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

We present a new approach for predicting spatial phase signals originated from photothermally excited metallic nanoparticles of arbitrary shapes and sizes. Heat emitted from the nanoparticle affects the measured phase signal via both the nanoparticle surrounding refractive index and thickness changes. Since 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, enabling more efficient phase imaging of plasmonic nanoparticles, and sparing trial and error experiments of using unsuitable nanoparticles. For the first time to our knowledge, using the proposed model, one can predict phase signatures from nanoparticles with arbitrary parameters. The proposed nonlinear model is based on a finite-volume method for geometry discretization, and an implicit backward Euler method for solving the transient inhomogeneous heat equation. To validate the model, we correlate its results with experimental results obtained for gold nanorods of various concentrations, which we acquired by a custom-built wide-field interferometric phase microscopy system.