Abstract

One of the most sensitive techniques for the measurements of very weak spectral absorption is integrating cavity spectroscopy (ICS). An integrating cavity is a closed container whose wall is a diffuse (Lambertian) reflector. The steady-state energy density in the cavity increases/decreases as the absorption coefficient of the sample in the cavity decreases/increases. Another approach to high-sensitivity absorption spectroscopy is cavity ring-down spectroscopy (CRDS), a very different but well-known technique. In CRDS, a sample is placed in a cavity formed by two high reflecting mirrors. A short pulse of light is injected into the cavity and bounces back and forth through the sample between the mirrors. The rate at which the light pulse decays depends on the sample absorption. Combining these two absorption spectroscopy techniques (ICS and CRDS) would provide an extremely powerful and useful new technology – integrating cavity ring-down spectroscopy (ICRDS). But, ICRDS has not previously been exploited because the diffuse reflectivity of all previously known materials was simply not high enough to do ring-down spectroscopy – the light pulse decayed too quickly due to the reflection. But a new diffuse reflecting material now makes ICRDS possible. This new breakthrough is examined and discussed.