Direct influence of aerosol particles on cavity enhanced spectroscopy: Modeling and first experimental results

This article presents the first experimental analysis of the influence of single, micrometer-sized aerosol particles on the reflectivity of a resonant Fabry-Pérot etalon (FPE). The results are presented along with a theoretical model capable of describing and predicting the particle influence on the transfer function of the etalon. The presence of the particle in the cavity alters the etalon reflectivity in the operating point up to several percent. Based on our findings, we show that the etalon reflectivity is mainly affected by light extinction by the particle. Phase effects due to interference of forward scattered light could not be observed experimentally in the investigated size range. However, extrapolations on the presented theoretical model show a significant dependence of the phase contribution on the collection angle of the etalon. Comparable magnitudes of the phase and extinction contribution are reached if the etalon geometry is slightly altered. This could be exploited to obtain the phase shift of the coherently forward scattered light. Cavity geometries can also be adapted to reduce the direct particle effect if it is undesired. In addition, the change in the transfer function of the etalon is analyzed and discussed, as it is especially relevant for measurement techniques such as photothermal interferometry.