Browsing by Subject "Tuning"
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Item Open Access Band-dropping via coupled photonic crystal waveguides(Optical Society of American (OSA), 2002) Bayındır, Mehmet; Özbay, EkmelWe observe the dropping of electromagnetic waves having a specific frequency or a certain frequency band in two-dimensional dielectric photonic crystals. The single frequency is dropped via cavity-waveguide coupling. Tunability of the demultiplexing mode can be achieved by modifying the cavity properties. The band-dropping phenomenon is achieved by introducing interaction between an input planar, or coupled-cavity, waveguide and the output coupled-cavity waveguides (CCWs). The dropping band can be tuned by changing the coupling strength between the localized cavity modes of the output CCWs. We also calculate the transmission spectra and the field patterns by using the finite-difference-time-domain (FDTD) method. Calculated results agree well with the microwave measurements. © 2002 Optical Society of America.Item Open Access CdSe/CdSe1-xTex core/crown heteronanoplatelets: tuning the excitonic properties without changing the thickness(American Chemical Society, 2017) Kelestemur Y.; Guzelturk, B.; Erdem, O.; Olutas M.; Erdem, T.; Usanmaz, C. F.; Gungor K.; Demir, Hilmi VolkanHere we designed and synthesized CdSe/CdSe1-xTex core/crown nanoplatelets (NPLs) with controlled crown compositions by using the core-seeded-growth approach. We confirmed the uniform growth of the crown regions with well-defined shape and compositions by employing transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. By precisely tuning the composition of the CdSe1-xTex crown region from pure CdTe (x = 1.00) to almost pure CdSe doped with several Te atoms (x = 0.02), we achieved tunable excitonic properties without changing the thickness of the NPLs and demonstrated the evolution of type-II electronic structure. Upon increasing the Te concentration in the crown region, we obtained continuously tunable photoluminescence peaks within the range of ∼570 nm (for CdSe1-xTex crown with x = 0.02) and ∼660 nm (for CdSe1-xTex crown with x = 1.00). Furthermore, with the formation of the CdSe1-xTex crown region, we observed substantially improved photoluminescence quantum yields (up to ∼95%) owing to the suppression of nonradiative hole trap sites. Also, we found significantly increased fluorescence lifetimes from ∼49 up to ∼326 ns with increasing Te content in the crown, suggesting the transition from quasi-type-II to type-II electronic structure. With their tunable excitonic properties, this novel material presented here will find ubiquitous use in various efficient light-emitting and -harvesting applications.Item Open Access Continuously tunable terahertz metamaterial employing magnetically actuated cantilevers(Optical Society of American (OSA), 2011) Ozbey, B.; Aktas O.Terahertz metamaterial structures that employ flexing microelectromechanical cantilevers for tuning the resonance frequency of an electric split-ring resonator are presented. The tuning cantilevers are coated with a magnetic thin-film and are actuated by an external magnetic field. The use of cantilevers enables continuous tuning of the resonance frequency over a large frequency range. The use of an externally applied magnetic field for actuation simplifies the metamaterial structure and its use for sensor or filter applications. A structure for minimizing the actuating field is derived. The dependence of the tunable bandwidth on frequency is discussed. © 2011 Optical Society of America.Item Open Access Dynamic tuning of plasmon resonance in the visible using graphene(The Optical Society, 2016) Balci, S.; Balci, O.; Kakenov, N.; Atar, F. B.; Kocabas, C.We report active electrical tuning of plasmon resonance of silver nanoprisms (Ag NPs) in the visible spectrum. Ag NPs are placed in close proximity to graphene which leads to additional tunable loss for the plasmon resonance. The ionic gating of graphene modifies its Fermi level from 0.2 to 1 eV, which then affects the absorption of graphene due to Pauli blocking. Plasmon resonance frequency and linewidth of Ag NPs can be reversibly shifted by 20 and 35 meV, respectively. The coupled graphene-Ag NPs system can be classically described by a damped harmonic oscillator model. Atomic layer deposition allows for controlling the graphene-Ag NP separation with atomic-level precision to optimize coupling between them.Item Open Access Implementation of high quality-factor on-chip tuned microwave resonators at 7GHz(WILEY, 2009) Melik, R.; Demir, Hilmi VolkanWe report on the design, analytical modeling, numerical simulation, fabrication, and experimental characterization of chip-scale microwave resonators that exhibit high quality-factors (Q-factors) in the microwave frequency range. We demonstrate high Q-factors by tuning these microwave resonators with the film capacitance of their LC tank circuits rather than the conventional approach of using external capacitors for tuning. Our chip-scale resonator design further minimizes energy losses and reduces the effect of skin depth leading to high Q-factors even for significantly reduced device areas. Using our new design methodology, we observe that despite the higher resonance frequency and smaller chip size, the Q-factor is improved compared with the previous literature using traditional approaches. For our 540 m 540 m resonator chip, we theoretically compute a Q-factor of 52.40 at the calculated resonance frequency of 6.70 GHz and experimentally demonstrate a Q-factor of 47.10 at the measured resonance frequency of 6.97 GHz. We thus achieve optimal design for microwave resonators with the highest Q-factor in the smallest space for operation at 6.97 GHz.Item Open Access A local search heuristic with self-tuning parameter for permutation flow-shop scheduling problem(IEEE, 2009) Dengiz, B.; Alabaş-Uslu, Ç.; Sabuncuoğlu, İhsanIn this paper, a new local search metaheuristic is proposed for the permutation flow-shop scheduling problem. In general, metaheuristics are widely used to solve this problem due to its NP-completeness. Although these heuristics are quite effective to solve the problem, they suffer from the need to optimize parameters. The proposed heuristic, named STLS, has a single self-tuning parameter which is calculated and updated dynamically based on both the response surface information of the problem field and the performance measure of the method throughout the search process. Especially, application simplicity of the algorithm is attractive for the users. Results of the experimental study show that STLS generates high quality solutions and outperforms the basic tabu search, simulated annealing, and record-to-record travel algorithms which are well-known local search based metaheuristics.Item Open Access Low-power thermooptical tuning of SOI resonator switch(IEEE, 2006) Kiyat, I.; Aydınlı, Atilla; Dagli, N.A wavelength selective optical switch is developed based on a high-Q racetrack resonator making use of the large thermooptic coefficient of silicon. The racetrack resonator was fabricated using a silicon-on-insulator (SOI) single-mode rib waveguide. The resonator shows a high Q factor of 38 000 with spectral sidelobes of 11 dB down and can be thermooptically scanned over its full free-spectral range applying only 57 mW of electrical power. A low power of 17 mW is enough to tune the device from resonance to off-resonance state. The device functions as a wavelength selective optical switch with a 3-dB cutoff frequency of 210 kHz.Item Open Access Tuning macroscopic sliding friction at soft contact interfaces: interaction of bulk and surface heterogeneities(Elsevier Ltd, 2016) Kılıç, K. İ.; Temizer, İ.Macroscopic frictional response of soft interfaces is strongly governed by the interaction of surface heterogeneities such as micro-texture features with bulk heterogeneities such as voids or inclusions beneath the highly deformable surface. This microscopic interaction manifests itself on the macroscale as an interface response that is reminiscent of stick-slip. Consequently, the accompanying macroscopic friction signal exhibits strong oscillations around a mean value, which itself significantly differs from its microscopic value due to finite deformations. In this work, a mechanism is proposed which enables the tuning of the macroscopic friction signal of soft interfaces. Specifically, it is demonstrated that optimally positioning subsurface particles in the vicinity of micro-texture features can significantly reduce observed oscillations, thereby allowing control of macroscopic sliding friction. © 2016 Elsevier LtdItem Open Access Ultrathin phase-change coatings on metals for electrothermally tunable colors(American Institute of Physics Inc., 2016-08) Bakan, G.; Ayas S.; Saidzoda, T.; Celebi, K.; Dana, A.Metal surfaces coated with ultrathin lossy dielectrics enable color generation through strong interferences in the visible spectrum. Using a phase-change thin film as the coating layer offers tuning the generated color by crystallization or re-amorphization. Here, we study the optical response of surfaces consisting of thin (5-40 nm) phase-changing Ge2Sb2Te5 (GST) films on metal, primarily Al, layers. A color scale ranging from yellow to red to blue that is obtained using different thicknesses of as-deposited amorphous GST layers turns dim gray upon annealing-induced crystallization of the GST. Moreover, when a relatively thick (>100 nm) and lossless dielectric film is introduced between the GST and Al layers, optical cavity modes are observed, offering a rich color gamut at the expense of the angle independent optical response. Finally, a color pixel structure is proposed for ultrahigh resolution (pixel size: 5 × 5 μm2), non-volatile displays, where the metal layer acting like a mirror is used as a heater element. The electrothermal simulations of such a pixel structure suggest that crystallization and re-amorphization of the GST layer using electrical pulses are possible for electrothermal color tuning.