Quantitative Photoacoustic Method for the Measurement of Absorption Coefficients of Highly Absorbing Samples

In this study, we present a quantitative photoacoustic (PA) method for performing absorption measurements on highly absorbing samples. Based on the thermoelastic mechanism, the relative changes in PA signal amplitude allowed the determination of absorption coefficients of materials in the 0.19-2500-cm^-1 range, with no prior knowledge of the material's optoacoustic properties required. We have tested our new methodology by performing absorption measurements on a series of planar liquid samples as well as gelatinized spherical samples. In this approach, laser-induced ultrasound waves were detected in transmission mode. With the model presented herein and a measurement of the relative change in amplitude of the PA signal at two different known concentrations, the absorption coefficient of the sample can be straightforwardly extracted. Three important advantages are highlighted by this analytical approach. First, no previous knowledge of the optical or acoustic properties of the sample is necessary. Second, only a small quantity of sample is required. Finally, our methodology includes both short- and long-pulse regimes, validating its use for any laser pulse duration so long as the requirement for thermal confinement is fulfilled. Remarkably, this new methodology performs best for thick, highly absorbing samples where traditional spectrophotometry is most challenging and unreliable, offering a promising alternative for quantification of the absorption properties of a range of diverse liquid, and gelatinous-state materials not amenable to conventional methods.