Chiral Metamaterial Platform with Tunable Near and Far Field Chiroptical Response

The development of artificially structured electromagnetic materials, called metamaterials, has led to the realization of optical phenomena that cannot be obtained with natural materials. Chiral plasmonic metamaterials have been proposed as a promising platform for optoelectronic devices with exotic applications, such as negative index of refraction materials and superlenses that can break the diffraction limit. Chiral metamaterials have unit cells that lack mirror symmetry and inversion centers, and exhibit asymmetries in response to circularly polarized light that are orders of magnitude greater than the ones observed from naturally occurring chiral molecules. In the far-field, these asymmetries are manifested in the circular dichroism (CD) signal that quantifies the difference in absorption of left and right handed light. Despite the extensive literature on the far-field optical response of chiral metamaterials, our understanding of the chiral electromagnetic light-matter interactions remains limited. To develop design principles for chiral metamaterials, it is important to characterize and tune the optical chirality C of the electromagnetic fields in the vicinity of the nanostructures in the chiral unit cell, and elucidate the complex relationship between the near- and far-field optical response of chiral systems.

Here, we used Finite-Difference Time-Domain calculations to simulate the optical response of a stacked gold L-shape resonator system. In this system, the sign of the CD spectrum can be controllably changed through lateral shifts in the relative position of two gold L- resonators, which tunes the interaction strength. These small structural reconfigurations change the energetic ordering of the hybridized modes and alter the CD response of the system without changing its handedness. We examined the sensitivity of this system to structural perturbations, and demonstrated that the CD spectrum can abruptly reverse under small (less than 1 nm) reconfigurations of the L-resonators.

We have also calculated the chirality of the local electromagnetic fields and revealed superchiral hotspots with optical chirality 15 times larger than that of circular polarized light. More importantly, our simulations demonstrated that the optical chirality and the volume of the superchiral fields are much larger for the mode with enhanced extinction cross section. Lateral shifts of the upper resonator relative to the bottom one tuned the magnitude, volume, and handedness -given by the sign of optical chirality- of the local superchiral fields excited by right and left circularly polarized light.

The ability of our chiral L-shape assembly to abruptly change the far-field chiroptical response makes it ideal for experimentally fabricating a fast switchable chiral platform with dynamically tunable CD response. Meanwhile, the tunable chiral local electromagnetic fields could be utilized to examine how the change in the chiral interactions between the nanostructures translates into a change in the far field response of chiral systems.