[show abstract][hide abstract] ABSTRACT: A novel gallium arsenide (GaAs) based micro-cooler design, previously analysed both experimentally and by an analytical Heat Transfer (HT) model, has been simu-lated using a self-consistent Ensemble Monte Carlo (EMC) model for a more in depth analysis of the thermionic cooling in the device. The best fit to the experimental data was found and was used in conjunction with the HT model to estimate the cooler-contact resistance. The cooling results from EMC indicated that the cooling power of the device is highly dependent on the charge distribution across the leading inter-face. Alteration of this charge distribution via interface extensions on the nanometre scale has shown to produce significant changes in cooler performance. C 2014 Au-thor(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4865251]
[show abstract][hide abstract] ABSTRACT: Wirelessly directed nerve regeneration: inductively powered electrical stimulation circuits on the biodegradable polymer, polycaprolactone, demonstrate directed regeneration of sensory neurons from a dorsal root ganglion. These circuits, produced using a unique transfer printing process, illustrate progress towards the use of electrical stimulation systems on biodegradable materials to improve peripheral nerve repair functional outcomes.
[show abstract][hide abstract] ABSTRACT: In this article a monolithic resonant terahertz sensor element with a noise equivalent power superior to that of typical commercial room temperature single pixel terahertz detectors and capable of close to real time read-out rates is presented. The detector is constructed via the integration of a metamaterial absorber and a micro-bolometer sensor. An absorption magnitude of 57% at 2.5 THz, a minimum NEP of and a thermal time constant of 68 ms for the sensor are measured. As a demonstration of detector capability, it is employed in a practical Nipkow terahertz imaging system. The monolithic resonant terahertz detector is readily scaled to focal plane array formats by adding standard read-out and addressing circuitry enabling compact, low-cost terahertz imaging.
[show abstract][hide abstract] ABSTRACT: We present a novel Nipkow disk design for terahertz (THz) single pixel imaging applications. A 100 mm high resistivity (ρ≈3k-10k Ω·cm) silicon wafer was used for the disk on which a spiral array of twelve 16-level binary Fresnel lenses were fabricated using photolithography and a dry-etch process. The implementation of Fresnel lenses on the Nipkow disk increases the THz signal transmission compared to the conventional pinhole-based Nipkow disk by more than 12 times thus a THz source with lower power or a THz detector with lower detectivity can be used. Due to the focusing capability of the lenses, a pixel resolution better than 0.5 mm is in principle achievable. To demonstrate the concept, a single pixel imaging system operating at 2.52 THz is described.
[show abstract][hide abstract] ABSTRACT: A new technique is demonstrated for the realisation of air bridges using one lithographic step. Gray scale lithography is used for the formation of 3D profiles on polymethyl methacrylate (PMMA) where a variable dose exposure is applied to create a trapezoid profile for the air bridge. In previous 3D electron beam lithography (EBL) methods the span area was exposed to a low dose or a low acceleration voltage [1,2,3]. Thus, the required discontinuity with the surrounding area for the lift-off process was created. In this technique, no exposure of the span area is needed. Another exposure of a gradient dose is applied to the sides of the highest part of the air bridge. The created profile, after developing the resist, is depicted in Figure 1. The surfaces with red and blue colour represent the metal to form the air bridge and the metal to lift-off, respectively. Using this configuration, the deposited metal at the sides of the top part of the air bridge is connected to the surrounding metal to lift-off and disconnect from the air bridge. The electron dose used in this area has to be smaller than the minimum dose that penetrates the total resist layer, so that the deposited metal does not reach the substrate. This method takes maximum advantage of the resist thickness for the fabrication of high structures, as no part of the resist is sacrificed
39th International Conference on Micro and Nano Engineering London, UK, September 2013., London; 09/2013
[show abstract][hide abstract] ABSTRACT: Multi-spectral imaging systems typically require the cumbersome integration of disparate filtering materials in order to work simultaneously in multiple spectral regions. We show for the first time how a single nano-patterned metal film can be used to filter multi-spectral content from the visible, near infrared and terahertz bands by hybridizing plasmonics and metamaterials. Plasmonic structures are well-suited to the visible band owing to the resonant dielectric properties of metals, whereas metamaterials are preferable at terahertz frequencies where metal conductivity is high. We present the simulated and experimental characteristics of our new hybrid synthetic multi-spectral material filters and demonstrate the independence of the metamaterial and plasmonic responses with respect to each other.
[show abstract][hide abstract] ABSTRACT: A self-consistent Ensemble Monte Carlo (EMC) model was developed to simulate the thermionic effect in heterostructure barrier coolers. The model was validated on an InGaAs-InGaAsP heterostructure device of variable barrier height and width, producing good quantitative agreement with previous literature results. The operation of the cooler was found to be a complex and intricate process depending on the field, conduction band and details of barrier structure. When applied to a GaAs-AlGaAs micro-cooler there was good agreement with the experimental results. Importantly, very small alterations in the barrier structure were found to lead to considerable changes in device performance. V C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4817087]
[show abstract][hide abstract] ABSTRACT: In this paper, we fabricated an anisotropically etched multilevel silicon diffractive lens. The operating frequency and focal length of the lens are 2.52 THz and 15 mm, respectively. Both 4- and 16-level lenses have been realized. The total etch depth and size of the lenses are 49 μm and 45 mm × 45 mm. Positive tone thick resist AZ 4562 was used as a mask material for dry etching the high resistivity silicon substrate. We have characterized the lens using a high power continuous wave far infrared laser operating at 2.52 THz. The measured results show good agreement with the designed specifications.
IEEE Transactions on Terahertz Science and Technology. 07/2013; 3(4):479-485.
[show abstract][hide abstract] ABSTRACT: We present the design, fabrication, and measurement of a 2-way modified Wilkinson divider constructed in a coplanar geometry exhibiting ultra wideband isolation, transmission, and port matching in the millimeter-wave frequency range. The proposed divider replaces the lumped resistor in the conventional Wilkinson divider with two quarter-wave length transmission lines, a phase inverter, and two 2Z0 resistors. Except for the three ports that are coplanar waveguides (CPWs), the main body of the divider uses coplanar striplines (CPS). The phase inverter is realized using a simple airbridge-based crossover which is compatible with a modern monolithic microwave integrated circuit process. The divider has a ring-like configuration fabricated on a 620 µm thick semi-insulating GaAs wafer using electron beam lithography (EBL) technology. Three-dimensional (3D) full-wave electromagnetic simulations have been carried out to optimize the design and investigate the possible effect of fabrication tolerance on the performance of the crossover and the divider. Two dividers working at center frequencies of 25 and 80 GHz have been designed, fabricated, and tested. They all show consistent performance in terms of bandwidth, isolation, and port matching. Experimental and simulation results are in excellent agreement.
International Journal of Microwave and Wireless Technologies 06/2013; 5(03). · 0.57 Impact Factor
[show abstract][hide abstract] ABSTRACT: This paper describes the simulation and fabrication
of the first planar Gunn diode based on InGaAs on InP
substrate. Gunn devices were simulated using the Sentaurus
Device software. The fabricated planar Gunn diodes are 1.3
μm long and 120 micron wide and the measured and simulated
results are in excellent agreement.
25th International Conference on Indium Phosphide and Related Materials, Kobe, Japan, June 2013, pp 1-2., Kobe, Japan, June 2013, pp 1-2.; 05/2013
[show abstract][hide abstract] ABSTRACT: Acoustic radiation force has been demonstrated as a method for manipulating micron-scale particles, but is frequently affected by unwanted streaming. In this paper the streaming in a multi-transducer quasi-standing wave acoustic particle manipulation device is assessed, and found to be dominated by a form of Eckart streaming. The experimentally observed streaming takes the form of two main vortices that have their highest velocity in the region where the standing wave is established. A finite element model is developed that agrees well with experimental results, and shows that the Reynolds stresses that give rise to the fluid motion are strongest in the high velocity region. A technical solution to reduce the streaming is explored that entails the introduction of a biocompatible agar gel layer at the bottom of the chamber so as to reduce the fluid depth and volume. By this means, we reduce the region of fluid that experiences the Reynolds stresses; the viscous drag per unit volume of fluid is also increased. Particle Image Velocimetry data is used to observe the streaming as a function of agar-modified cavity depth. It was found that, in an optimised structure, Eckart streaming could be reduced to negligible levels so that we could make a sonotweezers device with a large working area of up to 13mm×13mm.
[show abstract][hide abstract] ABSTRACT: An ultrasonic device for micro-patterning and precision manipulation of micrometre-scale particles is demonstrated. The device is formed using eight piezoelectric transducers shaped into an octagonal cavity. By exciting combinations of transducers simultaneously, with a controlled phase delay between them, different acoustic landscapes can be created, patterning micro-particles into lines, squares, and more complex shapes. When operated with all eight transducers the device can, with appropriate phase control, manipulate the two dimensional acoustic pressure gradient; it thus has the ability to position and translate a single tweezing zone to different locations on a surface in a precise and programmable manner.
[show abstract][hide abstract] ABSTRACT: When biased in the negative differential resistance regime, electroluminescence (EL) is emitted
from planar GaAs heterostructure Gunn diodes. This EL is due to the recombination of electrons in
the device channel with holes that are generated by impact ionisation when the Gunn domains reach
the anode edge. The EL forms non-uniform patterns whose intensity shows short-range intensity
variations in the direction parallel to the contacts and decreases along the device channel towards
the cathode. This paper employs Monte Carlo models, in conjunction with the experimental data, to
analyse these non-uniform EL patterns and to study the carrier dynamics responsible for them. It is
found that the short-range lateral (i.e., parallel to the device contacts) EL patterns are probably due
to non-uniformities in the doping of the anode contact, illustrating the usefulness of EL analysis on
the detection of such inhomogeneities. The overall decreasing EL intensity towards the anode is also
discussed in terms of the interaction of holes with the time-dependent electric field due to the transit
of the Gunn domains. Due to their lower relative mobility and the low electric field outside of the
Gunn domain, freshly generated holes remain close to the anode until the arrival of a new domain
accelerates them towards the cathode. When the average over the transit of several Gunn domains is
considered, this results in a higher hole density, and hence a higher EL intensity, next to the anode.
Journal of Applied Physics 03/2013; 113:124505. · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: A compact hybrid system has been developed to position and detect fluorescent micro-particles by combining a Single Photon Avalanche Diode (SPAD) imager with an acoustic manipulator. The detector comprises a SPAD array, light-emitting diode (LED), lenses, and optical filters. The acoustic device is formed of multiple transducers surrounding an octagonal cavity. By stimulating pairs of transducers simultaneously, an acoustic landscape is created causing fluorescent micro-particles to agglomerate into lines. The fluorescent pattern is excited by a low power LED and detected by the SPAD imager. Our technique combines particle manipulation and visualization in a compact, low power, portable setup.
[show abstract][hide abstract] ABSTRACT: Research in to label free methods for biological analysis have brought interesting developments. Cell impedance spectroscopy has been one of the promising outcomes. Here we show the development of an 8-well impedance measurement setup and studied the use of conducting polymers as electrode material in cell impedance spectroscopy. We have developed devices using PEDOT:PSS electrodes and shown its advantages (lower impedance and faster to reach electrochemical equilibrium) over conventional materials, such as gold. It is observed through electrochemical analysis that the lower interfacial impedance is due to the low charge transfer resistance of PEDOT:PSS. MDCK cell proliferation experiments were performed using both types of electrode materials to provide a comparative result. We applied electrical modeling methods to understand the cell–substrate interactions and shown its applications in cell impedance spectroscopy. This study presents the development and advantages of cell impedance spectroscopy using conducting polymer electrodes.
Sensors and Actuators B Chemical 01/2013; 176:667 - 674. · 3.54 Impact Factor
[show abstract][hide abstract] ABSTRACT: We describe the construction of an ultrasonic device capable of micro-patterning a range of microscopic particles for bioengineering applications such as targeted drug delivery. The device is formed from seven ultrasonic transducers positioned around a heptagonal cavity. By exciting two or three transducers simultaneously, lines or hexagonal shapes can be formed with microspheres, emulsions and microbubbles. Furthermore, phase control of the transducers allows patterning at any desired position in a controlled manner. The paper discusses in detail direct positioning of functionalised microspheres, emulsions and microbubbles. With the advantages of miniaturization, rapid and simple fabrication, ultrasonic tweezers is a potentially useful tool in many biomedical applications.
[show abstract][hide abstract] ABSTRACT: Electronic devices are shrinking in size and new materials such as gallium nitride (GaN) are being introduced, resulting in higher dissipated power densities. Therefore, new and novel technologies are required to remove the heat and to thermally characterise the devices. A particular example of a device that will require these thermal management technologies is the gallium arsenide (GaAs) planar Gunn diode which is being developed for millimetre-wave and terahertz (THz) frequencies, and in which the dissipated power density will be very high (approximately 106 W/cm2). This paper will review preliminary thermal and electrical characterisation of a GaAs electro-thermal micro-cooler with the aim of its integration with the planar Gunn diodes, thus increasing the efficiency of the latter. The thermal characterisation of these electro-thermo coolers brings its own set of unique measurement problems. A comparative temperature measurement has been developed using infra-red (IR) thermal microscopy and novel micro-particle sensors. The measurement technique will be described in detail and preliminary measurements on simple superlattice electro-thermal GaAs micro-coolers will be presented
[show abstract][hide abstract] ABSTRACT: We report on the design, fabrication, testing and packaging of a miniaturised system capable of detecting autofluorescence (AF) from mammalian intestinal tissue. The system comprises an application specific integrated circuit (ASIC), LED, optical filters, control unit and radio transmitter. The ASIC contains a high voltage charge pump and single photon avalanche diode detector (SPAD). The charge pump biases the SPAD above its breakdown voltage to operate in Geiger mode. The SPAD offers a photon detection efficiency of 37% at 520nm, which corresponds to the autofluorescence emission peak of the principle human intestinal fluorophore, flavin adenine dinucleotide (FAD). The ASIC was fabricated using a commercial triple-well high-voltage CMOS process. The complete device operates at 3V and draws an average of 7.1mA, enabling up to 23 hours of continuous operation from two 165mAh SR44 batteries.
IEEE transactions on bio-medical engineering 10/2012; · 2.15 Impact Factor