Quantum cascade laser spectroscopy: diagnostics to non-linear optics

Geoffrey Duxbury, Nigel Langford, Kenneth Hay, Nicola Tasinato

Research output: Contribution to journalArticle

20 Citations (Scopus)

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 (OCO)-O-18-C-12-O-16. 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.

LanguageEnglish
Pages2034-2048
Number of pages15
JournalJournal of Modern Optics
Volume56
Issue number18-19
DOIs
Publication statusPublished - 28 Oct 2009

Fingerprint

nonlinear optics
quantum cascade lasers
laser spectroscopy
gases
infrared lasers
low pressure
cells
molecular collisions
sweep frequency
oxides
nitrous oxides
infrared radiation
optical paths
lasers
molecules
ammonia
dipole moments
diodes
alignment
laser beams

Keywords

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

Cite this

Duxbury, Geoffrey ; Langford, Nigel ; Hay, Kenneth ; Tasinato, Nicola. / Quantum cascade laser spectroscopy: diagnostics to non-linear optics. In: Journal of Modern Optics. 2009 ; Vol. 56, No. 18-19. pp. 2034-2048.
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Quantum cascade laser spectroscopy: diagnostics to non-linear optics. / Duxbury, Geoffrey; Langford, Nigel; Hay, Kenneth; Tasinato, Nicola.

In: Journal of Modern Optics, Vol. 56, No. 18-19, 28.10.2009, p. 2034-2048.

Research output: Contribution to journalArticle

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