David Ziegler

Dynamic bending compensation while focusing through a multimode fiber

Multimode fiber endoscopes have recently been shown to provide sub-micrometer resolution, however, imaging through a multimode fiber is highly sensitive to bending. Here we describe the implementation of a coherent beacon source placed at the distal tip of the multimode fiber, which can be used to compensate for the effects of bending. In the first part of this paper, we show that a diffraction limited focused spot can be generated at the distal tip of the multimode fiber using the beacon. In the second part, we demonstrate focusing even when the fiber is bent by dynamically compensating for it. The speckle pattern at the proximal fiber end, generated by the beacon source placed at its distal end, is highly dependent on the fiber conformation. We experimentally show that by intensity correlation, it is possible to identify the fiber conformation and maintain a focus spot while the fiber is bent over a certain range. Once the fiber configuration is determined, previously calibrated phase patterns could be stored for each fiber conformation and used to scan the distal spot and perform imaging.

Textile Frequency Selective Surface

Frequency selective surfaces (FSSs) are ubiquitous on rigid substrates and increasingly on flexible polymeric substrates. Here, we developed a FSS on a textile which is neither rigid nor smooth. We fabricate a textile capable of rejecting the millimeter-wave radiation in a narrowband, while retaining desirable textile properties such as flexibility and breathability. The resonators and resonant wavelength are on the order of the weave pitch. Durability tests are performed and spectral response is measured.

Tunable millimeter and sub-millimeter spectral response of textile metamaterial via resonant states

We report on a new textile metamaterial created by adding metal wires directly into the polymer yarn. Split-ring resonator-like extended states are created. Simulations revealed that the extended states can be easily tuned via the geometry. Measurements of the transmittance spectrum as a function of the polarization angle in the low terahertz range were also performed and these peaks were ascribed to a polarization-dependent resonator model. The fabrics are viable candidates for flexible and deformable gigahertz and terahertz-enabled metamaterials.

Surface modification of nanocrystalline zinc oxide for bio-sensing applications

Zinc Oxide (ZnO) is a wide bandgap semiconductor that has been the subject of considerable research due to its potential applications in the areas of photonics, electronics and sensors. Nano-ZnO offers several advantages over existing biosensing platforms, most notably a large surface area for greater bio-functionalization and an inherent photoluminescence (PL) signal consisting of two emission peaks. One peak is in the UV, due to near band edge emission and the other is in the visible (green) region, due to oxygen vacancies caused by crystalline defects. Real-time detection of surface binding events may be possible if changes to the PL spectrum of a ZnO-based bio-sensor can be induced. Here we describe the surface modification of nanocrystalline zinc oxide (nano-ZnO) to introduce chemically reactive functionality for subsequent bio-functionalization. We have demonstrated through TEM-EDS that nano-ZnO powders have been surface modified with a heterobifunctional organosilane crosslinking agent that contains an amine-reactive aldehyde group. Furthermore, we have attached a fluorophore to the reactive aldehyde verifying the modified nano-ZnO surface is available for subsequent biomolecular covalent attachment. The introduction of a chemically reactive modifier to the surface of the nano-ZnO presents a template for the design of new, optically responsive bio-sensing platforms.

Optical properties of a retro-reflection fiber cross section formed via tri-component fiber extrusion

A retro-reflection, polymer fiber cross section is fabricated using a tri-component fiber extruder. The fiber cross section is comprised of a series of right angles. The right angles are retro-reflection features that run the entire length of the fiber. The retro-reflective features are formed by an extrusion process where the polymer fiber material is forced through a series of plates resulting in the cross section having the desired shape. Because the fiber cross sectional features form naturally by intersecting chords, the features scale naturally and have a tendency to maintain their form when the fiber is drawn to the desired diameter. Alternating the indices of refraction of the cross-sectional features allows for the realization of a number of unique and useful optical effects. The fiber cross section exhibits refraction and diffraction qualities as well as retro-reflection properties. As such, it exhibits prismatic and multiple-order diffraction interference. Hence, the fiber appears colorful when illuminated with white light. The colors can be controlled by a number of means: for example by the inclusion of dyes, nanoparticles, and by post-processing applications of thin films.

Nanoantennas with short wavelength resonance

Carbon nanotubes have been shown to exhibit light antenna behavior, such as polarization and length dependence, and enhancement of incident electromagnetic radiation at resonance. We study and model resonance effects from planar metallic nanoantennas, as a function of nanoantenna dimensions and material properties. We discuss the challenges of designing a two-dimensional nanoantenna array with resonance in the short wavelength (blue-green) region of the visible spectrum, constructed from different materials and in different environments.

Design and fabrication of extruded retroreflective polymer fibers

Extruded polymer fibers with submillimeter diameters are considered for tailored optical and near-IR properties. Retroreflection of light is demonstrated. Simulated and measured reflection spectra are compared and found to agree. Additional simulations suggest that retroreflection from gold coated 12-pointed star fibers could exceed retroreflection from microbeads on t-shirt. Finally, a novel, robust, extruded retroreflective fiber with performance approaching microbeads is presented.

Directionally reflective fiber from multicomponent fiber extrusion

Multicomponent fibers contain two or more distinct polymers within their cross section. They have been tailored to suit esthetic and functional requirements for numerous applications. For example, the ‘islands in the sea’ cross section, where a polymer is fed in individual streams into a ‘sea’ composed of another polymer (see Figure 1), has been studied extensively.1 The sea polymer is subsequently dissolved, generally after the fibers have been knit or woven into fabric. A typical island-to-sea ratio, e.g., of high-strength, 50nm-diameter nanofibers, is 80:20. In a multicomponent fiber extruder, the fiber cross sections are created by forcing melted polymers through an ensemble of plates called a ‘spin pack.’ Spin-pack hardware components have historically been manufactured by conventional methods such as milling and/or drilling. Alternatively, more modern systems use techniques similar to those used in printing circuit boards. Spin-pack components can very accurately distribute polymers in the extremely small area available to produce high-resolution cross-sectional features. There is general interest in developing fiber geometries that can provide a number of functionalities—such as a high-strength textile fiber that can resist wear and tear—and at the same time exhibit a useful optical response. Consider a textile made from a highly reflective fiber. It would provide a first responder with a measure of safety for use at night and help others identify the wearer. It is also conceivable that the fiber could function as a sensor to identify the presence of dangerous chemicals that may be present at an emergency scene. The high degree of geometric control available in multicomponent-fiber extrusion allows realization of such fibers. Figure 1. Cross section of bi-component ‘islands in the sea’ fibers produced through spinning.

Frequency Selective Surfaces offer new possibilities as reflectance filters in the NIR/visible spectrum

Frequency Selective Surfaces (FSS) are comprised of periodic, geometric, metallic patterns that act like an array of horizontal antennas. They were originally designed as band-pass/band-block filters. Nanofabrication techniques allow for the realization of FSS structures that operate in the near infrared (NIR) and visible portions of the electromagnetic spectrum. Thus it is possible to create arrays of light antenna filters possessing optical properties that are unlike those of dye, dielectric, or holographic filters that are in common use today. Recent studies of arrays of gold, dipole nanoantennas by our group and others offer an opportunity to compare modeled FSS response with experimental results elucidating the unique, off-normal reflectance stability of frequency selective surfaces operating in the NIR/visible portion of the spectrum.

Optical properties of woven arrays of bi-component extruded polymer fibers

Directionally reflective fiber arrays were woven with polymer fibers. Fiber cross sections produced using a tricomponent fiber extruder comprised standard circular as well as right angle retro-reflection features that run the entire length of the fiber. The fiber features are formed by an extrusion process where the polymer fiber material is forced through a series of plates resulting in the cross section having the desired shape. The woven fiber arrays were evaluated for their optical properties. Results for spectral reflectivity and spatial reflectance signature properties are presented. A comparison of the results obtained for the woven fiber arrays and those obtained for bundles of similar fibers are discussed. The implementation of fiber arrays through the use of weaving techniques allows for the realization of a number of unique and useful optical effects. As evidenced by the spectral measurements, the color of the woven arrays can be controlled by a number of means: for example by the inclusion of dyes, nanoparticles, and by post-processing applications of thin films. The present work represents a logical extension of previously reported experiments with unwoven fibers with retro-reflective features1.