Cavity Enhancement Techniques
Absorption spectroscopy is governed by the Beer-Lambert law: I=Ioe-σcL. Sensitivity can be increased by increasing the pathlength (L), which we do by constructing an optical cavity around the sample. In its simplest form, a cavity is a pair of highly reflective mirrors arranged such that radiation between the mirrors undergoes many reflections before leaking out of the cavity and onto the detector. Many cavity enhancement techniques exist, the most common of which are cavity ring-down spectroscopy (which measures the time dependent intensity of light within the cavity) and cavity enhanced absorption spectroscopy (which measures the time integrated transmitted intensity). We utilize such techniques to accurately detect trace gases such as NO2, C2H2, and H2S. These methods can Spherical microparticles can act as microcavities when radiation is confined to the surface. These can be trapped with optical tweezers. These whispering gallery modes, which are sensitive functions of size and refractive index of the microcavity, have been applied to studies of the Raman spectrum of aqueous aerosol droplets.
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