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Cavity enhanced spectroscopy –
Recent developments and new challenges
In a typical ‘cavity enhanced’ experiment a sample is put inside an optical cavity consisting of two very highly reflective mirrors. Monochromatic light, typically generated by pulsed or cw lasers, is coupled into the cavity and trapped for tens ofs, steadily leaking out and exhibiting an exponential decay that can be characterized by a so-called ‘ring down time’. The latter is a measure of the time that the light remains in the cavity. Consequently, a molecular absorption results in a shorter ring down time and by monitoring the ring down time as function of the laser frequency direct absorption spectra are recorded. The very long path lengths that are achieved in this way in combination with the fact that a rate of absorption is measured, rather than an absolute value, makes cavity ring down spectroscopy the most sensitive direct absorption technique currently available. In the last decennium many sophisticated detection schemes have been designed using both monochromatic and broadband light sources. Applications of cavity enhanced techniques are currently found in many different directions ranging from trace-gas and breath analysis to analytical chemistry, plasma and laboratory astrophysics. Both gas phase, fluids and the solid state samples are studied with more and more advanced cavity enhanced setups, such as evanescent wave detection, and the field is literally booming.
The winter school was visited by forty students. Six lecturers (Engeln, Berden, Orr-Ewing, Sigrist, Ruth and Linnartz) introduced the field, overviewed the different detection schemes, discussed applications in atmospheric, trace-gas and medical applications as well as astrochemistry, and compared the use of broad-band techniques with monochromatic light sources.
The workshop comprised six invited speakers (Hancock, Picque, Loock, Harren, Bründermann, Venables) and thirteen hot topic presentations, as well as a poster session with 19 contributions and an industrial session. A special focus was on recent developments in the field (e.g. the use of frequency combs for cavity enhanced experiments, the extension to the THz domain, and the use of micro-cavities) and new challenges (e.g. the application of broad band techniques for analytical purposes, the use of cavity ring down experiments to derive atmospherically relevant parameters or to search for the presence of explosives). In the industrial session information was provided on mirror preparation and quality management. A special session was dedicated to the use of cavity enhanced techniques in undergraduate research work. During the meeting the newest book on cavity ring down spectroscopy (Eds. Berden, Engeln, Wiley 2009) was presented.
With 70 participants this workshop was a ‘full house’. After
subsequent meetings in Heeze (NL), Lille (F),
Düsseldorf (D), Eindhoven (NL), Oxford (UK),
We look back to a very successful meeting.
Harold Linnartz (
(Laser Centre VU,
Andy Ruth (