分提供了必要的灵敏度，测量精度可达百万分之几(10^12中1)。我们还将测试每秒多达1000次的吸收测量方法，用于快速光谱分析应用，例如观察空气中的单个气溶胶颗粒(每个颗粒的尺寸小于1微米)。这种超灵敏光谱仪的应用范围很广，作为这一研究计划的关键部分，我们不仅将展示新方法，还将把它们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。等离子体是通过微波或射频电磁辐射激发或通过直流电放电部分电离的气体混合物。它们既用于化学气相沉积(从气体中生长钻石或碳纳米管等现代技术材料)，也用于材料的蚀刻(例如用于制造电子和计算机芯片的微米级半导体图案)，但发生在生长或蚀刻表面上方的气体环境中的大部分化学物质鲜为人知。通过仔细研究等离子体化学，依赖于这种技术的工业过程将变得更加有效。第二个应用的例子是测量空气中存在的各种重要化合物(无论是在室外，还是在建筑物或车辆内，人们大部分时间都在那里度过)，例如污染或自然产生的有机分子。在不同的流行条件下测量这些痕量成分对于正确理解地球大气的化学成分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.