PHOTONIC STRUCTURES AND DEVICES MOLDED ON SOFT POLYMER MATERIALS Public Deposited

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  • March 20, 2019
Creator
  • Tippets, Cary
    • Affiliation: College of Arts and Sciences, Department of Applied Physical Sciences, Materials Science Graduate Program
Abstract
  • Polymer materials are ubiquitous, relatively cheap, easy to process, and functionalize, making them interesting for many applications, in particular for optical systems that are traditionally fabricated from rigid and expensive materials. Polymer properties can be exploited to modulate the optical response of photonic structures. In this dissertation, I will discuss the fabrication and demonstration of several applications of soft polymers in the field of optics. Soft polymers can be used to fabricate structures with optical effects inaccessible using a single optical element created from standard materials. First, I employed a biomimetic approach to produce structural color similar to the bright blue of the Morpho Butterfly. Second, I used shape active polymers to reversibly modulate the height of an optical grating through heat. Lastly, I developed a varifocal polymer lens for an augmented reality system. Structural Color, as opposed to pigmented color, is the result of light interacting with structures with geometrical length scales comparable with the wavelength of visible light. There are many examples of structural color found in nature, from the various colors of the jewel beetles to the vibrant blue of the kingfisher bird. This structural effect can typically be identified by the iridescent nature of the coloration. I will discuss my approach toward biomimicry of the unique photonic structure found on the surface of the Morpho butterfly wings. This sub-micron sized structure is a ridge which in cross-sectional view resembles a tree, with a thin “trunk” and many periodic “branches” that produce a multilayer interference effect, strongly reflecting a brilliant blue color over a wide angular range. Biomimicry of the Morpho butterfly nanostructure has been attempted but the angular insensitivity has never been fully shown in a man-made replica. I will discuss the importance of the inherent randomness found within the Morpho structures that causes light to spread over such a large range. Here in, I will show two different fabrication approaches to integrate microstructure randomness and the consequence of such variations on the angular response. In structures that were fabricated using interference lithography a quasi-randomness (incomplete randomization) is induced through drying. Angular measurements show that a two-lobe reflection, much alike that produced by the true butterfly wing, is produced in angular space and is attributable to this quasi-random nanostructure. However, periodicity needs to be fully destroyed in order to overcome diffraction. To do this a direct-write lithography system was built and used to produce completely non-periodic structures. The results showed a more pronounced a two-lobe reflection at oblique angles. Finite-difference time-domain (FDTD) simulations were employed to understand this reflection signature and to determine effect of other geometric features. From these simulations a photonic structure, capable of spreading light in similar fashion to the butterfly, and that can be fabricated with standard microfabrication techniques is proposed. In connection to the use of polymers in diffractive structures, I will discuss my work with shape active polymers. Shape memory polymers offer a unique approach for application that demand multipurpose parts and have been utilized as heart stents and actuators. The applicability of these shape memory polymers as optical elements is demonstrated by examining the optical response of a shape shifting diffraction grating. As the height of the diffraction grating is reversibly changed the intensity of diffracted light is modulated. This constitutes a simple device realization that nevertheless illustrates the materials and optical issues that arise from the application of shape memory polymer in more complex photonic shapes will lead to the optical systems with versatile components. Finally, the use of elastomeric polymers as shape active lens will be explored. Varifocal lenses have shown the potential to solve an inherent problem in virtual and augmented reality headsets. In augmented and virtual reality headsets, the human eye will focus on the screen several inches from the face but images for both eyes are off set in order cause the users eyes to converge at a certain angle, imitating distance. In the real-world focus and vergence are in sync but these headsets encounter what is known as vergence–accommodation conflict and it is the source of major user discomfort. Vergence–accommodation conflict prevents the wide spread adoption of these potentially impactful technologies. I will present my work in developing a varifocal half silvered mirror for use in an augmented reality system. The system was validated by a perception test that showed users having increased success when the system was properly focused
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Rights statement
  • In Copyright
Advisor
  • Samulski, Edward T.
  • Sheiko, Sergei
  • Klotsa, Daphne
  • Lopez, Rene
  • Rubinstein, Michael
  • Falvo, Michael
Degree
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill Graduate School
Graduation year
  • 2017
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