Thermal lens Z-scan method for assessing photothermal and fluorescence properties in suspensions and solutions

The single-beam Z-scan thermal lens technique is applied to investigate absorption efficiencies of spherical gold nanoparticles or fluorescence quantum yield in various suspensions and solutions with high-moderate absorption. Gold NPs of different diameters were analyzed at three distinct wavelengths (blue, green, red). The method extends traditional Gaussian beam analysis to top-hat beams and allows for higher absorption conditions. The experimental data, validated against Mie theory, confirms that photothermal efficiency increases with smaller diameters and shorter wavelengths. Furthermore, the TL technique is leveraged to evaluate fluorescence quantum yield in solutions exhibiting either strong fluorescence or high thermal efficiency with weak fluorescence. Analytical calculations were performed to estimate random errors introduced by optical detectors, revealing significant uncertainty in low-fluorescence solutions, while stronger fluorescence helps minimize measurement errors. The study further discusses the challenges posed by random errors when multiple measurements are required to obtain precise fluorescence estimations. These combined findings contribute to refining optical measurement techniques and improving accuracy in photothermal and fluorescence studies.