Single-Shot Aspect Ratio and Orientation Imaging of Nanoparticles

Plasmonic nanoparticles with surface plasmon resonance (SPR) and scattering response dependent on their geometry and surrounding environment are predestinated to be used as optical probes for sensing and imaging. Optical microscopy is capable of observing nanoparticles in various media, but their geometry remains hidden below the diffraction limit. Here, a wide-field optical imaging technique is demonstrated, restoring the aspect ratio and orientation of individual nanoparticles via the polarization anisotropy (PA) measurement of the scattered light. The PA is mapped into a single nanoparticle image, formed by decomposing the scattered light into longitudinal and transverse SPR modes and manipulating their angular momentum. The wide-field images provide the aspect ratio and orientation of many deposited nanoparticles allowing their assessment in heterogeneous suspensions or time-resolved measurements. In calibration experiments, orientation measurement accuracy and excellent sensitivity to nanoparticles with specific aspect ratios are demonstrated. Subsequently, the method is deployed in the automatic shape-dependent categorization of hundreds of nanoparticles in a heterogeneous mixture. The single-shot capability is demonstrated in the time-resolved imaging of the electrophoretic deposition process

Quantitative 3D Phase Imaging of Plasmonic Metasurfaces

Coherence-controlled holographic microscopy (CCHM) is a real-time, wide-field, and quantitative light-microscopy technique enabling 3D imaging of electromagnetic fields, providing complete information about both their intensity and phase. These attributes make CCHM a promising candidate for performance assessment of phase-altering metasurfaces, a new class of artificial materials that allow to manipulate the wavefront of passing light and thus provide unprecedented functionalities in optics and nanophotonics. In this paper, we report on our investigation of phase imaging of plasmonic metasurfaces using holographic microscopy. We demonstrate its ability to obtain phase information from the whole field of view in a single measurement on a prototypical sample consisting of silver nanodisc arrays. The experimental data were validated using FDTD simulations and a theoretical model that relates the obtained phase image to the optical response of metasurface building blocks. Finally, in order to reveal the full potential of CCHM, we employed it in the analysis of a simple metasurface represented by a plasmonic zone plate. By scanning the sample along the optical axis we were able to create a quantitative 3D phase map of fields transmitted through the zone plate. The presented results prove that CCHM is inherently suited to the task of metasurface characterization. Moreover, as the temporal resolution is limited only by the camera framerate, it can be even applied in analysis of actively tunable metasurfaces