Publications with Fulltext
Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/6
Browse
12 results
Search Results
Publication Open Access Hexagonal boron nitride incorporation to achieve high performance Li4Ti5O12 electrodes(American Institute of Physics (AIP) Publishing, 2020) Department of Electrical and Electronics Engineering; Ergen, Onur; Department of Electrical and Electronics Engineering; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Engineering; Graduate School of Sciences and Engineering; 272106There is an increasing demand for fast charging and high capacity lithium ion batteries. However, conventional Li-ion battery chemistries cannot meet the stringent requirements of these demands due to the poor performance of graphite anodes, especially on safety during fast charging. Finding the right anode material that can replace conventional graphite while providing high capacity is very challenging. Today, lithium titanium oxide (LTO) is considered one of the most attractive anode materials that can provide the desired ultra-fast charging ability (>10C) with high safety. However, it has many serious drawbacks when compared to the existing graphite anodes, including poor intrinsic conductivity, narrow electrochemical window, etc. Extensive research has been done to overcome these problems, especially in developing new LTO composite materials with reduced graphene oxide. However, even these methods have rapid capacity fading at high current densities, >5C, due to increased internal resistance and polarization losses. Here, we demonstrate an effective way to improve LTO composite materials by developing unique nanoengineered three-dimensional frameworks with hexagonal boron nitride (h-BN) addition. Li-ion cells with h-BN incorporation exhibit excellent performance and operational stability, especially at fast and ultra-fast charging rates, >10C.Publication Open Access Morphology-dependent resonances of optical microsphere resonators for the realization of passive wavelength-division multiplexing components(Society of Photo-optical Instrumentation Engineers (SPIE), 2004) Department of Physics; Department of Electrical and Electronics Engineering; İşçi, Şenol; Bilici, Temel; Serpengüzel, Ali; Kurt, Adnan; Faculty Member; Teaching Faculty; Department of Physics; Department of Electrical and Electronics Engineering; College of Sciences; N/A; N/A; 27855; 194455Morphology-dependent resonances of microspheres can provide the necessary optical feedback for applications in spectroscopy, laser science, and optical communications. The elastic scattering of focused light from dielectric microspheres is understood by the localization principle and the generalized Lorenz-Mie theory. We excited the morphology-dependent resonances of glass microspheres by a tunable distributed-feedback laser and detected the elastically scattered signal. Efficient coupling to morphology-dependent resonances is achieved using an optical fiber half coupler. Resonance peaks in the elastic scattering spectra and associated dips in the transmission spectra are observed experimentally. Simulation results of elastic scattering spectra of glass microspheres in the C-band are presented. (C) 2004 Society of Photo-Optical Instrumentation Engineers.Publication Open Access Photonic integrated circuit-assisted optical time-domain reflectometer system(TÜBİTAK, 2022) Department of Electrical and Electronics Engineering; Onbaşlı, Mehmet Cengiz; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; 258783Optical time-domain reflectometers (OTDR) are photonic systems that consist of an interrogator, a receiver and a fiber optical cable and have applications in telecommunications, security, environmental monitoring, distributed temperature and strain sensing. Since OTDR systems are bulk optical setups that consume multiple Watts of power, have large mass and volume footprint and are vulnerable to thermal drift, deployment of OTDR systems in the field is expensive, complicated and may not necessarily yield accurate sensing results. Thus, a compact, low-power, inexpensive and thermal drift-free OTDR system needs to be developed for improving the accuracy and the viability of OTDR in the field. In this study, I present the design and modeling of a photonic integrated OTDR system design based on IMEC's iSiPP50G silicon integrated photonic process design kit. The photonic integrated circuit includes a photonic modulator and a photodetector. Photonic power link budgets and the corresponding electronic signal-to-noise ratios are analyzed for 5-110 km fiber optical OTDR systems and power-efficient OTDR system designs are presented for inexpensive multiproject wafer fabrication.Publication Open Access Orthogonal frequency division multiplexing with power distribution index modulation(Wiley, 2020) Department of Electrical and Electronics Engineering; Doğukan, Ali Tuğberk; Başar, Ertuğrul; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 149116Orthogonal frequency division multiplexing with index modulation (OFDM-IM) is a promising multicarrier transmission technique that exploits both conventional modulation and the indices of active subcarriers for data transmission. In this Letter, the authors propose OFDM with power distribution index modulation (OFDM-PIM) where a unique power distribution technique is applied to index modulated subcarriers. Furthermore, we propose OFDM with in-phase/quadrature PIM (OFDM-I/Q-PIM) which performs the same power distribution process on the real and imaginary parts of the data symbols, independently. Average bit error performances of OFDM-PIM and OFDM-I/Q-PIM are obtained and the superiority of the proposed schemes over reference schemes is demonstrated via extensive computer simulations.Publication Open Access Attenuation constant measurements of clear glass samples at the low terahertz band(Wiley, 2020) Department of Electrical and Electronics Engineering; Yılmaz, Türker; Akan, Özgür Barış; Faculty Member; Department of Electrical and Electronics Engineering; College of EngineeringThe technical performance requirements from wireless communication networks are continuously rising. One method to satisfy the demands is increasing the carrier frequency to the millimetre wave or low terahertz band spectrum to utilise wider operation bandwidth. In order to facilitate the studies in this frequency range, the corresponding electromagnetic (EM) wave properties, channel attributes and material characteristics need to be analytically formulated. In line with these, this Letter initially presents the theoretical expressions governing the EM wave transmission across a conducting medium. Then, by using the relative S21 parameter quantities in the proposed attenuation constant (alpha) computation technique, the alpha results of the measurements performed between 260 and 350 GHz for the clear window glass samples of different thicknesses are given. This Letter concludes with the evaluation of the outcomes.Publication Open Access Thin film rare earth iron garnets with perpendicular magnetic anisotropy for spintronic applications(American Institute of Physics (AIP) Publishing, 2019) N/A; Department of Electrical and Electronics Engineering; Zanjani, Saeedeh Mokarian; Onbaşlı, Mehmet Cengiz; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 258783Perpendicular magnetic anisotropy (PMA) in garnet thin films is important for achieving numerous spintronic applications including spin-orbit switching. In this study, we computationally investigated how to control PMA by tuning substrate strain in Holmium Iron Garnet (HoIG) films grown on five different (111) single crystal garnet substrates of Gadolinium Gallium Garnet (GGG, Gd3Ga5O12), Yttrium Aluminum Garnet (YAG, Y3Al5O12), Terbium Gallium Garnet (TGG, Tb3Ga5O12), Substituted Gadolinium Gallium Garnet (sGGG, Gd3Sc2Ga3O12), and Neodymium Gallium Garnet (NGG, Nd3Ga5O12). The negative sign of effective anisotropy energy density, K-eff < 0, and anisotropy field, H-a < 0, determines the easy magnetization axis of the film to be perpendicular to the film surface. Here, we show that magnetoelastic anisotropy energy density determines the sign of the total anisotropy and it can be manipulated by altering the lattice parameter mismatch of the film and its substrate. Based on this study, HoIG is predicted to have PMA when grown on GGG, TGG and YAG among all five substrates mentioned. Moreover, the saturation field magnitude is calculated as an order of several hundreds of Oersteds, which is feasible in practical applications to saturate rare earth iron garnets with perpendicular magnetic anisotropy.Publication Open Access Evidence for ferromagnetic coupling at the doped topological insulator/ferrimagnetic insulator interface(American Institute of Physics (AIP) Publishing, 2016) Liu, Wenqing; He, Liang; Zhou, Yan; Murata, Koichi; Ross, Caroline A.; Jiang, Ying; Wang, Yong; Xu, Yongbing; Zhang, Rong; Wang, Kang. L.; Department of Electrical and Electronics Engineering; Onbaşlı, Mehmet Cengiz; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; 258783One of the major obstacles of the magnetic topological insulators (TIs) impeding their practical use is the low Curie temperature (T-c). Very recently, we have demonstrated the enhancement of the magnetic ordering in Cr-doped Bi2Se3 by means of proximity to the high-T-c ferrimagnetic insulator (FMI) Y3Fe5O12 and found a large and rapidly decreasing penetration depth of the proximity effect, suggestive of a different carrier propagation process near the TI surface. Here we further present a study of the interfacial magnetic interaction of this TI/FMI heterostrucutre. The synchrotron-based X-ray magnetic circular dichroism (XMCD) technique was used to probe the nature of the exchange coupling of the Bi2-xCrxSe3/Y3Fe5O12 interface. We found that the Bi2-xCrxSe3 grown on Y3Fe5O12(111) predominately contains Cr3+ cations, and the spin direction of the Cr3+ is aligned parallel to that of tetrahedral Fe3+ of the YIG, revealing a ferromagnetic exchange coupling between the Bi2-xCrxSe3 and the Y3Fe5O12. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.Publication Open Access Analysis and optimization of diode end-pumped solid-state lasers: applicationto Nd3+ : YVO4 lasers at 1064 and 1342 nm(Society of Photo-optical Instrumentation Engineers (SPIE), 2005) Buhours, S.; Department of Physics; Department of Electrical and Electronics Engineering; Sennaroğlu, Alphan; Kurt, Adnan; Faculty Member; Teaching Faculty; Department of Physics; Department of Electrical and Electronics Engineering; College of Sciences; 23851; 194455We describe a systematic procedure that uses experimental and numerical methods to analyze the continuous-wave power performance of diode end-pumped solid-state lasers. For the general case, saturation, excited-state absorption, and thermal lensing effects are considered and integral equations are derived to study the evolution of the pump and laser beams in the gain medium. As an application of the method, we consider two different diode end-pumped Nd3+:YVO4 lasers operating at 1064 and 1342 nm. Experimental efficiency data were first analyzed to determine the stimulated emission cross sections and the resonator losses. The best-fit laser parameters were then used to calculate the optimum crystal length that maximizes the output power of the laser. The described method should prove useful in the design of a wide range of efficient diode-pumped solid-state lasers.Publication Open Access Fuzzy framework for unsupervised video content characterization and shot classification(Society of Photo-optical Instrumentation Engineers (SPIE), 2001) Ferman, A. Müfit; Department of Electrical and Electronics Engineering; Tekalp, Ahmet Murat; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; 26207In this paper we present a fuzzy framework for domain-dependent analysis of video sequences. Fuzzy clustering and cluster validation methods are first employed to determine the number of distinct shot patterns and construct a reference model for a program or video domain of interest, using an appropriate training set. This model is subsequently utilized to assign new input data to the available classes by a fuzzy minimum-distance classifier. Additional domain-specific information can be introduced after classification to further enhance the annotations associated with every shot. The main advantage of the approach is that it builds a model for the input video automatically from training data, and thus eliminates the need for extensive user supervision. The fuzzy representation method improves the interpretability of the results, and reduces the number of erroneous classifications, since the continuous class affiliations of each input sample provide a confidence measure for the final assignments. The proposed approach presents a computationally efficient, unsupervised method for building browsable semantic descriptions of video sequences. Specifically, the algorithm can be used to generate various components of an MPEG-7-compliant description. © 2001 SPIE and IS&T.Publication Open Access Inter-numerology interference in OFDM-IM systems(Wiley, 2021) Doğan Tusha, Seda; Tusha, Armed; Althunibat, Saud; Qaraqe, Khalid; Arslan, Hüseyin; Department of Electrical and Electronics Engineering; Başar, Ertuğrul; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; 149116In 5G and beyond communication systems, distinct numerologies can coexist to serve diverse requirements for users and applications. However, the inter-numerology interference (INI) is a main challenge that significantly impacts the system performance. Therefore, the performance under INI has become an essential evaluation metric for the suitability of the different transmission schemes in the future communication systems. This paper analyzes the impact of INI on the performance of orthogonal frequency division multiplexing with index modulation (OFDM-IM) systems. Specifically, an analytical expression of the INI level in OFDM-IM systems is presented as a function of the subcarrier activation ratio (SAR) and subcarrier activation probability (SAP). Furthermore, aiming at reducing the INI level, an adaptive subcarrier mapping scheme (SMS) is proposed based on the conventional combinatorial mapping scheme. Moreover, analysis and evaluation of SAR and SAP are performed regarding the requirements of 5G and beyond services. It is proved that the INI level in OFDM-IM systems is highly dependent not only on the number of active subcarriers but also on their position in an OFDM block.