Optical nanoantenna optimization for local field enhancement at visible wavelengths

Projecte realitzat en col.laboració amb l'Institut de Ciències FotòniquesDipole nanoantennas are studied by means of numerical calculations, over a broad frequency range (250-600 THz), in order to evaluate the benefits of its integration in NSOM microscopy systems. Operational wavelength, radius and material were systematically varied to study the antenna’s resonant length, the field confinement, enhancement and decay. To perform more accurate calculations we implemented an algorithm to include real metal dispersion, at optical wavelengths, in the simulation. The simulations’ results suggest that the antenna’s performance depends strongly on the material’s dielectric properties; even aspects that we would relate only with the antenna geometry like field confinement. By comparing gold and aluminum antennas we found a crossover wavelength for the antenna performance. Below this wavelength aluminum is a more suitable material while for larger wavelengths gold gives better results

All-digital self-adaptive PVTA variation aware clock generation system for DFS

An all-digital self-adaptive clock generation system capable of adapt the clock frequency to compensate the effects of PVTA variations on the IC propagation delay and satisfy an externally set propagation length condition is presented. The design uses time-to-digital converters (TDCs) to measure the propagation length and a variable length ring oscillator (VLRO) to synthesize the clock signal. The VLRO naturally adapts its frequency to the PVTA variations suffered by its logic gates while the TDCs are used to track these variations across the chip and modify the VLRO length in order to adapt the clock frequency to them. The system measurements, for a 45nm FPGA, show that it adapts the VLRO length, and therefore the clock frequency, to satisfy the propagation length condition. Measurements also prove the system capabilities to act as a dynamic frequency scaling clock source since the propagation length condition value act as a frequency selection input and a strong linear relation between the input value and the resultant clock period is present.Peer ReviewedPostprint (author’s final draft

Closed loop controlled ring oscillator: a variation tolerant self-adaptive clock generation architecture

Postprint (published version

Flexible, transparent electrodes using carbon nanotubes

We prepare thin single-walled carbon nanotube networks on a transparent and flexible substrate with different densities, using a very simple spray method. We measure the electric impedance at different frequencies Z(f) in the frequency range of 40 Hz to 20 GHz using two different methods: a two-probe method in the range up to 110 MHz and a coaxial (Corbino) method in the range of 10 MHz to 20 GHz. We measure the optical absorption and electrical conductivity in order to optimize the conditions for obtaining optimum performance films with both high electrical conductivity and transparency. We observe a square resistance of 1 to 8.5 k[greek capital letter omega] for samples showing 65% to 85% optical transmittance, respectively. For some applications, we need flexibility and not transparency: for this purpose, we deposit a thick film of single-walled carbon nanotubes on a flexible silicone substrate by spray method from an aqueous suspension of carbon nanotubes in a surfactant (sodium dodecyl sulphate), thereby obtaining a flexible conducting electrode showing an electrical resistance as low as 200 [greek capital letter omega]/sq. When stretching up to 10% and 20%, the electrical resistance increases slightly, recovering the initial value for small elongations up to 10%. We analyze the stretched and unstretched samples by Raman spectroscopy and observe that the breathing mode on the Raman spectra is highly sensitive to stretching. The high-energy Raman modes do not change, which indicates that no defects are introduced when stretching. Using this method, flexible conducting films that may be transparent are obtained just by employing a very simple spray method and can be deposited on any type or shape of surface.Postprint (published version

Transparent, flexible electrodes and sensors based on carbon nanotube thin films

Hem obtingut capes primes de Nanotubs de Carboni d'una sola paret (CNT) sobre un substrat amb un mètode molt simple, que poden ser emprades com elèctrodes flexibles i transparents en dispositius electrònics. Per tal d'obtenir dispositius reproduïbles amb propietats similars, en particular amb similar impedància Z(ω), és important relacionar les propietats elèctriques amb la quantitat de CNTs presents en la capa. Per això, hem realitzat capes primes de CNTs sobre substrats flexibles i transparents (PPC, policarbonat de propilè) amb diferents densitats de CNT. A partir d'un mètode iteratiu matemàtic i de l'Anàlisi Tèrmogravimètric (TGA) de cada mostra, hem pogut determinar la quantitat de CNTs presents en cada mostra. També n'hem fet una estimació a partir de l'espectroscòpia d'absorció òptica. Hem vist que els dos mètodes donen resultats coherents. Hem analitzat les diferents mostres mesurant la impedància elèctrica a diferents frequències, fins 110 MHz. Les capes primes amb poca densitat de CNTs són semiconductores, en canvi les denses són metàl·liques, i prou conductores per ser utilitzades com elèctrode de treball en un procés electroquímic. Podem obtenir així composites CNT-polímer conductor o CNT-metall, electroquímicament. Amb l'objectiu de les aplicacions per a sensors, utilitzant les capes primes de CNT com elèctrode de treball hem obtingut composites CNT-polímer conductor, depositant-hi electroquímicament un polímer conductor, polipirrol o polianilina. Hem analitzat les propietats del dispositiu com a sensor electroquímic, observant la seva resposta en funció del pH, mesurant el potencial en circuit obert en funció del pH de la solució, entre 1 i 13. Els resultats mostren una bona sensibilitat, linearitat i estabilitat. Per això, els dispositius CNT/polipirrol i CNT/polianilina poden tenir aplicacions com a sensors o biosensors en estat sòlid, depositats sobre qualsevol superfície de forma arbitrària, que pot ser transparent i flexible.We obtained thin films of single-walled carbon nanotubes (CNTs), which may be used as transparent, flexible electrodes in electronic devices, on a substrate using a very simple method. In order to construct reproducible devices with similar properties, in particular with similar impedance Z(ω), it is important to associate the electrical properties with the number of CNTs in a network. We prepared thin CNT networks on transparent, flexible substrates (PPC, polypropylene carbonate) with different CNT densities. The number of CNTs was estimated using a mathematical method based on the data obtained from thermo-gravimetric analysis (TGA). We were able to estimate the relative number of CNTs using optical absorption spectroscopy. These two methods are in good agreement. We also analysed the various samples using electrical impedance measurements at frequencies of up to 110 MHz. Low-density networks are semiconductors, whilst high-density networks behave like metals and are sufficiently good conductors to be used as working electrodes in electrochemical processes. It is thus possible to obtain CNT polymer and metal composite conductors electrochemically. With sensor applications in mind, we used CNT thin films as a working electrode to obtain a composite CNT-conducting polymer. This was performed by electrochemically depositing a conducting polymer ? polypyrrole or polyaniline ? on the electrode. The pH dependence of the device was measured by recording its open circuit potential in various buffer solutions. This enabled us to analyse the properties of the device as an electrochemical sensor. The results showed a good sensitivity, linearity and stability in both cases. Thus, the CNT/polypyrrole and CNT/polyaniline devices could have applications as solidstate gas sensors or biosensors when they are deposited on transparent and flexible surfaces of any shape

Optical nanoantenna optimization for local field enhancement at visible wavelengths

Projecte realitzat en col.laboració amb l'Institut de Ciències FotòniquesDipole nanoantennas are studied by means of numerical calculations, over a broad frequency range (250-600 THz), in order to evaluate the benefits of its integration in NSOM microscopy systems. Operational wavelength, radius and material were systematically varied to study the antenna’s resonant length, the field confinement, enhancement and decay. To perform more accurate calculations we implemented an algorithm to include real metal dispersion, at optical wavelengths, in the simulation. The simulations’ results suggest that the antenna’s performance depends strongly on the material’s dielectric properties; even aspects that we would relate only with the antenna geometry like field confinement. By comparing gold and aluminum antennas we found a crossover wavelength for the antenna performance. Below this wavelength aluminum is a more suitable material while for larger wavelengths gold gives better results

Measuring the tolerance of self-adaptive clocks to supply voltage noise

Simultaneous switching noise has become an important issue due to its signal integrity and timing implications. Therefore a lot of time and resources are spent during the PDN design to minimize the supply voltage variation. This paper presents the self-adaptive clock as an alternative to tolerate the critical path delay variation due to supply noise thanks to its self-adaptable nature. A self-adaptive clock generation circuit is proposed in this paper and its benefits, in terms of clock period reduction, are assessed under a realistic supply noise obtained through simulation for different switching activities.Peer Reviewe