Beyond Geometrical Symmetry: Revealing Near-Field Optical Chirality on Achiral Gold Nanoparticles under Linear Polarization Excitation

Chirality plays a crucial role in the interactions between light and matter. While the majority of research has focused on the interaction of chiral structures with chiral light, recent studies have demonstrated that achiral plasmonic nanostructures can already exhibit chiral near fields under linearly polarized excitation. Building on this insight, we demonstrate that linearly polarized light alone can generate and control near-field chirality on geometrically achiral, C_3v-symmetric gold nanotriangles. We employ plasmon-assisted two-photon polymerization as a tip-free near-field recorder that converts transient near fields into permanent 3D polymer topographies. This approach directly imprints the optical near field into polymer structures, enabling direct readout of its evolution with incident polarization angle and wavelength. Planar (2D) symmetry breaking is quantified through the in-plane chirality factor V_min, defined from the loss of mirror symmetry of the polymer with respect to the three mirror planes of the nanotriangle and evaluated from both SEM images and FDTD simulations. Near-field dissymmetry maps derived from simulations and AFM topographies further resolve the redistribution of chiral hot spots. By jointly tuning polarization and wavelength, we reveal a controllable transition between achiral and chiral polymer configurations correlated with the modal composition of the plasmonic response (dipolar versus edge-dominated higher-order modes). These results establish a practical route to engineer and permanently record polarization-tunable planar dissymmetry in achiral nanoantennas, with implications for chiral sensing, enantioselective photochemistry, and nanophotonic devices.