On the H∞ controller design for a magnetic suspension system model

This paper deals with the H∞ optimal controller design for a magnetic suspension system model derived in Knospe and Zhu [2011], with added input/output delay. The plant is a fractional order system with time delay i.e., the transfer function of the plant involves infinite dimensional terms including a rational function of √/s and e-hs, where h > 0 represents the delay. The H∞ optimal controller is designed by using the recent formulation given in Ozbay [2012] for the mixed sensitivity minimization problem for unstable infinite dimensional plants with low order weights. The effect of time delay on the achievable performance level is illustrated. © 2013 IFAC

Observation of cavity structures in composite metamaterials

We investigated the cavity structure by the deformation of a unit cell of a Composite Metamaterial (CMM) structure. We considered different cavity structures with different resonance frequencies and Q-factors. We observed the Q-factor of the cavity resonance as 108 for a CMM based single cavity wherein the cavity structure is a closed ring structure. We investigated the reduced photon lifetime and observed that at the cavity resonance, the effective group velocity was reduced by a factor of 20 for a CMM based single cavity compared to the electromagnetic waves propagating in free space. Since the unit cells of metamaterials are much smaller than the operation wavelength, subwavelength localization is possible within these metamaterial cavity structures. We found that the electromagnetic field is localized into a region of/8, where is the cavity resonance wavelength. Subsequently, we brought two cavities together with an intercavity distance of two metamaterial unit cells and then investigated the transmission spectrum of CMM based interacting 2-cavity system. Finally, using the tight-binding picture we observed the normalized group velocity corresponding to the coupled cavity structure. © 2010 Society of Photo-Optical Instrumentation Engineers

Forward tunneling current in Pt/p-InGaN and Pt/n-InGaN Schottky barriers in a wide temperature range

The current-transport mechanisms of the Pt contacts on p-InGaN and n-InGaN were investigated in a wide temperature range (80-360 K) and in the forward bias regime. It was found that the ideality factor (n) values and Schottky barrier heights (SBHs), as determined by thermionic emission (TE), were a strong function of temperature and Φb0 show the unusual behavior of increasing linearly with an increase in temperature from 80 to 360 K for both Schottky contacts. The tunneling saturation ( JTU(0)) and tunneling parameters (E 0) were calculated for both Schottky contacts. We observed a weak temperature dependence of the saturation current and a fairly small dependence on the temperature of the tunneling parameters in this temperature range. The results indicate that the dominant mechanism of the charge transport across the Pt/p-InGaN and Pt/n-InGaN Schottky contacts are electron tunneling to deep levels in the vicinity of mixed/screw dislocations in the temperature range of 80-360 K. © 2012 Elsevier B.V. All rights reserved

Defect structures in metallic photonic crystals

Cataloged from PDF version of article.We have investigated metallic photonic crystals built around a layer‐by‐layer geometry. Two different crystal structures (face‐centered‐tetragonal and tetragonal) were built and their properties were compared. We obtained rejection rates of 7–8 dB per layer from both metallic crystals. Defect modes created by removing rods resulted in high peak transmission (80%), and high quality factors (1740). Our measurements were in good agreement with theoretical simulations. © 1996 American Institute of Physic

Improved selectivity from a wavelength addressable device for wireless stimulation of neural tissue

Electrical neural stimulation with micro electrodes is a promising technique for restoring lost functions in the central nervous system as a result of injury or disease. One of the problems related to current neural stimulators is the tissue response due to the connecting wires and the presence of a rigid electrode inside soft neural tissue. We have developed a novel, optically activated, microscale photovoltaic neurostimulator based on a custom layered compound semiconductor heterostructure that is both wireless and has a comparatively small volume (<0.01 mm3). Optical activation provides a wireless means of energy transfer to the neurostimulator, eliminating wires and the associated complications. This neurostimulator was shown to evoke action potentials and a functional motor response in the rat spinal cord. In this work, we extend our design to include wavelength selectivity and thus allowing independent activation of devices. As a proof of concept, we fabricated two different microscale devices with different spectral responsivities in the near-infrared region. We assessed the improved addressability of individual devices via wavelength selectivity as compared to spatial selectivity alone through on-bench optical measurements of the devices in combination with an in vivo light intensity profile in the rat cortex obtained in a previous study. We show that wavelength selectivity improves the individual addressability of the floating stimulators, thus increasing the number of devices that can be implanted in close proximity to each other. © 2014 Seymour, Freedman, Gökkavas, Özbay, Sahinand Ünlü

Ultrafast and highly efficient resonant cavity enhanced photodiodes

In this talk, we will review our research efforts on resonant cavity enhanced (RCE) high-speed high-efficiency photodiodes (PDs) operating in the 1st and 3rd optical communication windows. Using a microwave compatible planar fabrication process, we have designed and fabricated GaAs and InGaAs based RCE photodiodes. For RCE GaAs Schottky type photodiodes, we have achieved peak quantum efficiencies of 50% and 75% with semi-transparent (Au) and transparent (indium-tin-oxide) Schottky layers respectively. Along with 3-dB bandwidths of 50 and 60 GHz, these devices exhibit bandwidth-efficiency (BWE) products of 25 GHz and 45 GHz respectively. By using a postprocess recess etch, we tuned the resonance wavelength of an RCE InGaAs PD from 1605 to 1558 nm while keeping the peak efficiencies above 60%. The maximum quantum efficiency was 66% at 1572 nm which was in good agreement with our theoretical calculations. The photodiode had a linear response up to 6 mW optical power, where we obtained 5 mA photocurrent at 3 V reverse bias. The photodetector had a temporal response of 16 psec at 7 V bias. After system response deconvolution, the 3-dB bandwidth of the device was 31 GHz, which corresponds to a bandwidth-efficiency product of 20 GHz

Planar bilayer metamaterial with left-handed transmission and negative refraction at microwave frequencies

A planar composite metamaterial consisting of bilayers of metal cutwire pairs and long wire pairs which are separated by a thin dielectric layer is designed and fabricated for microwave frequencies. The simulated and experimentally measured transmission spectra of the metamaterial and its individual components (cutwire-only and wire-only) indicates that the metamaterial exhibits a transmission band within the common stop bands of its components, and thus acts as a medium with negative index of refraction. The existence of n < 0 is further supported by a refraction experiment. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA

Plasmonic nanoparticle based nanobiosensors and nanophotodetectors

Plasmonics mainly deals with light-matter interactions in metallic nanostructures. It has gathered interest since its discovery due to the benefits it provides when compared with photonics and electronics. It owes its popularity to the tremendous number of applications it serves for. In this paper, we review how plasmonic nanoparticles can be utilized in applications such as localized surface plasmon resonance based biosensing and enhancing performance of photodetectors. © 2013 SPIE

Plasmonic enhanced terahertz time-domain spectroscopy system for identification of common explosives

In this study, we present a classification algorithm for terahertz time-domain spectroscopy systems (THz-TDS) that can be trained to identify most commonly used explosives (C4, HMX, RDX, PETN, TNT, composition-B and blackpowder) and some non-explosive samples (lactose, sucrose, PABA). Our procedure can be used in any THz-TDS system that detects either transmission or reflection spectra at room conditions. After preprocessing the signal in low THz regime (0.1-3 THz), our algorithm takes advantages of a latent space transformation based on principle component analysis in order to classify explosives with low false alarm rate. © 2017 SPIE

SILVER nano-cylinders designed by EBL used as label free LSPR nano-biosensors

Localized Surface Plasmon Resonance (LSPR) is based on the electromagnetic-field enhancement of metallic nano-particles. It is observed at the metal-dielectric interface and the resonance wavelength can be tuned by the size, shape, and periodicity of the metallic nanoparticles and the surrounding dielectric environment. This makes LSPR a powerful candidate in bio-sensing. In the present work, the size and period dependency of the LSPR wavelength was studied through simulations and fabrications. The surface functionalization, that transforms the surface into a sensing platform was done and verified. Finally, the concentration dependency of the LSPR shifts was observed. All the measurements were done by a transmission set-up. The study is at an early stage, however results are promising. The detection of specific bacteria species can be made possible with such a detection method. © 2011 SPIE