Browsing by Author "Koc, A."

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Open Access
Comparative study of electroabsorption in InGaN/GaN quantum zigzag heterostructures with polarization-induced electric fields
(AIP Publishing LLC, 2008-05-19) Sari, E.; Ozel, T.; Koc, A.; Ju, J. W.; Ahn, H.-K.; Lee, I.-H.; Baek, J.-H.; Demir, Hilmi Volkan
We present a comparative study on InGaN/GaN quantum zigzag structures embedded in p-i-n diode architecture that exhibit blue-shifting electroabsorption in the blue when an electric field is externally applied to compensate for the polarization-induced electric field across the wells. With the polarization breaking their symmetry, the same InGaN/GaN quantum structures redshift their absorption edge when the external field is applied in the same direction as the well polarization. Both computationally and experimentally, we investigate the effects of polarization on electroabsorption by varying compositional content and structural parameters and demonstrate that electroabsorption grows stronger with weaker polarization in these multiple quantum well modulators.
Open Access
Digital computation of fractional Fourier and linear canonical transforms and sparse image representation
(IEEE, 2018) Koc, A.; Özaktaş, Haldun M.; Bartan, B.; Gundogdu, E.; Çukur, Tolga
Fast and accurate digital computation of the fractional Fourier transform (FRT) and linear canonical transforms (LCT) are of utmost importance in order to deploy them in real world applications and systems. The algorithms in O(NlogN) to obtain the samples of the transform from the samples of the input function are presented for several different types of FRTs and LCTs, both in 1D and 2D forms. To apply them in image processing we consider the problem of obtaining sparse transform domains for images. Sparse recovery tries to reconstruct images that are sparse in a linear transform domain, from an underdeter- mined measurement set. The success of sparse recovery relies on the knowledge of domains in which compressible representations of the image can be obtained. In this work, we consider two- and three-dimensional images, and investigate the effects of the fractional Fourier (FRT) and linear canonical transforms (LCT) in obtaining sparser transform domains. For 2D images, we investigate direct transforming versus several patching strategies. For the 3D case, we consider biomedical images, and compare several different strategies such as taking 2D slices and optimizing for each slice and direct 3D transforming.
Open Access
Influence of La and Si promoters on the anaerobic heterogeneous catalytic decomposition of ammonium dinitramide (ADN) via alumina supported iridium active sites
(Elsevier, 2022-02-25) Kurt, Merve; Kap, Zeynep; Senol, Sinem; Ercan, Kerem Emre; Sika-Nartey, Abel Tetteh; Kocak, Yusuf; Koc, A.; Esiyok, H.; Caglayan, B. S.; Aksoylu, A. E.; Ozensoy, Emrah
Structural origins of the promotional effects of the La or Si doping of alumina supported Ir catalysts in anaerobic ammonium dinitramide decomposition were investigated. Our findings reveal that Ir/Al2O3 and Ir/La-Al2O3 favorably lower the onset temperature of the ADN decomposition reaction, whereas Si doping boosts the pressure generation during the reaction. Formation of mostly metallic Ir nanoparticles for Ir/Al2O3 and Ir/La-Al2O3 enables the lowering of the activation energy of the reaction. On the other hand, enhancement due to Si promotion is associated to the generation of small oxidic Irnx+ clusters which are strongly interacting with the SiOx-AlOx surface domains of the support material. Fundamental structure-functionality relationships unraveled in the current work may allow design of novel catalytic systems for aerospace monopropellant propulsion systems with higher performance by simultaneous exploitation of Ir active sites with different electronic properties.
Open Access
Risk-averse ambulance redeployment via multi-armed bandits
(IEEE, 2018) Sahin, U.; Yucesoy, V.; Koc, A.; Tekin, Cem
Ambulance redeployment comprises the problem of deploying ambulances to certain locations in order to minimize the arrival times to possible calls and plays a significant role in improving a country's emergency medical services and increasing the number of lives saved during an emergency. In this study, unlike the existing optimization methods in the literature, the problem is cast as a multi-armed bandit problem. Multi-armed bandit problems are a part of sequential online learning methods and utilized in maximizing a gain function (i.e. reward) when the reward distributions are unknown. In this study, in addition to the objective of maximizing rewards, the objective of minimizing the expected variance of rewards is also considered. The effect of risk taken by the system on average arrival times and number of calls responded on time is investigated. Ambulance redeployment is performed by a risk-averse multi-armed bandit algorithm on a data-driven simulator. As a result, it is shown that the algorithm which takes less risk (i.e. that minimizes the variance of response times) responds to more cases on time.
Open Access
Two exact formulations for disassembly line balancing problems with task precedence diagram construction using an AND/OR graph
(Taylor & Francis, 2009) Koc, A.; Sabuncuoglu, I.; Erel, E.
In this paper, the disassembly line balancing problem, which involves determining a line design in which used products are completely disassembled to obtain useable components in a cost-effective manner, is studied. Because of the growing demand for a cleaner environment, this problem has become an important issue in reverse manufacturing. In this study, two exact formulations are developed that utilize an AND/OR Graph (AOG) as the main input to ensure the feasibility of the precedence relations among the tasks. It is also shown that traditional task precedence diagrams can be derived from the AOG of a given product structure. This procedure leads to considerably better solutions of the traditional assembly line balancing problems; it may alter the approach taken by previous researchers in this area.