(439a) 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.