Abstract

A three-dimensional point-source localization technique is demonstrated using two-photon photoluminescence and four-wave mixing nonlinear optical signals from plasmonic gold nanorods (AuNRs), imaged at the single-particle level. Introduction of position-dependent latitudinal astigmatisms into the imaging system, in combination with a change point detection (CPD) algorithm, resulted in localization of single particles with high precision in three dimensions. Astigmatisms were generated using axial sample-position displacements spanning the range from $\pm 10$ to $\pm 90\mathrm{nm}$ with a minimum step-size resolution of $\pm 3\mathrm{nm}$. Based on the current data, 20 nm point source localization was achieved in the axial dimension using a single imaging objective. This technique is named variable displacement–change point detection (VD-CPD). The influence of plasmon enhancement on achievable axial localization was also quantified. Two AuNR systems with different length-to-diameter aspect ratios (AR, where $\mathrm{AR}=1.86$ and 3.90) were selected for this purpose; the $\mathrm{AR}=1.86$ and $\mathrm{AR}=3.90$ had nonresonant and resonant longitudinal surface plasmon resonances (LSPR) energies, respectively, with the laser fundamental. Matching the fundamental wave LSPR energies resulted in increased axial localizations. Power-dependent analysis of the LSPR-mediated nonlinear optical images revealed that resonantly excited AuNRs results in third-order signals. The axial localization provided by VD-CPD exceeds what could be obtained using astigmatic imaging alone by a factor of 2.5. This advance will facilitate the in-depth study of photonic materials and complex biological environments that can benefit from increased axial position determinations.

© 2018 Optical Society of America

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