Effects of optical attenuation, heat diffusion and acoustic coherence in photoacoustic signals produced by nanoparticles

TL;DR

This study investigates how nanoparticle concentration affects photoacoustic signals, revealing a linear increase up to a saturation point caused by optical attenuation and interference effects, with implications for optimizing photoacoustic imaging.

Contribution

The paper models and experimentally validates the combined effects of optical attenuation and interference on photoacoustic signal saturation in nanoparticle samples.

Findings

Photoacoustic amplitude increases linearly with concentration up to optical density 0.5.

Saturation is caused by optical attenuation and destructive interference.

Linear behavior is limited to optical densities ≤0.5.

Abstract

Behavior of the photoacoustic signal produced by nanoparticles as a function of their concentration was studied in detail. As the concentration of nanoparticles is increased in a sample, the peak-to-peak photoacoustic amplitude increases linearly up to a certain value, after which an asymptotic saturated behavior is observed. To elucidate the mechanisms responsible for these observations, we evaluate the effects of nanoparticles concentration, the optical attenuation and the effects of heat propagation from nano-sources to their surroundings. We found that the saturation effect of the photoacoustic signal as a function the concentration of nanoparticles is explained by a combination of two different mechanisms. As has been suggested previously, but not modeled correctly, the most important mechanism is attributed to optical attenuation. The second mechanism is due to an interference destructive process attributed to the superimposition of the photoacoustic amplitudes generated for each nanoparticle, this explanation is reinforced through our experimental and simulations results; based on this, it is found that the linear behavior of the photoacoustic amplitude could be restricted to optical densities $\le0.5$.