Localized Spontaneous Chiroptical Response in Disordered Plasmonic Nanoaggregates

Metamaterials exhibiting orders-of-magnitude en-hanced artificial chiroptical response than natural counterparts have been intensely investigated. The underlying principle of these chiral responses heavily hinges on an elaborate nanostructure fabrication to generate either intrinsic geometry chirality or handedness-dependent mode interference. Here, we unveil that localized spontaneous chiroptical response can be leveraged by plasmonic nanoaggregates with a disordered coupling effect. The near-field coupling among disordered individual gold nanoparticles leads to localized hot spots that are sensitive to the handedness of light. As a consequence, the nearby nanoparticles showcase near-unity circular dichroism in linear absorption to left-and right-circularly polarized light. It directly leads to a subsequent selective photothermal deformation of plasmonic nanoparticles, which produces prominent nonlinear circular dichroism in two-photon fluorescence by circular polarization beams. Such localized spontaneous chirality is then experimentally evidenced by nanoscale multiplexing watermarks in polychromatic beams with left-and right-circular polarizations. Our results might open a new strategy to tailor the chiroptical response in disordered photonic systems and facilitate the study of chiral light-matter interactions at the deep-subwavelength regime.