Graduation Year

2020

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Zhimin Shi, Ph.D.

Co-Major Professor

Jiangfeng Zhou, Ph.D.

Committee Member

Libin Ye, Ph.D.

Committee Member

Andreas Muller, Ph.D.

Committee Member

Jing Wang, Ph.D.

Keywords

Chirality, Polarization, Metamaterials, Gratings, CPA

Abstract

Electromagnetic metamaterials are materials that have designed electromagnetic properties. In general metamaterials are subwavelength and can be treated as homogeneous media with an effective permittivity and permeability. Early metamaterial designs were developed to exhibit electromagnetic responses not typically found in naturally occuring materials, but this is not a strict requirement. Metamaterials have shown promise for applications throughout the electromagnetic spectrum. The goal of this work is to present a coherent methodology for metamaterials design, fabrication and application while also describing all of the relevant physics of the designed metamaterials.

Chapter 1 is comprised of an introduction into the field of metamaterials. A description of the development of metamaterials from the intial works investigating media with a negative index of refraction to current investigations into actively controlled metamaterials is presented. Further, a general description of the physics of early metamaterials and the concepts used in their design is discussed.

In Chapter 2, a metamaterial composed of a nanocomposite slab designed for application as a photomask absorber in Extreme-Ultraviolet (EUV) Lithography is presented. Rigorous computational simulations are performed to calculate the electromagnetic response of the nanocomposite and the results for various sized nanoparticle inclusions are compared. It is further demonstrated that the material's enhanced response compared with a single-element-based absorber is due to increased absorption caused by the nanoparticle inclusions. An effective medium theory is then combined with a transfer matrix method to rapidly calculate the electromagnetic response of the nanocomposite metamaterial as a function of absorber thickness and volume of nanoparticle inclusions. Last, the lithography process is simulated for the metamaterial nanocomposite absorber simulations and it's performance in reducing the bias required for horizontal and vertical printed features is discussed.

Chapter 3 introduces a planar chiral metamaterial that is proposed for use in the THz regime as a polarization rotator. It is demonstrated through simulations that the proposed metamaterial is capable of rotating a linearly polarized input beam by $90^\circ$. Further, inductive coupling between cut-wire pairs in adjacent layers is shown to be responsible for the giant optical activity demonstrated by the metamaterial. A study is presented on the metamaterial's electromagnetic response as a function of geometric feature size of the design components and the effects of common fabrication errors. The electromagnetic response of the metamaterial is shown to be robust against fabrication imperfections. Last, a fabrication process using common microlithography techniques is presented and demonstrated. Fabricated metamaterial samples are then presented and future studies into the metamaterial are discussed.

Chapter 4 describes the application of the interferometric control method, Coherent Perfect Absorption (CPA), to 4-fold rotationally symmetric chiral media. A thorough derivation is provided to describe the controllable output from a 2-port scattering process involving such a chiral material. It is shown that the chiral metamaterial described in Chapter 3 is a suitable 4-fold rotationally symmetric chiral media that can be used in the CPA control scheme. The performance of the metamaterial under the CPA scheme is calculated and it is shown that selective absorption of left and right-handed circularly polarized light can be achieved. As a result, the chiral metamaterial is shown to demonstrate active control of the output polarization state when illuminated under the CPA control scheme.

In Chapter 5, the coherent control interferometric method is applied to loss-free dielectric anisotropic media. It is shown that a material with suitable anisotropy has two potential applications: coherent polarization beam splitting and active control of the output polarization state. A subwavelength High-Index-Contrast (HICM) Metastructure grating is then presented as a candidate with suitable geometric symmetries to achieve these responses. A robust scan of the grating geometric features using Rigorous Coupled Wave Analysis is performed to find structures that can achieve the anisotropy required to demonstrate coherent control functionalities. A study of the field response within the grating structures under a single input beam is analyzed to understand physical mechanism responsible for the HICM grating response. Last, a rapid design process using an effective index of refraction calculated by a Bloch theorem is presented and a refractive index retrieval method is then applied to calculate the effective index of refraction of the anisotropic media for comparison.

Appendix A and B are comprised of mathematical formulations for some of the simulations used in each of the chapters of this dissertation. Appendix A describes how to formulate the transfer matrix method using Maxwell's Equations and describes all of the relevant assumptions and physics for both the Transverse-Electric (TE) and Transverse-Magnetic (TM) cases. Appendix B provides a mathematical derivation of Rigorous Coupled Wave Analysis for both the TE and TM cases and describes all of the relevant assumptions and physics used in it's development and computational programming.

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