Enhancing chiroptical signals from metamaterials via nonlinear excitation

Abstract

As natural chiral materials demonstrate limited circularly dichroic contrasts, enhancement of these polarization dependent signals has long been the focus of chiral metamaterial research. By manipulating the geometric chirality of resonant plasmonic nanostructures, we are capable of enhancing light confinement to amplify chiral modified, nonlinear signals from quantum emitters. The metamaterial demonstrates a linear transmission contrast of 0.5 between left and right circular polarizations and a 20× contrast between second harmonic responses from the two incident polarizations. Nonlinear and linear response images probed with circularly polarized lights show strongly defined contrast. As a second set of experimentation, the chiral center of the metamaterial is opened, providing direct access to place emitters to occupy the most light-confining and chirally sensitive regions. The resulting two-photon emission profiles from circularly polarized excitation displays mirrored symmetry for the two enantiomer structures. The efficiency of the nonlinear signal directly correlates to the chiral resonance of the linear regime. The nonlinear emission signal is enhanced by 40× that of the emitters not embedded in the metamaterial and displays a 3× contrast for the opposite circular polarization. Such manipulations of nonlinear signals with metamaterials open pathways for diverse applications where chiral selective signals are monitored, processed, and analyzed.