Garth Wells

Large-area, freestanding, single-layer graphene-gold: a hybrid plasmonic nanostructure.

Graphene-based plasmonic devices have recently drawn great attention. However, practical limitations in fabrication and device architectures prevent studies from being carried out on the intrinsic properties of graphene and their change by plasmonic structures. The influence of a quasi-infinite object (i.e., the substrate) on graphene, being a single sheet of carbon atoms, and the plasmonic device is overwhelming. To address this and put the intrinsic properties of the graphene-plasmonic nanostructures in focus, we fabricate large-area, freestanding, single-layer graphene-gold (LFG-Au) sandwich structures and Au nanoparticle decorated graphene (formed via thermal treatment) hybrid plasmonic nanostructures. We observed two distinct plasmonic enhancement routes of graphene unique to each structure via surface-enhanced Raman spectroscopy. The localized electronic structure variation in the LFG due to graphene-Au interaction at the nanoscale is mapped using scanning transmission X-ray microscopy. The measurements show an optical density of ∼0.007, which is the smallest experimentally determined for single-layer graphene thus far. Our results on freestanding graphene-Au plasmonic structures provide great insight for the rational design and future fabrication of graphene plasmonic hybrid nanostructures.

A Memory Controlled Feedforward Linearizer Suitable for MMIC Implementation

This paper presents the design considerations for a microstrip feedforward linearizer. The linearizer is suitable for MMIC implementation and can be adjusted using a memory circuit. A wide operating frequency, input level and temperature range is, therefore, feasible. Measured results for a 900 MHz linearizer show a 25 dB carrier to intermodulation noise improvement over a wide operating range.

A Circuit Model for the Design of Self-Excited EBG Resonator Antennas With Miniaturized Unit Cells

A circuit model based on Bloch theory is introduced to simplify analysis and design of antennas composed of thick metal electromagnetic band-gap (EBG) cells with large intercell coupling capacitance. The cells are composed of thick metal patches periodically deployed on a metal-backed dielectric slab. Two versions of cells are presented that provide large intercell capacitance, one with narrow high aspect ratio (HAR) gaps between cells and the other with interdigitated gaps between cells. This large capacitance reduces the antenna resonance and dramatically miniaturizes the EBG cells. Three cascaded unit cells are used to demonstrate the applicability of the circuit model to characterize the recently introduced self-excited EBG resonator antenna. Full-wave numerical analysis and experimentation validate the robustness and accuracy of the model over large variations in electrical/physical cell dimensions.

Short and Open Circuited EBG Resonator Antennas: Miniaturization With a Shorting Plate and Dielectric Loading

A method is proposed to decrease the matching frequency of the recently introduced self-excited electromagnetic bandgap (EBG) resonator antenna (SE-EBG-RA). The method is based on the application of a metal shorting plate at the open end of the antenna which reduces the resonance. EBG unit cells are comprised of thick metal patches on top of a PEC-backed substrate, which are separated by tiny high aspect ratio (HAR) gaps. Two to six cells, each electrically much smaller than wavelength, are deployed in one-dimensional (1-D) as a fragment of EBG microstripline with high radiation properties. Both open-circuit (OC) and short-circuit (SC) versions are presented, the SC version being electrically smaller. The miniaturization effect of dielectric loading of HAR gaps is also examined. The characteristics of proposed structures as improved radiators in terms of bandwidth (BW), size, gain, and efficiency are demonstrated through parametric and comparative analyses and also prototyping. The efficiency, BW, and footprint of the smallest dielectric-loaded version, a two-cell SC SE-EBG-RA, are 96%, 3.5%, and 0.22λ × 0.28λ, respectively.

Signal distortion caused by tree foliage in a 2.5 GHz channel

Fixed terrestrial broadband wireless systems work very well when a clear line-of-sight is available. In near line-of-sight transmission where a few foliated trees block the line-of-sight, the signal can experience large and rapid fading. This paper investigates the time-variant nature of the frequency response of a 6 MHz wireless channel obstructed by a few fully foliated trees. Measurements were taken on fixed wireless paths with trees in the vicinity of the receive antenna, and under different weather conditions. It was found that signal fading is a function of the foliage density, the amount of wind, the multipath in the channel, and whether the leaves are wet or dry. The fades were largely flat across the band but with some frequency selective fading. Fading rates of 0.5 to 2 fades/s were measured, and occasionally the slope reached 50 dB/s.

Thick Metal EBG Cells with Narrow Gaps and Application to the Design of Miniaturized Antennas

The paper presents a methodology to achieve e-cient low-proflle electromagnetic bandgap (EBG) antennas based on thick EBG unit cells. The EBG cells are composed of thick metal patches separated by narrow high aspect ratio (HAR) gaps, and positioned on a PEC-backed substrate. This approach yields new miniaturized EBG cells with considerably reduced electrical size. The miniaturized cells are employed to demonstrate new compact self-excited EBG resonator antennas with considerably reduced operating frequencies. Full-wave simulations and experimental results demonstrate the design approach.

Machine Protection System for the Stepper Motor Actuated SyLMAND Mirrors

SyLMAND, the Synchrotron Laboratory for Micro and Nano Devices at the Canadian Light Source, consists of a dedicated X‐ray lithography beamline on a bend magnet port, and process support laboratories in a clean room environment. The beamline includes a double mirror system with flat, chromium‐coated silicon mirrors operated at varying grazing angles of incidence (4 mrad to 45 mrad) for spectral adjustment by high energy cut‐off. Each mirror can be independently moved by two stepper motors to precisely control the pitch and vertical position. We present in this paper the machine protection system implemented in the double mirror system to allow for safe operation of the two mirrors and to avoid consequences of potential stepper motor malfunction.

Numerical and experimental thermal analysis of polyimide-based x-ray masks at the Canadian Light Source

In deep x-ray lithography (DXRL), synchrotron radiation is applied to transfer absorber patterns on an x-ray mask into thick photoresist to generate high quality microstructures. Fabrication of the required x-ray masks is a demanding process sequence and constitutes a bottleneck in DXRL technology. Polymer-based mask membranes offer many benefits during mask fabrication and operation, but usually suffer from large thermal distortions during x-ray exposure. These are due to the low thermal conductivity of most polymers (approximately 0.2 W/m K), which results in inefficient heat transfer to the cooled areas around mask and substrate. The power tuning capabilities at the Synchrotron Laboratory for Micro and Nano Devices beamline, Canadian Light Source, however, allow the beam power to be adjusted and consequently limit thermal distortions. In this study, x-ray masks based on 30 μm thick polyimide membranes are studied. Numerical simulations of the thermal and thermoelastic behavior were performed using the software package ansys r14.5. The beam power input parameters were calculated with the software lex-d. For experimental verification, a process to fabricate simple test masks was developed. The polymer membranes were processed on stainless steel sacrificial wafers and were patterned with 80 μm thick nickel absorbers in two macroscopic layouts. Five chromel/alumel (K-type) thermocouples where then bonded to the absorbers to measure the heat distribution. The measurements generally validated the numerical results. The simulated thermal distributions consistently overestimate the experimental values by approximately 4–6 K, which is mainly attributed to uncertainties in the experimental proximity gap settings. The thermal simulation results indicate that the dominant heating mechanism of the resist is conduction: Energy absorbed in the mask absorbers is conducted through the helium gas in the proximity gap to the nonexposed poly(methyl methacrylate) (PMMA) areas and the substrate (cooled to 18 °C). For 500 μm thick PMMA resist, exposed with a synchrotron beam power of 19.6 W, maximum temperatures in the mask are 31.0 and 25.8 °C in the resist below. Maximum single-axis resist deformations in the mask plane amount to 4.12 μm. At 250 μm resist thickness, the observed temperatures are only 25.4 °C in the mask and 22.3 °C in the resist, with maximum mask plane deformations of about 2.3 μm. Integrated over the entire absorber size of 60 mm, these deformations roughly double. Local structure accuracy results were obtained by measuring distortions in a micropatterned polyimide mask. Deformations verify simulation results, vary with the position on the layout, and scale with the incident beam power. At 3.3 W incident beam power, typical deformations around 1–1.5 μm and maximum deformations of 2.3 μm were measured in 100 μm thick resist.In deep x-ray lithography (DXRL), synchrotron radiation is applied to transfer absorber patterns on an x-ray mask into thick photoresist to generate high quality microstructures. Fabrication of the required x-ray masks is a demanding process sequence and constitutes a bottleneck in DXRL technology. Polymer-based mask membranes offer many benefits during mask fabrication and operation, but usually suffer from large thermal distortions during x-ray exposure. These are due to the low thermal conductivity of most polymers (approximately 0.2 W/m K), which results in inefficient heat transfer to the cooled areas around mask and substrate. The power tuning capabilities at the Synchrotron Laboratory for Micro and Nano Devices beamline, Canadian Light Source, however, allow the beam power to be adjusted and consequently limit thermal distortions. In this study, x-ray masks based on 30 μm thick polyimide membranes are studied. Numerical simulations of the thermal and thermoelastic behavior were performed using the ...

Attenuated Total Reflection Fourier Transform Infrared (ATR FT-IR) Spectromicroscopy Using Synchrotron Radiation and Micromachined Silicon Wafers for Microfluidic Applications

A custom-designed optical configuration compatible with the use of micromachined multigroove internal reflection elements (μ-groove IREs) for attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectroscopy and imaging applications in microfluidic devices is described. The μ-groove IREs consist of several face-angled grooves etched into a single, monolithic silicon chip. The optical configuration permits individual grooves to be addressed by focusing synchrotron sourced IR light through a 150 µm pinhole aperture, restricting the beam spot size to a dimension smaller than that of the groove walls. The effective beam spot diameter at the ATR sampling plane is determined through deconvolution of the measured detector response and found to be 70 µm. The μ-groove IREs are highly compatible with standard photolithographic techniques as demonstrated by printing a 400 µm wide channel in an SU-8 film spin-coated on the IRE surface. Attenuated total reflection FT-IR mapping as a function of sample position across the channel illustrates the potential application of this approach for rapid prototyping of microfluidic devices.