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Item Open AccessGeneration of cylindrical vector beams with few-mode fibers excited by Laguerre–Gaussian beams(Elsevier, 2004-07-01) Volpe, Giovanni; Petrov, D.; Volpe, GiovanniWe propose a novel method to efficiently produce light beams with radial, azimuthal, and hybrid polarization, through a few-mode fiber excited by a Laguerre–Gaussian beam. With different input polarization we can selectively excite different combinations of modes from the LP11 group. We propose to show how to transform the output beam into a cylindrical vector beam in free-space through various polarization transformations. Item Open AccessSpectral negative refraction and focusing analysis of a two-dimensional left-handed photonic crystal lens(American Physical Society, 2004-11) Güven, Kaan; Aydın, Koray; Alıcı, Kamil Boratay; Soukoulis, C. M.; Özbay, Ekmel; Güven, Kaan; Aydın, Koray; Alıcı, Kamil Boratay; Özbay, EkmelWe report the spectral refraction analysis and focusing properties of a two-dimensional, dielectric photonic crystal (PC) slab in freespace. A transverse electric polarized upper band of the crystal is used. The measured refraction spectra indicates that a highly isotropic negative index of refraction is present in the measured frequency range of the band. We demonstrate experimentally and numerically the focusing of the field emitted from an omnidirectional source placed in front of the crystal. Both the source and the focus pattern are away from the PC interfaces of the order of several wavelengths. The focus pattern mimics the arbitrary lateral and longitudinal shifts of the source, which is a manifestation of true flat lens behavior. Item Open AccessNegative refraction and subwavelength focusing using photonic crystals(SPIE, 2005-01) Özbay, Ekmel; Aydın, Koray; Alıcı, Kamil Boratay; Güven, Kaan; Özbay, Ekmel; Aydın, Koray; Güven, Kaan; Alıcı, Kamil BoratayWe review certain novel optical properties of two-dimensional dielectric photonic crystals (PCs) which exhibit negative refraction behavior. We investigate two mechanisms which utilize the band structure of the PC and lead to a negative effective index of refraction (neff < 0). The negative refraction phenomenon is demonstrated experimentally and by simulations when the incident beam couples to a photonic band with neff < 0. Further, the PC slab acts like a focusing lens to an omnidirectional source where the properties of focusing depends on the details of the band structure. In one case, by utilizing the TM polarized first band, an image of the source can be formed in the vicinity of the interface with subwavelength resolution. In another case, a TE polarized upper band is used which is able to focus the omnidirectional field far away from the interface with a resolution on par with the wavelength. In the latter case, we explicitly show the flat lens behavior of the structure. These examples indicate that PC based lenses can surpass limitations of conventional lenses and greatly enhance and extend optics applications. Item Open AccessExperimental analysis of true left-handed behaviour and transmission properties of composite metamaterials(Elsevier, 2005-12) Güven, Kaan; Aydın, Koray; Özbay, Ekmel; Güven, Kaan; Aydın, Koray; Özbay, EkmelWe report the true left-handed transmission of a composite metamaterial (CMM) consisting of periodically stacked split-ring resonator (SRR) and wire elements. The negative permeability (μ < 0) gap is demonstrated explicitly by comparing SRR and closed-ring resonator structures. We confirm experimentally that the plasma cut-off frequency of the CMM is determined by the combined dielectric response of SRR and wire elements, and it is much lower than that of the wire-only medium. This is crucial to identify the left-handed transmission bands of the CMM. We further investigate the effect of intralayer and interlayer disorder on the transmission spectrum of CMM arising from misaligned fabrication and stacking of the SRR layers. We found that the intralayer disorder affects the μ < 0 gap of SRRs and the left-handed transmission band of CMM significantly, whereas the SRR transmission is rather immune to interlayer disorder. Item Open AccessSurface plasmon radiation forces(The American Physical Society, 2006-06) Volpe, Giovanni; Quidant, R.; Badenes, G.; Petrov, D.; Volpe, GiovanniWe report the first experimental observation of momentum transfer from a surface plasmon to a single dielectric sphere. Using a photonic force microscope, we measure the plasmon radiation forces on different polystyrene beads as a function of their distance from the metal surface. We show that the force magnitude at resonance is strongly enhanced compared to a nonresonant illumination. Measurements performed as a function of the probe particle size indicate that optical manipulation by plasmon fields has a strong potential for optical sorting. Item Open AccessTorque detection using brownian fluctuations(The American Physical Society, 2006-11-24) Volpe, Giovanni; Petrov, D.; Volpe, GiovanniWe report the statistical analysis of the movement of a submicron particle confined in a harmonic potential in the presence of a torque. The absolute value of the torque can be found from the auto- and cross-correlation functions of the particle’s coordinates. We experimentally prove this analysis by detecting the torque produced onto an optically trapped particle by an optical beam with orbital angular momentum. Item Open AccessPrinted multilayer superstructures of aligned single-walled carbon nanotubes for electronic applications(American Chemical Society, 2007-10) Kang, S. J.; Kocabaş, Coşkun; Kim, H.-S.; Cao, Q.; Meitl, M. A.; Khang, D.-Y.; Rogers, J. A.; Kocabaş, CoşkunWe developed means to form multilayer superstructures of large collections of single-walled carbon nanotubes (SWNTs) configured in horizontally aligned arrays, random networks, and complex geometries of arrays and networks on a wide range of substrates. The approach involves guided growth of SWNTs on crystalline and amorphous substrates followed by sequential, multiple step transfer of the resulting collections of tubes to target substrates, such as high-k thin dielectrics on silicon wafers, transparent plates of glass, cylindrical tubes and other curved surfaces, and thin, flexible sheets of plastic. Electrical measurements on dense, bilayer superstructures, including crossbars, random networks, and aligned arrays on networks of SWNTs reveal some important characteristics of representative systems. These and other layouts of SWNTs might find applications not only in electronics but also in areas such as optoelectronics, sensors, nanomechanical systems, and microfluidics. Item Open AccessMolecular scale buckling mechanics in individual aligned single-wall carbon nanotubes on elastomeric substrates(American Chemical Society, 2008) Khang, D. -Y.; Xiao, J.; Kocabaş, Coşkun; MacLaren, S.; Banks, T.; Jiang, H.; Huang, Y. Y.; Rogers, J. A.; Kocabaş, CoşkunWe have studied the scaling of controlled nonlinear buckling processes in materials with dimensions in the molecular range (i.e., ∼1 nm) through experimental and theoretical studies of buckling in individual single-wall carbon nanotubes on substrates of poly(dimethylsiloxane). The results show not only the ability to create and manipulate patterns of buckling at these molecular scales, but also, that analytical continuum mechanics theory can explain, quantitatively, all measurable aspects of this system. Inverse calculation applied to measurements of diameterdependent buckling wavelengths yields accurate values of the Young’s moduli of individual SWNTs. As an example of the value of this system beyond its use in this type of molecular scale metrology, we implement parallel arrays of buckled SWNTs as a class of mechanically stretchable conductor. Item Open AccessComparison of electron and hole charge-discharge dynamics in germanium nanocrystal flash memories(AIP Publishing, 2008-02) Akça, İmran B.; Dâna, Aykutlu; Aydınlı, Atilla; Turan, R.; Akça, İmran B.; Dâna, Aykutlu; Aydınlı, AtillaElectron and hole charge and discharge dynamics are studied on plasma enhanced chemical vapor deposition grown metal-oxide-silicon germanium nanocrystal flash memory devices. Electron and hole charge and discharge currents are observed to differ significantly and depend on annealing conditions chosen for the formation of nanocrystals. At low annealing temperatures, holes are seen to charge slower but to escape faster than electrons. They discharge slower than electrons when annealing temperatures are raised. The results suggest that discharge currents are dominated by the interface layer acting as a quantum well for holes and by direct tunneling for elec-trons. Item Open AccessElectro-optic and electro-absorption characterization of InAs quantum dot waveguides(Optical Society of America, 2008-03) Akça, İmran B.; Dana, Aykutlu; Aydınlı, Atilla; Rossetti, M.; Li, L.; Fiore, A.; Dağlı, N.; Akca, İmran B.; Dana, Aykutlu; Aydınlı, AtillaAbstract Optical properties of multilayer InAs quantum dot waveguides, grown by molecular beam epitaxy, have been studied under applied electric field. Fabry-Perot measurements at 1515 nm on InAs/GaAs quantum dot structures yield a significantly enhanced linear electro-optic efficiency compared to bulk GaAs. Electro-absorption measurements at 1300 nm showed increased absorption with applied field accompanied with red shift of the spectra. Spectral shifts of up to 21% under 18 Volt bias was observed at 1320 nm. (C) 2008 Optical Society of America. Item Open AccessStable ℋ∞ controller design for time-delay systems(Taylor & Francis, 2008-04) Gümüşsoy, S.; Özbay, Hitay; Özbay, HitayThis paper investigates stable suboptimal [image omitted] controllers for a class of single-input single-output time-delay systems. For a given plant and weighting functions, the optimal controller minimizing the mixed sensitivity (and the central suboptimal controller) may be unstable with finitely or infinitely many poles in +. For each of these cases, search algorithms are proposed to find stable suboptimal [image omitted] controllers. These design methods are illustrated with examples. Item Open AccessSpectroscopic ellipsometric study of Ge nanocrystals embedded in SiO 2 using parametric models(Wiley, 2008-05) Basa, P.; Petrik, P.; Fried, M.; Dâna, Aykutlu; Aydınlı, Atilla; Foss, S.; Finstad, T. G.; Dâna, Aykutlu; Aydınlı, AtillaGe-rich SiO2 layers on top of Si substrates were deposited using plasma enhanced chemical vapour deposition. Ge nanocrystals embedded in the SiO2 layers were formed by high temperature annealing. The samples were measured and evaluated by spectroscopic ellipsometry. Effective medium theory (EMT) and parametric semiconductor models have been used to model the dielectric function of the layers. Systematic dependences of the layer thickness and the oscillator parameters have been found on the annealing temperature (nanocrystal size). Item Open AccessA 500 MHz carbon nanotube transistor oscillator(American Institute of Physics, 2008-09) Pesetski, A. A.; Baumgardner, J. E.; Krishnaswamy, S. V.; Zhang, H.; Adam, J. D.; Kocabaş, Coşkun; Banks, T.; Rogers, J. A.; Kocabaş, CoşkunOperation of a carbon nanotube field effect transistor (FET) oscillator at a record frequency of 500 MHz is described. The FET was fabricated using a large parallel array of single-walled nanotubes grown by chemical vapor deposition on ST-quartz substrates. Matching of the gate capacitance with a series inductor enabled greater than unity net oscillator loop gain to be achieved at 500 MHz. Item Open AccessSlowing down surface plasmons on a Moiré surface(The American Physical Society, 2009-02) Kocabaş, Aşkın; Şenlik, S. Seçkin; Aydınlı, Atilla; Kocabaş, Aşkın; Şenlik, S. Seçkin; Aydınlı, AtillaWe have demonstrated slow propagation of surface plasmons on metallic Moiré surfaces. The phase shift at the node of the Moiré surface localizes the propagating surface plasmons and adjacent nodes form weakly coupled plasmonic cavities. Group velocities around vg=0.44c at the center of the coupled cavity band and almost a zero group velocity at the band edges are observed. A tight binding model is used to understand the coupling behavior. Furthermore, the sinusoidally modified amplitude about the node suppresses the radiation losses and reveals a relatively high quality factor (Q=103). Item Open AccessHigh-frequency performance of submicrometer transistors that use aligned arrays of single-walled carbon nanotubes(American Chemical Society, 2009-04-08) Kocabaş, Coşkun; Dunham, S.; Cao, Q.; Cimino, K.; Ho, X.; Kim, H.-S.; Dawson, D.; Payne, J.; Stuenkel, M.; Zhang, H.; Banks, T.; Feng, M.; Rotkin, S. V.; Rogers, J. A.; Kocabaş, CoşkunThe unique electronic properties of single-walled carbon nanotubes (SWNTs) make them promising candidates for next generation electronics, particularly in systems that demand high frequency (e.g., radio frequency, RF) operation. Transistors that incorporate perfectly aligned, parallel arrays of SWNTs avoid the practical limitations of devices that use individual tubes, and they also enable comprehensive experimental and theoretical evaluation of the intrinsic properties. Thus, devices consisting of arrays represent a practical route to use of SWNTs for RF devices and circuits. The results presented here reveal many aspects of device operation in such array layouts, including full compatibility with conventional small signal models of RF response. Submicrometer channel length devices show unity current gain (ft) and unity power gain frequencies (fmax) as high as ∼5 and ∼9 GHz, respectively, with measured scattering parameters (S-parameters) that agree quantitatively with calculation. The small signal models of the devices provide the essential intrinsic parameters: saturation velocities of 1.2 × 107 cm/s and intrinsic values of ft of ∼30 GHz for a gate length of 700 nm, increasing with decreasing length. The results provide clear insights into the challenges and opportunities of SWNT arrays for applications in RF electronics. Item Open AccessQuantitative assessment of non-conservative radiation forces in an optical trap(Institute of Physics Publishing, 2009-05) Pesce, G.; Volpe, Giovanni; De Luca, A. C.; Rusciano, G.; Volpe, G.; Volpe, GiovanniThe forces acting on an optically trapped particle are usually assumed to be conservative. However, the presence of a non-conservative component has recently been demonstrated. Here, we propose a technique that permits one to quantify the contribution of such a non-conservative component. This is an extension of a standard calibration technique for optical tweezers and, therefore, can easily become a standard test to verify the conservative optical force assumption. Using this technique, we have analyzed optically trapped particles of different size under different trapping conditions. We conclude that the non-conservative effects are effectively negligible and do not affect the standard calibration procedure, unless for extremely low-power trapping, far away from the trapping regimes usually used in experiments. Item Open AccessTunable surface plasmon resonance on an elastomeric substrate(Optical Society of American (OSA), 2009-05) Olcum, Selim; Kocabaş, Aşkın; Ertaş, Gülay; Atalar, Abdullah; Aydınlı, Atilla; Olcum, Selim; Kocabaş, Aşkın; Ertaş, Gülay; Atalar, Abdullah; Aydınlı, AtillaIn this study, we demonstrate that periods of metallic gratings on elastomeric substrates can be tuned with external strain and hence are found to control the resonance condition of surface plasmon polaritons. We have excited the plasmon resonance on the elastomeric grating coated with gold and silver. The grating period is increased up to 25% by applying an external mechanical strain. The tunability of the elastomeric substrate provides the opportunity to use such gratings as efficient surface enhanced Raman spectroscopy substrates. It's been demonstrated that the Raman signal can be maximized by applying an external mechanical strain to the elastomeric grating. © 2009 Optical Society of America. Item Open AccessGrating based plasmonic band gap cavities(Optical Society of American (OSA), 2009-08) Şenlik, S. Seçkin; Kocabaş, Aşkın; Aydınlı, Atilla; Şenlik, S. Seçkin; Kocabaş, Aşkın; Aydınlı, AtillaWe report on a comparative study of grating based plasmonic band gap cavities. Numerically, we calculate the quality factors of the cavities based on three types of grating surfaces; uniform, biharmonic and Moiré surfaces. We show that for biharmonic band gap cavities, the radiation loss can be suppressed by removing the additional grating component in the cavity region. Due to the gradual change of the surface profile in the cavity region, Moiré type surfaces support cavity modes with higher quality factors. Experimentally, we demonstrate the existence of plasmonic cavities based on uniform gratings. Effective index perturbation and cavity geometries are obtained by additional dielectric loading. Quality factor of 85 is obtained from the measured band structure of the cavity. © 2009 Optical Society of America. Item Open AccessAlignment controlled growth of single-walled carbon nanotubes on quartz substrates(American Chemical Society, 2009-09-11) Xiao, J.; Dunham, S.; Liu, P.; Zhang, Y.; Kocabaş, Coşkun; Moh, L.; Huang, Y.; Hwang, K. -C.; Lu, C.; Huang, W.; Rogers, J. A.; Kocabaş, CoşkunSingle-walled carbon nanotubes (SWNTs) possess extraordinary electrical properties, with many possible applications in electronics. Dense, horizonally aligned arrays of linearly configured SWNTs represent perhaps the most attractive and scalable way to implement this class of nanomaterial in practical systems. Recent work shows that templated growth of tubes on certain crystalline substrates yields arrays with the necessary levels of perfection, as demonstrated by the formation of devices and full systems on quartz. This paper examines advanced implementations of this process on crystalline quartz substrates with different orientations, to yield strategies for forming diverse, but welldefined horizontal configurations of SWNTs. Combined experimental and theoretical studies indicate that angle-dependent van der Waals interactions can account for nearly all aspects of alignment on quartz with X, Y, Z, and ST cuts, as well as quartz with disordered surface layers. These findings provide important insights into methods for guided growth of SWNTs, and possibly other classes of nanomaterials, for applications in electronics, sensing, photodetection, light emission, and other areas. Item Open AccessNovel perspectives for the application of total internal reflection microscopy(Optical Society of America, 2009-12) Volpe, Giovanni; Brettschneider, T.; Helden, L.; Bechinger, C.; Volpe, GiovanniTotal Internal Reflection Microscopy (TIRM) is a sensitive non-invasive technique to measure the interaction potentials between a colloidal particle and a wall with femtonewton resolution. The equilibrium distribution of the particle-wall separation distance z is sampled monitoring the intensity I scattered by the Brownian particle under evanescent illumination. Central to the data analysis is the knowledge of the relation between I and the corresponding z, which typically must be known a priori. This poses considerable constraints to the experimental conditions where TIRM can be applied (short penetration depth of the evanescent wave, transparent surfaces). Here, we introduce a method to experimentally determine I(z) by relying only on the distance-dependent particle-wall hydrodynamic interactions. We demonstrate that this method largely extends the range of conditions accessible with TIRM, and even allows measurements on highly reflecting gold surfaces where multiple reflections lead to a complex I(z).