Circular Differential Two-Photon Luminescence from Helically Arranged Plasmonic Nanoparticles

We report the observation of a circular differential two-photon photoluminescence (TPPL) response from a three-dimensional chiral metamaterial, comprising a system of achiral (spherical) metal nanoparticles arranged on a chiral (helical) dielectric template. The enhanced dipolar response of the individual particles arising from their strong electromagnetic coupling resulted in strong photoluminescence under peak illumination intensities as low as 2 × 10³ W/cm². The TPPL signal was of approximately equal magnitude but of opposite sign, which depended on both the circular polarization state of the incident beam and the handedness of the helical geometry. The strong chiro-optical effect observed in these experiments may be relevant to technologies related to nonlinear plasmonics, in particular imaging applications where control over the polarization state of the imaged photons may be desirable.