Author ORCID Identifier

https://orcid.org/0000-0003-4474-1284

Date Available

10-23-2021

Year of Publication

2021

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Arts and Sciences

Department/School/Program

Physics and Astronomy

First Advisor

Dr. Jeffrey Todd Hastings

Second Advisor

Dr. Joseph Straley

Abstract

Traditional optical elements, such as refractive lenses, mirrors, phase plates and polarizers have been used for various purposes such as imaging systems, lithographic printing, astronomical observations and display technology. Despite their long-term achievements, they can be bulky and not suitable for miniaturization. On the other hand, recent nanotechnology advances allowed us to manufacture micro and nanoscale devices with ultra-compact sizes. Metasurfaces, 2D engineered artificial interfaces, have emerged as candidates to replace traditional refractive lenses with ultra-thin miniaturized optical elements. They possess sub-wavelength unit cell structures with a specific geometry and material selection. Each unit cell can uniquely tailor the phase, transmission and polarization state of the light. A pre-determined distribution of the unit cells in the transverse plane can provide certain functionality to mimic the traditional optical elements with a few wavelength thicknesses. Due to the requirement of sub-wavelength periodicity, they provide high sampling which can be critical for high numerical aperture requirement. Regarding different functionality and fabrication methods, several base material options exist. Due to ohmic resistance, plasmonic unit cells in the transmission mode may suffer from high loss and operate with low efficiency. On the other hand, all-dielectric metasurfaces are free of ohmic losses and suitable for standard lithographic fabrication techniques. The most common options are electron-beam and photo-lithography. Despite its common availability, electron-beam lithography is costly and slow in general. Additionally, the longitudinal degree of freedom on the unit cell geometry is not accessible, which causes high-index material requirements to achieve full control on wavefront shaping. This thesis details the design and fabrication techniques of low-index metasurfaces fabricated by two-photon lithography techniques. Contrary to electron-beam lithography, two-photon lithography allows us to explore a larger design space including the longitudinal degree of freedom. Improvement on the design space through two-photon lithography allows us to explore different design techniques such as doublet structures separated by a true air gap, which is not easily accessible through electron-beam lithography. First, we demonstrate a hybrid achromatic metalens (HAML) based on a combination of phase plate and nanopillar structures. We achieve achromatization in the near-infrared wavelength regime with high efficiency and diffraction-limited performance. Achromatization is achieved by a fast ray-tracing algorithm that provides target phase shift derivatives. We theoretically develop an analytical way to express the target phase shift over the interface. Experimental verification is done using several lens geometries. We show that introducing a true air gap between the metasurface elements allows us to explore different portions of the design space within the fabrication limits. Next, we adapted the same ray-tracing algorithm to an ultra-broadband achromatic metalens from the visible into the short-wave infrared with diffraction-limited performance. We replace nanopillars with nanoholes to obtain a smaller period and higher durability. Experimental verification is done through several performance tests including high-resolution imaging and verification of achromatization over the whole spectrum. Finally, we extend our design and fabrication techniques to model tunable metasurface doublets. We experimentally verify that the focal length of the doublet structure changes as the function of the mutual rotation angle of the elements. Finally, we present high-index dielectric metasurface doublets generating different vortex modes as the function of discrete rotation angles. We explore both converging and Bessel beams yielding various vortex modes.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2021.045

Funding Information

This work was supported in part by Intel Corporation (2018-2021). Additional support for this work was provided by the Reese S. Terry professorship in Electrical Engineering at the University of Kentucky (2018-2021). This work was performed in part at the U.K. Center for Nanoscale Science and Engineering and the U.K. Electron Microscopy Center, members of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (ECCS-1542164). This work used equipment supported by National Science Foundation Grant No. CMMI-1125998 (2018-2021).

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