Frequency-Resolved Single-Particle Photothermal Interferometry
FELIX STOLLBERGER, Michael Gleichweit, Ruth Signorell, Alexander Bergmann, Graz University of Technology
Abstract Number: 45
Working Group: Instrumentation and Methods
Abstract
Photoacoustic spectroscopy (PAS) is a well-established technique to probe optical or physicochemical properties of ambient aerosols and single particles. Theoretic studies have shown that the frequency-dependency of the photoacoustic signal contains information about evaporation-condensation and thermal equilibrium effects. This dependency could be exploited to measure heat capacity, absorption cross-section, or related heat-transport phenomena of aerosols.
Such measurements have not been possible due to the restrictions imposed by acoustic resonators necessary for PAS. The closely related photothermal interferometry (PTI) is not subject to such constraints, as the employed optical resonator allows free tuning of the modulation frequency. However, this property has only been used to optimize the SNR of the photothermal signal and not to investigate the frequency dependency of the photothermal effect systematically. With this study, we present a novel experimental method for performing frequency-resolved PTI on single aerosol particles.
The experimental setup features counter-propagating optical tweezers to trap single, micrometer-sized tetraethylene glycol (TEG) droplets within the cavity of a Fabry-Pérot interferometer. The particles are excited by a sinusoidally modulated infrared laser aligned with one of the trapping beams. While the particle is evaporating, the modulation frequency of the infrared laser is periodically swept in a range of 10 Hz to 100 kHz. Lock-in detection is used to record the photothermal amplitude (PTA) and phase (PTP), which are correlated with the particle radius obtained from static light scattering after the experiment.
Our experimental results show a clear size and frequency dependence of the PTA, with a decrease proportional to 1/f2 for modulation frequencies above 2 kHz. This contradicts the previously modelled and measured 1/f behavior of ensemble PTI measurements. The PTP exhibits the theoretically predicted size and frequency dependency and saturates with strong fluctuations above 5 kHz.
Overall, we prove the applicability of frequency-resolved PTI and present the first experimental data from single particles in the Knudsen transition regime. The proposed method could be used to measure the heat capacity or absorption cross-sections on single-particle levels by fitting the data with a theoretical model.