Date of Award

2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Cherie R. Kagan

Abstract

Electromagnetic metamaterials (EMs) are artificial materials constructed from sub-wavelength building blocks that are tailored in their size, shape, composition, and arrangement in order to control the amplitude, phase, polarization, and propagation of light. Although theoretical research has provided most of the design rules for EMs, fabrication of EMs is still quite challenging, especially as the necessity to be smaller than optical wavelengths requires nanoscale meta-atoms. Conventional nanofabrication methods have limited most optical metamaterials to 2D or, often with multiple patterning steps, simple 3D meta-atoms. These methods are also time-consuming processes and not compatible with high throughput, large-area fabrication. In this thesis, we create nanofabrication methods that overcome most of the challenges mentioned above by leveraging the unique properties of colloidal nanocrystals (NCs).

Colloidal NCs are hybrid materials composed of inorganic cores capped with organic ligands. Ligand exchange methods can replace as-synthesized long ligands with more compact ligands. Originally, these ligand exchange methods were developed to reduce interparticle distance and enhance electronic coupling in NC assemblies, which is desirable for NC-based electronic and optical devices. In addition to the electronic and optical properties, the ligand-exchange-induced reduction in interparticle distance can introduces a large misfit strain along the interface by constructing NC/bulk heterostructures, enabling chemo-mechanical 2D to 3D shape transformation. We systematically studied the evolution of the mechanical properties of NC assemblies and put forward design rules for fabrication. We showed complex 3D meta-atoms through one-step patterning, either suspended in solution or anchored on rigid substrates. We also demonstrated the tunability in structure and function of these metamaterials by tailoring composition, incorporating metal or magnetic NCs, and size and shape through the 2D pattern design and their shape transformation via chemical and thermal treatments.

This fabrication methodology is exploited to create chiro-photonic devices. We demonstrated that metamaterials with 3D chiral meta-atoms can generate giant chiroptical responses that largely scatter right(left) hand circularly polarized light (RCP(LCP)) and transmit LCP(RCP). By designing the shape of the meta-atoms and the periodicity of the meta-atom arrays, we successfully showed ultra-thin photonic devices that are capable of switchable chirality and generating broadband circularly polarized light.

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