High-Q Circular Dichroism Resonances in Plasmonic Lattices with Chiral Unit Cells

Plasmonic chiral nanostructures have attracted a great deal of attention in chirality-based biosensing, enantioselective separation, and recently photonic orbital angular momentum-based quantum information processing. However, the intrinsic Ohmic losses of metals and significant radiative scattering of plasmonically resonant nanostructures result in small quality factors and hence weak optical chirality (such as circular dichroism, CD). Here, it is reported that propagating surface plasmon polaritons (SPPs)—an achiral electromagnetic surface wave—can significantly enhance the Q-factors of localized surface plasmon resonance (LSPR) related CD in plasmonic lattices with chiral unit cells. It is demonstrated experimentally and theoretically that mode interaction between the highly dispersive achiral SPPs and the nondispersive chiral LSPR results in the formation of hybrid chiral SPPs, enabling efficient tuning of the resultant transmission CD dispersion and signal intensity. A maximum CD signal of 0.8 is experimentally observed with a Q-factor of 45 in the visible spectral region, showing good agreement with theoretical calculations. This study provides an effective yet facile approach for engineering both the strength and dispersion of optical chirality, paving the way for realizing large-scale, low-cost, and high-performing chiral plasmonic and dielectric metasurfaces for a variety of sensing, imaging, and information manipulation.