A new optical technique developed in the department of Chemical Engineering and Biotechnology for measuring molecular species at trace level concentrations was recently featured as a Research Highlight in the September issue of Nature Photonics.
The original paper referred to, Cavity enhanced absorption spectroscopy of multiple trace gas species using a supercontinuum radiation source, appeared in the July issue of Optics Express. The work is a collaborative effort between the Laser Analytics Group headed by Clemens Kaminski, and the Atmospheric Chemistry Group, headed by Professor Rod Jones in the Chemistry department.
The method pioneers the use of supercontinuum (SC) radiation generated in photonic crystal fibres in a technique called cavity enhanced absorption spectroscopy (CEAS). The broadband supercontinuum light is coupled into a 1.15 meter long optical cavity formed by two high reflectivity mirrors (99.995% reflectivity). The light entering the cavity bounces between the mirrors a large number of times before leaving the cavity, thus achieving effective absorption lengths up to 20 km, which enhances the sensitivity of the technique. The light transmitted through the cavity is analysed with a spectrometer.
The method represents a step change in trace gas sensing: In contrast to traditional implementations of the technique SC-CEAS is not limited to the measurement at single discrete wavelengths but spectra covering 100 nm or more can be captured all at once. Such massively multiplexed measurements permit a multitude of species to be targeted at once with good spectral resolution. The groups used it for the quantitative measurement NO2 and NO3, species of importance in atmospheric chemistry. In the case of NO3, a sensitivity of three parts per trillion by volume (pptv) was achieved for a measurement lasting less than 2s.
There is great potential for the technology for applications in the life sciences, in environmental and process diagnostics applications. This research has been promoted and enabled by CamBridgeSens, an EPSRC funded network to unite sensor research across the University of Cambridge.