Prediction of Photothermal Phase Signatures from Arbitrary Plasmonic Nanoparticles and Experimental Verification

We present a new approach for predicting spatial phase signals originating from photothermally excited metallic nanoparticles of arbitrary shapes and sizes. The heat emitted from such a nanoparticle affects the measured optical phase signal via changes in both the refractive index and thickness of the nanoparticle surroundings. Because these particles can be bio-functionalized to bind certain biological cell components, they can be used for biomedical imaging with molecular specificity, as new nanoscopy labels, and for photothermal therapy. Predicting the ideal nanoparticle parameters requires a model that computes the thermal and phase distributions around the particle, thereby enabling more efficient phase imaging of plasmonic nanoparticles and avoiding trial-and-error experiments while using unsuitable nanoparticles. The proposed nonlinear model is the first to enable the prediction of phase signatures from nanoparticles with arbitrary parameters. The model is based on a finite-volume method for geometry discretization and an implicit backward Euler method for solving the transient inhomogeneous heat equation, followed by calculation of the accumulative phase signal. To validate the model, we compared its results with experimental results obtained for gold nanorods of various concentrations, which we acquired using a custom-built wide-field interferometric phase microscopy system.