Coupling between modulated Mie scattering and photoacoustic signal generation in optically trapped, single aerosol particles

Abstract

Photoacoustic spectroscopy and photothermal spectroscopy are two common methods to probe aerosol particle absorption coefficients and can be performed both on aerosol ensembles and on the single particle level. With photothermal spectroscopy typically changes in the particle’s light scattering pattern upon heating or cooling are observed with photo-diodes or cameras. In photoacoustic spectroscopy, the acoustic response to periodic light absorption is recorded e.g. with a microphone. Although both methods are closely related through their excitation process, the detection pathways are quintessentially different. In our single particle optical trapping setup, however, we observe a previously unnoticeable, unidirectional coupling between modulated Mie scattering (result of the photothermal effect) and photoacoustic spectroscopy. The coupling manifests itself via differently shaped, sudden features in the acoustic signal. Our analysis suggests a non-trivial interaction between light scattering of single, optically trapped particles and the photoacoustic signal generation based on interactions of light with the acoustic resonator’s walls. Measurements over several trapping powers and photoacoustic excitation powers support this conclusion. How the coupling manifests itself, such as shape and strength, can be conclusively explained by the structure of the particle’s momentary phase function (scattering intensity) calculated by classical Mie theory. This allows us to formulate conditions to either utilise or minimise the coupling effects in future experiments.