Photothermal Correlation Spectroscopy of Gold Nanoparticles in Solution

We demonstrate correlation analysis of the photothermal signal from gold nanoparticles diffusing in solution. Our experimental setup is primarily optimized for large detection volumes and therefore has a relatively low numerical aperture (0.25). This requirement limits light collection and detection efficiency, but we can nevertheless detect particles as small as 20 nm in diameter in 100 μs. Photothermal correlation spectroscopy (PCS) in water and in water:glycerol mixtures gives access to characteristic diffusion times of some tens of milliseconds. Diffusion is enhanced for high pump intensities, due to the increased temperature around the particle. The photothermal signal at the detector arises from the interference of the scattered probe beam and a reference field. In the forward configuration, the reference is the transmitted probe beam itself. In the backward configuration, the reference wave is a reflection by a nearby interface. It has an optical phase difference depending on the distance of the particle to the reflecting interface. The backward signal also includes the directly modulated scattering signal. Potential applications of photothermal correlation in biological labeling are demonstrated on bacteriophage virus particles carrying 20 nm gold labels.