Raymond C. Rumpf

Broadband Microwave Frequency Characterization of 3-D Printed Materials

3-D printing allows increased design flexibility in the fabrication of microwave circuits and devices and is reaching a level of maturity that allows for functional parts. Little is known about the RF and microwave properties of the standard materials that have been developed for 3-D printing. This paper measures a wide variety of materials over a broad spectrum of frequencies from 1 MHz to 10 GHz using a variety of well-established measurement methods.

Thermo-mechanical Characterization of Metal/Polymer Composite Filaments and Printing Parameter Study for Fused Deposition Modeling in the 3D Printing Process

New metal/polymer composite filaments for fused deposition modeling (FDM) processes were developed in order to observe the thermo-mechanical properties of the new filaments. The acrylonitrile butadiene styrene (ABS) thermoplastic was mixed with copper and iron particles. The percent loading of the metal powder was varied to confirm the effects of metal particles on the thermo-mechanical properties of the filament, such as tensile strength and thermal conductivity. The printing parameters such as temperature and fill density were also varied to see the effects of the parameters on the tensile strength of the final product which was made with the FDM process. As a result of this study, it was confirmed that the tensile strength of the composites is decreased by increasing the loading of metal particles. Additionally, the thermal conductivity of the metal/polymer composite filament was improved by increasing the metal content. It is believed that the metal/polymer filament could be used to print metal and large-scale 3-dimensional (3D) structures without any distortion by the thermal expansion of thermoplastics. The material could also be used in 3D printed circuits and electromagnetic structures for shielding and other applications.

Fabrication of three-dimensional micro-photonic structures on the tip of optical fibers using SU-8.

A method is reported for fabricating truly three-dimensional micro-photonic structures directly onto the end face of an optical fiber using the cross-linkable resist SU-8. This epoxide-based material is well suited for micro-device fabrication because it is photo-processed as a solid and the cross-linked material is mechanically robust, chemically resistant, and optically transparent. Yet, procedures commonly used to process SU-8, particularly spin-coating, are impractical when the intended fabrication substrate is the end-face of an optical fiber. A melt-reflow process was developed to prepare optical fibers having SU-8 resin deposited at controlled thickness on the fiber end-face. Multi-photon direct laser writing was then used to fabricate various refractive lenses, a compound lens system, and a woodpile photonic crystal within the resin on the end-face of the optical fiber. Data are presented that show how the refractive lenses can be used to alter the output of the optical fiber. This work opens a new path to low-profile integrated photonic devices.

3D PRINTING OF ANISOTROPIC METAMATERIALS

Material properties in radio frequency and microwave regimes are limited due to the lack of molecular resonances at these frequencies. Metamaterials are an attractive means to realize a prescribed permittivity or permeability function, but these are often prohibitively lossy due to the use of ine-cient metallic resonators. All-dielectric metamaterials ofier excellent potential to overcome these losses, but they provide a much weaker interaction with an applied wave. Much design freedom can be realized from all-dielectric structures if their dispersion and anisotropy are cleverly engineered. This, however, leads to structures with very complex geometries that cannot be manufactured by conventional techniques. In this work, artiflcially anisotropic metamaterials are designed and then manufactured by 3D printing. The efiective material properties are measured in the lab and agree well with model predictions.

Fully three-dimensional modeling of the fabrication and behavior of photonic crystals formed by holographic lithography.

A comprehensive and fully three-dimensional model of holographic lithography is used to predict more rigorously the geometry and transmission spectra of photonic crystals formed in Epon SU-8 photoresist. It is the first effort known to the authors to incorporate physics of exposure, postexposure baking, and developing into three-dimensional models of photonic crystals. Optical absorption, reflections, standing waves, refraction, beam coherence, acid diffusion, resist shrinkage, and developing effects combine to distort lattices from their ideal geometry. These are completely neglected by intensity-threshold methods used throughout the literature to predict lattices. Numerical simulations compare remarkably well with experimental results for a face-centered-cube (FCC) photonic crystal. Absorption is shown to produce chirped lattices with broadened bandgaps. Reflections are shown to significantly alter lattice geometry and reduce image contrast. Through simulation, a diamond lattice is formed by multiple exposures, and a hybrid trigonal-FCC lattice is formed that exhibits properties of both component lattices.

Synthesis of spatially variant lattices

It is often desired to functionally grade and/or spatially vary a periodic structure like a photonic crystal or metamaterial, yet no general method for doing this has been offered in the literature. A straightforward procedure is described here that allows many properties of the lattice to be spatially varied at the same time while producing a final lattice that is still smooth and continuous. Properties include unit cell orientation, lattice spacing, fill fraction, and more. This adds many degrees of freedom to a design such as spatially varying the orientation to exploit directional phenomena. The method is not a coordinate transformation technique so it can more easily produce complicated and arbitrary spatial variance. To demonstrate, the algorithm is used to synthesize a spatially variant self-collimating photonic crystal to flow a Gaussian beam around a 90° bend. The performance of the structure was confirmed through simulation and it showed virtually no scattering around the bend that would have arisen if the lattice had defects or discontinuities.

Guided Mode Resonance Filter as a Spectrally Selective Feedback Element in a Double-Cladding Optical Fiber Laser

In this work, a spectrally selective optical element is introduced based on a 2-D guided mode resonance filter (GMRF) as an external feedback element. The GMRF was designed to provide a highly efficient narrow linewidth reflection within the gain bandwidth of the fiber laser, while transmitting the pump beam. These features enabled the fiber laser to operate in an external cavity configuration to provide a wavelength-stabilized and narrow linewidth output within the optical -band.

All-Dielectric Frequency Selective Surface for High Power Microwaves

In this work, an all-dielectric frequency selective surface was developed for high power microwaves. By avoiding the use of metals, arcing at field concentration points and heating in the conductors was avoided. To do this in a compact form factor while still producing a strong frequency response, we based our design on guided-mode resonance (GMR). To make this approach viable for radio and microwave frequencies, we overcame three major challenges. First, conventional GMR devices have less than 1% fractional bandwidth and we extended this to 16%. Second, conventional GMR devices have a field-of-view less than 1° and we extended this to over 40°. Third, conventional GMR devices must be composed of hundreds of periods to operate, but our device operated very well with only eight. In this paper, we present our design and experimental results at 1.7 GW/m2.

SIMPLE IMPLEMENTATION OF ARBITRARILY SHAPED TOTAL-FIELD/SCATTERED-FIELD REGIONS IN FINITE- DIFFERENCE FREQUENCY-DOMAIN

The total-fleld/scattered-fleld (TF/SF) formulation is a popular technique for incorporating sources into electromagnetic models like the flnite-difierence frequency-domain (FDFD) method. It is versatile and simplifles calculation of waves scattered from a device. In the context of FDFD, the TF/SF formulation involves modifying all of the flnite-difierence equations that contain fleld terms from both the TF and SF regions in order to make the terms compatible. While simple in concept, modifying all of the equations for arbitrarily shaped TF/SF regions is tedious and no solution has been ofiered in the literature to do it in a straightforward manner. This paper presents a simple and e-cient technique for implementing the TF/SF formulation that allows the TF/SF regions to be any shape and of arbitrary complexity. Its simplicity and versatility are demonstrated by giving several practical examples including a difiraction grating, a waveguide problem, and a scattering problem with a cylindrical wave source.

3-D Printed All-Dielectric Frequency Selective Surface With Large Bandwidth and Field of View

In this paper, an all-dielectric frequency selective surface (ADFSS) was developed using genetic algorithms and fast Fourier transforms (FFTs) to generate random geometries. This device showed a stop-band fractional bandwidth (FBW) of 54% and a field of view of 16°. The optimized FSS was manufactured by three-dimensional (3-D) printing and the frequency response was measured in the laboratory. This device was also tested in the pass band at a high pulsed microwave power of 45.26 MW/m2 and no damage was observed. This is the first known demonstration of a 3-D printed ADFSS.