Multi-Functional Materials and Devices

Current Projects: Past Projects:

Additive Manufacturing of 3D Metaoptics

Personnel: Wenyuan Chen (Ph.D. student in Electrical Engineering)

Metasurfaces, consisting of subwavelength scatters that tailor the phase at will with high spatial resolution, provide a versatile platform that enables the redesign of conventional bulky optical components into compact, multifunctional optics. While metasurfaces have been demonstrated in numerous devices such as lenses, polarization convertors, and orbital angular momentum generators, it has been reported that arranging metasurfaces into multilayered 3D structures would enable multifunctionalities and overcome fundamental limitations of a single-layer metasurface while still preserving the wavelength-scale compactness. Current metasurfaces are fabricated through traditional lithography methods and therefore the fabrication of multilayered devices can be challenging.

The advent of additive manufacturing has enabled fabricating complex optical components with sub-micron precision and would allow for the fabrication of 3D metaoptics without the need of cleanroom-based environment and techniques such as atomic layer deposition and electron-beam lithography. Notably, mechanical properties, such as fatigue-resistance, could potentially be incorporated into these judiciously designed architected structures for mechanically robust optics that could, for example, remove the need of protective layers that degrade imaging quality. This work aims to investigate the design, AM-based fabrication in material platforms such as TiO2, and applications of 3D metaoptics that are ultracompact, multifunctional, and mechanically robust. This framework serves to develop 3D nanophotonic platforms through additive manufacturing, with the potential to reduce device footprint, system complexity, and fabrication complexity.

Degradable Plastics

Personnel: Lien Nguyen (Ph.D. student in Chemistry)

The carbon-carbon backbone in polyolefins is highly resistant to degradation, leading to these polymers' wide use in building and packaging materials. Unfortunately, this resistance to degradation that makes polyolefins so useful during their functional lifetimes also gave rise to a plastics crisis as over 40% of plastic post-consumer waste ends up in landfills. Although many degradable and recyclable polymers sourced from plant-based materials have been reported in recent years, the majority are not attractive candidates for use in food packaging, which accounts for a third of the global packaging market. Renewably-sourced materials, such as lactic acid and cellulose, have poor barrier properties towards gas and vapor, leaving the packaged foods vulnerable to oxidation.

Cyclic ketene acetals (CKAs) have been reported to enable degradability in polyolefins by introducing ester groups into the polymer backbone. CKAs are attractive candidates for labile units in degradable polymers for 1) their versatile molecular structures enabling fine tuning polymeric microstructures and physical properties, 2) chemical versatility allowing copolymerization with a broad range of vinyl monomers, 3) fine tuning material degradability by simply adjusting initial monomer concentrations, and 4) biocompatibility allowing a variety of possible applications.

We aim to develop CKA copolymer-based resins for use in interference lithography to design novel degradable films with low gas and vapor permeabilities for use in food packaging.

Electroactive Polymers for Refreshable Braille Devices

Personnel: Sammy Shaker (Ph.D. student in Biology and Biological Engineering) and Akash Dhawan (Ph.D student in Medical Engineering)

Refreshable braille devices allow the blind community to rapidly engage with written materials; however, the two-line restrictions of most commercial devices limit the amount of information and the type of information that can be displayed. In order to expand the amount of displayable information beyond two lines and the types of displayable information from Braille to advanced graphics, researchers have looked to electroactive polymers (EAPs). EAPs include various types of polymers displaying coupling between electrical and mechanical properties, wherein electrical stimuli can lead to mechanical responses and mechanical stimuli can likewise induce potential differences. In contrast to existing devices utilizing piezoelectric cantilevers, EAP-based systems could be produced in smaller sizes, allow for more precise textures, and lower the costs of production for a full-screen system. EAP systems, however, require further work to increase their pressure generation under a given voltage difference, decrease the strength of the required electric field, and increase their durability.

Our research aims to explore a new type of electroactive polymer we have synthesized in the form of interpenetrating networks of oppositely charged polymers. Within this system, we observe swelling upon the imposition of a potential difference, which we postulate is due to intrachain like-charge repulsion. We are investigating this proposed mechanism and the basic mechanical properties of the polymer gel while modulating the required potential difference, extent of response, and speed of swelling, in addition to attempting to generate electroactive polymer monoliths via additive manufacturing techniques.

Past Projects

Architected Micro-scale Shape Memory Polymers for Biomedical Applications

Personnel: Dr. Luizetta Elliott (alumna)

20μm scale shape memory polymer structure
20-micron scale shape memory polymer structure

Shape memory polymers (SMPs) can generate programmable movement in response to heat, enabling deployable structures such as stents and clot removal devices. If utilized at the micro-scale in complex 3D geometries, these materials could facilitate miniaturized surgical applications such as clot removal from ~ 200um diameter ocular veins and minimally invasive neural implant positioning for long term neural recording.

We developed a photosensitive resin that polymerizes into programmable 3D architected shapes with minimal dimensions of 650nm using two photon lithography direct laser wiring (TPL-DLW). We synthesized this resin for shape memory actuation via a glass transition, i.e. the produced structures transform from glassy to rubbery state in response to increased temperature to enable shape programming and recovery. We characterized the temperature dependent mechanical properties of this material through dynamic nanomechanical analysis (DnMA) to identify a glass transition region. Based on this characterization, we shape memory programmed these structures, achieving 86 +/- 4% recovery in response to heat. An example of this actuation is demonstrated here with a microscale flower structure.

High-Refractive-Index, Nanoarchitected Titanium Dioxide for 3D Dielectric Photonic Crystals

Personnel: Dr. Andrey Vyatskikh (alumnus) and Dr. Ryan Ng (alumnus)

Additive manufacturing at small scales enables advances in micro- and nanoelectromechanical systems, micro-optics, and medical devices. Materials that lend themselves to AM at the nanoscale, especially for optical applications, are limited. State-of-the-art AM processes for high-refractive-index materials typically suffer from high porosity and poor repeatability and require complex experimental procedures. We developed an AM process to fabricate complex 3D architectures out of fully dense titanium dioxide (TiO2) with a refractive index of 2.3 and nanosized critical dimensions. Transmission electron microscopy (TEM) analysis proves this material to be rutile phase of nanocrystalline TiO2, with an average grain size of 110 nm and <1% porosity. Proof-of-concept woodpile architectures with 300–600 nm beam dimensions exhibit a full photonic band gap centered at 1.8–2.9 μm, as revealed by Fourier-transform infrared spectroscopy (FTIR) and supported by plane wave expansion simulations. The developed AM process enables advances in 3D MEMS, micro-optics, and prototyping of 3D dielectric PhCs.

Ultra-Thin Si Nanopillar Arrays for Polarization-Independent Spectral Filters in the Near-IR

Personnel: Dr. Ryan Ng (alumnus)

Spectral filters have a wide range of sensing applications ranging from environmental (hazardous waste, oil, etc) to surveillance. In sensing, detectors are sensitive to anywhere from several to hundreds of electromagnetic bands. Based on the number of bands and bandwidth, these systems are separated into multispectral and hyperspectral imaging systems with multispectral systems capturing under 10 bands and hyperspectral imaging capturing hundreds to thousands of bands of narrow width (around 10-20 nm) that allow for a continuous measurement across a spectrum.

Hyperspectral Filters - Pillar Array
Pillar Array

Subwavelength dielectric nanopillar arrays have potential for such spectral filtering applications as band pass and notch filters. In these arrays, rapid spectral variations in reflectivity and transmission are observed when incident light couples via a grating vector to a leaky waveguide mode propagating perpendicular to the surface and are reradiated, leading to sharp near-unity reflectivity resonances. The band width, amplitude, and peak wavelength are easily controlled through array fabrication parameters such as the pillar height, radius, and array periodicity.