Strong chiral optical response from planar arrays of subwavelength metallic structures supporting surface plasmon resonances

A strong differential response in the scattering of left and right circularly polarized light from surface plasmons in planar metal nanostructures is investigated theoretically and experimentally. We show that a strong chiral optical response can be obtained by an interference effect arising from a combination of resonance phase shifts in the metal structures and phase shifts associated with the rotation of the electric field vector. The effect is modeled using an analytical theory of localized surface plasmon resonances which predicts a maximum in the chiral response when the nanostructure exhibits at least two resonant modes, separated in frequency by Γ/√3, where Γ is the FWHM of the resonance, and when the angle between the dipole moments of the modes is oriented at 45 degrees. The predictions of the model agree well with numerical simulations based on the finite-difference time-domain method. The interference effect is demonstrated by optical measurements on planar metamaterials consisting of subwavelength arrays of gold rods. The effect is not related to optical activity, circular dichroism, diffraction, or phase shifts on propagation. The failure to satisfy the conditions for interference explains why some geometrically chiral structures show little or no differential scattering with circularly polarized incident light. This work provides criteria for designing new plasmonic nanostructures with strong chiral optical response.