High-Sensitivity\nDetection of Chiro-Optical Effects\nin Single Nanoparticles by Four-Wave Mixing Interferometry

The field of chiral nanoparticles is rapidly expanding,\nyet measuring\nthe chirality of single nano-objects remains a challenging endeavor.\nHere, we report a technique to detect chiro-optical effects in single\nplasmonic nanoparticles by means of phase-sensitive polarization-resolved\nfour-wave mixing interferometric microscopy. Beyond conventional circular\ndichroism, the method is sensitive to the particle polarizability,\nin amplitude and phase. First, we demonstrate its application on single\nchiral nanohelices fabricated by focused ion beam induced deposition.\nWe examined the combination of detected fields, which measures the\nparticle polarizability, and showed that this is a sensitive reporter\nof chirality, providing dissymmetry factors (<i>g</i><sub>α</sub>) impressively approaching unity. We then applied the\nmethod to a set of individual small gold nanoparticles near the dipole\nlimit (60 nm nominal size), having correspondingly small chiral effects\nfrom the intrinsic lattice defects and nonperfectly spherical shape.\nWe find that <i>g</i><sub>α</sub> is randomly distributed\nin the population, consistent with its nondeterministic origin, but\nagain exhibits remarkably high values, an order of magnitude higher\nthan those obtained using conventional light absorption. Considering\nthe importance of chiral plasmonic nanoparticles in fields ranging\nfrom catalysis to metamaterials, this technique offers a powerful\nway to quantify chiro-optical effects at the single particle level\nwith unprecedented sensitivity.