Direct Influence of Aerosol Particles in Cavity Enhanced Spectroscopy

FELIX STOLLBERGER, Michael Gleichweit, Ruth Signorell, Alexander Bergmann, Graz University of Technology

     Abstract Number: 158
     Working Group: Aerosol Physics

Abstract
Cavity-enhanced spectroscopic techniques provide superior sensitivity compared with cavity-free methods. Consequently, they are well suited for the in-situ study of aerosol microphysics and for identifying the environmental impact of aerosols, which still needs to be quantified with the desired accuracy. Among those techniques are cavity-ring-down spectroscopy, cavity-enhanced Raman spectroscopy, and photothermal interferometry to probe particle extinction, chemical information, absorption, and physicochemical properties.

Introducing an aerosol particle into a cavity directly affects the optical properties of a cavity in terms of a position dependent extinction and phase contribution. The phase contribution is based on interference of coherently forward scattered light. Both must be considered if the reflectivity or transmissivity of the cavity is monitored for sensing purposes. We present a theoretical model for this cavity-particle interaction based on a Fabry-Pérot etalon, considering cavity geometry, particle size, refractive index, and particle position within the intra-cavity standing wave. We validate our theoretical findings with measurements of single, optically trapped aerosol particles inside a Fabry-Pérot etalon (FPE). Excellent agreement between the model and experiment was achieved. Based on our model, we were able to show that the effect of extinction exceeds the phase contribution due to coherent forward scattering by nearly two orders of magnitude in the observed size range of 600 nm to 3.2μm. However, small changes in the etalon geometry significantly influence the ratio of the two effects. Minor variations in the etalon collection angle and beam diameter increased the phase contribution by a factor of 64.

Overall, we could successfully model the influence of a single micrometer-sized particle on the reflectivity of an FPE and provide the first direct observation of this effect by measurements. Our findings will increase the accuracy of cavity-enhanced methods by accounting for the bias created by the presence of the aerosol particle. Furthermore, the method can be used as a novel analysis method for retrieving complex scattering information from isolated particles.