Derrick Langley

Topography from Topology: Photoinduced Surface Features Generated in Liquid Crystal Polymer Networks

Films subsumed with topological defects are transformed into complex, topographical surface features with light irradiation of azobenzene-functionalized liquid crystal polymer networks (azo-LCNs). Using a specially designed optical setup and photoalignment materials, azo-LCN films containing either singular or multiple defects with strengths ranging from |½| to as much as |10| are examined. The local order of an azo-LCN material for a given defect strength dictates a complex, mechanical response observed as topographical surface features.

Electrostatically Tunable Meta-Atoms Integrated With In Situ Fabricated MEMS Cantilever Beam Arrays

Two concentric split ring resonators (SRRs) or meta-atoms designed to have a resonant frequency of 14 GHz are integrated with microelectromechanical systems cantilever arrays to enable electrostatic tuning of the resonant frequency. The entire structure was fabricated monolithically to improve scalability and minimize losses from externally wire-bonded components. A cantilever array was fabricated in the gap of both the inner and outer SRRs and consisted of five evenly spaced beams with lengths ranging from 300 to 400 μm. The cantilevers pulled in between 15 and 24 V depending on the beam geometry. Each pulled-in beam increased the SRR gap capacitance resulting in an overall 1-GHz shift of the measured meta-atom resonant frequency.

Inexact computing with approximate adder application

This paper presents an analysis of an inexact N-bit ripple carry adder architecture. Results show that a 30 percent power reduction is achieved for several approximate adders while maintaining a root-mean square error of 16 percent.

MEMS integrated metamaterial structure having variable resonance for RF applications

Metamaterial structures for RF applications are becoming essential in the race to reduce the footprint of antenna and components necessary for RF systems. Metamaterials provide a viable option to engineer structures from commonly used materials and processes to reduce the weight and size requirements for systems that normally operate at ¼ wavelength or greater in size for optimal performance. The Split ring resonators (SRR) first developed by Pendry, et al., has proven to be a viable component necessary to create negative index material structures. A fabricated SRR has a specific

Integrated Computational Investigation of Photoconductive Semiconductor Switches in Pulsed Power Radio Frequency Applications

Successful integration of linear-mode photoconductive semiconductor switches (PCSS) into RF pulsing antenna sources may yield higher switching speeds and improved power efficiency over conventional switching methods, while also reducing size/weight. This paper presents a multiphysics computational methodology for studying linear-mode PCSS in a high-power antenna system, with a focus on extrinsically triggered wide-bandgap materials. A comparison with the existing literature reveals crucial performance sensitivities to photon absorption models, as well as encouraging results for the potential of PCSS as high-power switching devices.

High power ultrawideband short pulse (UWBSP) electromagnetics (EM) application for wide band gap (WBG) photoconductive semiconductor switches (PCSS)

UWBSP EM systems typically rely on spark gap or vacuum tube-based devices because of the significant current and voltage magnitudes involved. Historically, solid-state devices have not been able to resist dielectric breakdown and the subsequent catastrophic failure modes that result in device damage when the large, pulsed bias voltages associated with UWBSP applications are applied. Recent advances in semi-insulating WBG materials have the potential to create switches that relieve the breakdown problem, while also allowing for more predictable, controllable, stable, and efficient switching at a reduced size and weight than what has been possible with traditional high power switching technologies. These WBG switches are operated by virtue of optical excitation of charge carriers from dopants deep within the band gap, also known as extrinsic photoconduction. This paper presents a hybrid computational approach for investigating a PCSS-based UWBSP system, focusing on the source and accompanying antenna radiation. The PCSS is simulated through Synopsys Sentaurus, using an array of fundamental and user-defined solid-state physics models. Both conventional and WBG materials are examined, and compared to published data in existing literature on comparable spark gap and PCSS systems. The results of the PCSS are used to excite two lossy UWB antenna designs, and the radiated output signals are compared both to each other and to existing literature. The results indicate that PCSS possess very high potential for not only matching but also exceeding the performance of spark gap based devices. The thick dipole UWB antenna appears to be a better suited design for radiating the transient signal switched by the PCSS than the bow tie antenna, with a higher amplitude of radiated power and far electric field. The magnitude of the radiation presents a compelling case for further investigation into the prospect of further developing solid-state driving sources for high power electromagnetic radiating systems. More advanced material measurement and characterization of the physical process involved with critical aspects of the switch, such as extrinsic photoconduction, are required in order to produce a higher fidelity model for future research.

Subtractive plasma-etch process for patterning high performance ZnO TFTs

Fig 2 shows a cross sectional SEM image of the channel area after etching tungsten prior to stripping the PMMA etch mask. The thickness of the ZnO film measured ~52 nm, both in the channel area and underneath the source/drain pads, indicating that the ZnO film was not thinned during the plasma-etch process. Fig 3(a) and (b) show the I_D-V_D output characteristics of a ZnO TFT with a 155 nm and 425 nm channel, respectively. The maximum observed drain current density (I_D) and transconductance (g_m) for the 155 nm devices was 830 mA/mm and 121 mS/mm, respectively. However, the devices do not saturate at high V_D values, due to lateral breakdown limitations of the short channels. Fig 3(c) shows I_D plotted as a function of V_G on both a log- and linear-scale for devices operating in the linear region with measured channel lengths of 155, 215, 325 and 425 nm. The devices show a near linear dependence on I_D versus L_C. However, a negative shift in the on-voltage (V_on), defined as the gate voltage when I_D begins its initial increase the log(I_D)-V_G plot [4] is observed as L_C is decreased. Fig 4(a) shows the extracted on-resistance (R_on) plotted as a function of channel length. From the data, the total parasitic source/drain resistance (R_SD) can be extracted by gated-TLM method [5], shown in Fig 4(b). The devices exhibit a width normalized R_SD of 2.1 Ω·mm indicating the tungsten/ZnO interface resistance is low.

Flexible Terahertz Metamaterials for Frequency Selective Surfaces

While recent years have seen great advances in the generation, detection, and application of terahertz frequency radiation, this region of the electromagnetic spectrum still suffers from a lack of efficient and effective frequency specific optical components. While such terahertz devices do exist, they are often limited by the materials they are based on and a lack of frequency selectivity and tunability. Metamaterial devices can provide frequency resonant behavior in the form of transmissive and reflective filters. Such a frequency selective surface can also be made tunable via the use of a flexible substrate. In this talk, we will highlight work involving the design, fabrication, and characterization of terahertz metamaterial devices based on flexible substrates. Finite element method simulations have been utilized to design a split-ring resonator (SRR) structure on a flexible SU8 polymer substrate with a targeted 250 GHz resonant response. Multiple configurations of SRR arrays have been fabricated on free standing SU8 substrates. These devices have subsequently been characterized using terahertz time-domain spectroscopy and imaging systems. The metamaterial devices have shown selective transmission and reflection over a narrow range of frequencies near the targeted resonance at 250 GHz. Details of both the design, fabrication, and characterization will be discussed.

Using Inductance as a Tuning Parameter for RF Meta-atoms

The resonant frequency of metamaterials structured with split ring resonator (SRR) meta-atoms is determined primarily through the capacitance and inductance of the individual meta-atoms. Two designs that vary inductance incrementally were modeled, simulated, fabricated, and tested to investigate the role inductance plays in metamaterial designs. The designs consisted of strategically adding sections to the SRR to increase the inductance, but in a manner that minimized capacitance variations. Each design showed a shift in resonant frequency that was proportional to the length of the added section. As the length of each section was increased, the resonant frequency shifted from 2.78 GHz to 2.18 GHz.

Low-loss meta-atom for improved resonance response

Measurements of a meta-atom integrated with a low noise amplifier into the split-ring resonator are presented. A comparison is made between baseline meta-atoms and one integrated with a GaAs low noise amplifier. S-parameter measurements in a RF strip-line show the resonant frequency location. The resonance null is more prominent for the integrated meta-atom. Biasing the low noise amplifier from 0 to 7 VDC showed that the resonant null improved with biasing voltage. As the biasing voltage increases, the transmission null reduced from -11.82 to -23.21 dB for biases from 0 to 7 VDC at resonant frequency.