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Chiral Scatterometry on Chemically Synthesized Single Plasmonic Nanoparticles.

Wide-spread application of nanoparticles requires large scale fabrication techniques. Being intrinsically scalable, bottom-up nanoparticle synthesis shows an ever-growing control over particle morphology, enabling even chirally selective shapes. Significant efforts have been undertaken to refine the synthesis in order to decrease the structural spread of the particles and to purify and maximize the resulting handedness. So far, imaging technologies such as electron microscopy are mostly used to investigate the quality of the synthesis. However, for nanophotonic and plasmonic applications the optical properties are, in fact, key. In this work, we show that single particle chiral scatterometry holds great potential as a feedback to characterize the (chir-)optical quality of chemically synthesized nanoparticles. The spectra of single helicoid nanoparticles reveal a diverse set of chiroptical responses with hugely varying absolute chiral asymmetry in spite of the well-controlled morphology of the particles. Averaging over the single nanoparticles reproduces the solution ensemble measurement remarkably well. This demonstrates that the single particles, despite their morphological and consequently chiroptical differences, exhibit a clearly pronounced chiral spectral and structural feature. We find that the g-factor, i.e., the degree of asymmetry of chiral light scattering of single nanoparticles can be up to four times larger than the ensemble g-factor. This proves that chiral scatterometry can be a highly important optical feedback for bottom-up nanoparticle synthesis as it reveals that the asymmetry of the ensemble solution can be further increased and maximized by appropriate refinement methods or by post-fabrication purification.