High-Sensitivity Detection of Chiro-Optical Effects in Single Nanoparticles by Four-Wave Mixing Interferometry

The field of chiral nanoparticles is rapidly expanding, yet measuring the chirality of single nano-objects remains a challenging endeavor. Here, we report a technique to detect chiro-optical effects in single plasmonic nanoparticles by means of phase-sensitive polarization-resolved four-wave mixing interferometric microscopy. Beyond conventional circular dichroism, the method is sensitive to the particle polarizability, in amplitude and phase. First, we demonstrate its application on single chiral nanohelices fabricated by focused ion beam induced deposition. We examined the combination of detected fields, which measures the particle polarizability, and showed that this is a sensitive reporter of chirality, providing dissymmetry factors (g_α) impressively approaching unity. We then applied the method to a set of individual small gold nanoparticles near the dipole limit (60 nm nominal size), having correspondingly small chiral effects from the intrinsic lattice defects and nonperfectly spherical shape. We find that g_α is randomly distributed in the population, consistent with its nondeterministic origin, but again exhibits remarkably high values, an order of magnitude higher than those obtained using conventional light absorption. Considering the importance of chiral plasmonic nanoparticles in fields ranging from catalysis to metamaterials, this technique offers a powerful way to quantify chiro-optical effects at the single particle level with unprecedented sensitivity.