Quantum cascade laser spectroscopy: diagnostics to non-linear optics

Abstract

In many examples of the use of mid-infrared quantum cascade (QC) lasers for gas detection or process monitoring, an assumption is made that their use is an obvious extension of tuneable diode laser spectroscopy. We wish to show that making such an assumption is not necessarily justified when the frequency sweep rate is rapid, as is down-chirped QC laser infrared radiation. This is demonstrated via a series of experiments designed to investigate the physics of the interaction of chirped infrared laser radiation with low pressure gases. The unusual signals, which characterise the rapid passage of the down-chirped radiation through a low pressure gas, are due to two main effects, the laser sweep rate, and the long path length of the refocusing cells used. The sweep rate of the laser frequency may be faster than the inter-molecular collision frequency, allowing the build up of a strong molecular alignment within the gas. The long optical path lengths in the refocusing absorption cells, used to facilitate sensitive detection of trace gases, allow the build up of a large macroscopic polarisation within the gas cell. We give examples of this behaviour in molecules with large transition dipole moments, ammonia and nitrous oxide, and with a very small one ¹⁸O¹²C¹⁶O. We also outline the use of Maxwell–Bloch calculations to investigate the origins of this behaviour, and hence to define operating conditions where the concentration of trace molecules may be determined.

Keywords:

• rapid passage
• infrared spectroscopy
• quantum cascade lasers
• molecular collision processes
• chirped lasers
• Maxwell–Bloch equations

Acknowledgements

The authors would like to thank the United Kingdom Engineering and Physical Research Council for funding the initial part of this work through the research Grant GST/M6911 1999, and for the research studentship to KGH. We are also very grateful to the Leverhulme trust for the award of an Emeritus Fellowship to GD. We are also indebted to the National Environmental Sciences Research Council (NERC) for funding the main part of this work through the Grant NER/T/S2002/00052, and to the NERC Airborne Support Facility (ARSF) for the development flights on their Dornier aircraft, and to Carl Joseph and David Davies for their help before and during the flight campaign. We acknowledge the award of a grant from the EU Accord programme for the evaluation of new types of QC lasers. We are also grateful (NT) to Universita Ca Foscari di Venezia for a studentship, and to Cascade Technologies for supplying the prototype four laser spectrometer. We would like to acknowledge the help of the staff of Cascade Technologies, in particular Ian Howieson, Ruth Lindley and Michael McCulloch for their help in preparing the equipment for the projects.