Tuning the Chiral Optical Response of Metamaterial and Metamaterial-Semiconductor Nanocrystal Hybrid Systems

Abstract

Chiral metamaterials have been proposed as a promising platform for exotic optoelectronic applications such as ultrasensitive sensors, 3D displays, and ultrafast optical circuits. The functionality of such devices depends on their ability to dynamically change their optical response when a stimulus is applied. However, there are few examples and strategies for designing chiral systems with dynamically tunable optical response without necessitating reconfiguration of the chiral assembly. This thesis presents nanostructures with chiroptical response that can be tuned by modifying the refractive index of non-metallic components, and examines the effect of different design parameters on both circular dichroism and circularly polarized photoluminescence (PL). We show a chiral metamaterial system with metallic and dielectric components, where the refractive index of the dielectric component tunes the dissymmetry in transmission of right and left circularly polarized light (RCP, LCP). We then study the polarization of PL from chiral gold nanorod dimer arrays coated with poly(lauryl methacrylate) - CdSe/CdS quantum dot (QD) composite films. For these studies, we constructed a Fourier space polarimeter and demonstrated how changing the pitch of the periodic array, altering the luminescent material, introducing a dielectric spacer layer, and modulating the refractive index of the underlying substrate affects the handedness and directionality of the PL of the QD film. Finally, we show using finite-difference time-domain simulations that the placement of luminescent nanostructures within the unit cell of metallic arrays leads to enhanced degrees of circularly polarized PL compared to luminescent films that coat the metallic arrays uniformly. In this fashion, metamaterials with highly tailored directionality and polarization of PL can be designed and built. We fabricate assemblies of gold nanorods with QD nanopillars as well as assemblies of nanostructured QD solids via direct-write electron beam lithography, and show that these assemblies exhibit substantial chiroptical response. The results of this thesis encourage the integration of dielectric, phase change, or other materials with switchable optical properties in the design of chiral optical metamaterials, and expand the range of architectures and strategies for dynamically tunable chiroptical properties.