Prediction of photothermal phase signatures from arbitrary plasmonic nanoparticles and experimental verification

Abstract

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. A nonlinear computational model can accurately predict thermally induced phase changes originated from arbitrarily shaped plasmonic nanoparticles. The model, developed by Omry Blum and Natan Shaked of Tel Aviv University in Israel, first numerically calculates the thermal distributions around the photothermally excited nanoparticles, and then generates the associated phase maps for light transmitted through the particle sample. Predictions generated by the model agreed well with both previous thermal-distribution studies and new experimental results for gold nanorods obtained using interferometric phase microscopy. Since plasmonic nanoparticles can be used as either imaging-contrast agents or heat-therapy agents in biomedical applications, using such a model can predict the performance of arbitrary shaped plasmonic nanoparticles even prior to their actual synthesis. Thus, this model can be used to enhance the nanoparticle performance in phase-imaging or heat-therapy techniques.