Daniel J. Gregoire

Fractal design concepts for stretchable electronics

Stretchable electronics provide a foundation for applications that exceed the scope of conventional wafer and circuit board technologies due to their unique capacity to integrate with soft materials and curvilinear surfaces. The range of possibilities is predicated on the development of device architectures that simultaneously offer advanced electronic function and compliant mechanics. Here we report that thin films of hard electronic materials patterned in deterministic fractal motifs and bonded to elastomers enable unusual mechanics with important implications in stretchable device design. In particular, we demonstrate the utility of Peano, Greek cross, Vicsek and other fractal constructs to yield space-filling structures of electronic materials, including monocrystalline silicon, for electrophysiological sensors, precision monitors and actuators, and radio frequency antennas. These devices support conformal mounting on the skin and have unique properties such as invisibility under magnetic resonance imaging. The results suggest that fractal-based layouts represent important strategies for hard-soft materials integration.

Wideband Artificial Magnetic Conductors Loaded With Non-Foster Negative Inductors

We examine how the bandwidth of artificial magnetic conductors (AMCs) can be greatly increased when loaded with negative-inductance non-Foster circuits. This increase in bandwidth is achieved by enhancing the structural inductance of the AMC by combining it in parallel with a negative inductance, thus achieving a bandwidth that exceeds what is possible with passive approaches. A prototype VHF-UHF active AMC was fabricated and measured, which achieved a bandwidth greater than 80% at a resonant frequency of 263 MHz.

Surface-Wave Waveguides

We present simulations and measurements of surface-wave waveguides (SWGs) for guiding surface waves (SWs) along a constrained path. In its simplest form, the SWG is a two-dimensional analog to a dielectric waveguide where a low-index material surrounds a high-index material. The most common method for realizing materials with differing SW index is to use a grid of metallic patches of varying size on a dielectric substrate. Using asymmetric patches results in a tensor SW index. We show simulations of how the phase front of the guided surface wave can be precisely controlled using a combination of SW-index grading and rotation of the SW-index tensor. We present measurements of straight and curved SWGs showing how SWG width and curvature affect guiding properties.

Electromagnetic-wave absorption in highly collisional plasmas

An experimental and theoretical investigation of basic plasma‐physics processes relating to the absorption of electromagnetic waves in collisional plasmas is described. One‐way absorption of 63 dB at 4 GHz was demonstrated in a section of plasma‐loaded rectangular C‐band waveguide. The ultraviolet photoionization plasma‐production technique employed permits independent variation of the plasma‐density profile and electron‐collision frequency. A theoretical model for the absorption and scattering processes, which includes scattering contributions from the plasma‐vacuum interface, and partial reflections and collisional absorption in the bulk plasma, is in reasonable agreement with the experimental results.

PLASMA-ANODE ELECTRON GUN

The plasma‐anode electron gun (PAG) is an electron source in which the thermionic cathode is replaced with a cold, secondary‐electron‐emitting electrode. Electron emission is stimulated by bombarding the cathode with high‐energy ions. Ions are injected into the high‐voltage gap through a gridded structure from a plasma source (gas pressure ≤50 mTorr) that is embedded in the anode electrode. The gridded structure serves as both a cathode for the plasma discharge and as an anode for the PAG. The beam current is modulated at near ground potential by modulating the plasma source, eliminating the need for a high‐voltage modulator system. During laboratory tests, the PAG has demonstrated square‐wave, 17‐μs‐long beam pulses at 100 kV and 10 A, and it has operated stably at 70 kV and 2.5 A for 210 μs pulse lengths without gap closure.

Observation of power instability and multimode behavior in a far‐infrared free‐electron laser

Measurements of the time dependence and the frequency spectrum of the output power in the far‐infrared free‐electron laser at the University of California at Santa Barbara are reported. In typical light pulses of 20–50 μs we have observed unexpected oscillations of the laser output with a characteristic period of 5 μs. At the same time, the laser frequency swept over a discrete set of frequency modes, separated by 1.3 GHz. We also present measurements of the gain and loss of the optical mode and discuss the problem of the accelerator terminal voltage drop in a single electron beam pulse with relation to the light spectrum.

3-D Conformal Metasurfaces

This letter presents a method for mapping and fabricating metasurfaces on arbitrary 3-D surfaces. Metasurfaces are used for holographic leaky-wave antennas, reflectarray antennas, coatings for soft horn antennas, artificial magnetic conductors, high-impedance surfaces, frequency selective surfaces, electromagnetic band-gap structures, and surface-wave waveguides. They are typically fabricated as 2-D planar structures by printing a grid of metallic patches on a dielectric substrate, but for some applications, it is desirable to create a metasurface conformal to a 3-D surface. The methods presented here can be used with direct-write printing or with standard printed circuit board (PCB) techniques. The PCB method involves mapping the metasurface to a set of 2-D bands that are laminated to a 3-D substrate. As an example, a 3-D conformal, ellipsoidal, holographic, leaky-wave antenna was designed, fabricated, and measured.

MM-wave wireless access technology for the wideband wireless local loop applications

We describe and demonstrate a wideband wireless local loop (W-WLL) testbed concept based on local multipoint distribution services (LMDS) millimeter wireless technology but with a modified access and backbone architecture. The testbed is intended as a demonstration platform for broadband wireless services particularly for high speed internet and shared multimedia applications. The investigation is focused on the radio link design, network architecture, system integration, and compatible interface to the existing ATM fiber and satellite core networks.

A Coaxial TEM Cell for Direct Measurement of UHF Artificial Magnetic Conductors [AMTA Corner]

We have demonstrated a coaxial TEM (transverse electromagnetic wave) cell for direct measurement of artificial magnetic conductors (AMCs) in the VHF-UHF range. The advantages of the coaxial TEM cell are 1) it is a compact, bench-top device; and 2) it provides rapid, direct measurement of artificial-magnetic-conductor reflection response over a broad frequency range. We present measurements of various artificial-magnetic-conductor structures measured in the coaxial TEM cell, including 1) fixed-response, capacitor-loaded artificial magnetic conductors; 2) tunable varactor-loaded artificial magnetic conductors; and 3) wideband, tunable artificial magnetic conductors loaded with non-Foster, negative-inductance circuits. The measurements of the various artificial magnetic conductors were consistent with simulations. The coaxial TEM cell detailed here was 50 cm long with a 10.4 in diameter at the DUT, tapering to an N connector at the feed port. It was single-moded below 500 MHz. The design can be scaled to extend to other frequencies and other artificial-magnetic-conductor geometries.

3D artificial impedance surfaces

Artificial impedance surfaces (AISs) fabricated with an array of metallic patches are used for holographic leaky-wave antennas, artificial magnetic conductors, high-impedance surfaces, frequency selective surfaces, etc. AISs are typically fabricated as 2D planar structures, but for some applications, it is desirable to have an AIS on a 3D surface. We present here a method for mapping AIS patterns onto arbitrary 3D surfaces with minimal distortion. This method can be used with direct-write printing. We also outline a method for using standard 2D printed circuit board techniques for realizing 3D AISs.