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Item Open Access1018 nm Yb-doped high-power fiber laser pumped by broadband pump sources around 915 nm with output power above 100 W(OSA - The Optical Society, 2017) Midilli, Y.; Efunbajo, O. B.; Şimşek, B.; Ortaç, B.We demonstrate a 1018 nm ytterbium-doped all-fiber laser pumped by tunable pump sources operating in the broad absorption spectrum around 915 nm. In the experiment, two different pump diodes were tested to pump over a wide spectrum ranging from 904 to 924 nm by altering the cooling temperature of the pump diodes. Across this so-called pump wavelength regime having a 20 nm wavelength span, the amplified stimulated emission (ASE) suppression of the resulting laser was generally around 35 dB, showing good suppression ratio. Comparisons to the conventional 976 nm-pumped 1018 nm ytterbium-doped fiber laser were also addressed in this study. Finally, we have tested this system for high power experimentation and obtained 67% maximum optical-to-optical efficiency at an approximately 110 W output power level. To the best of our knowledge, this is the first 1018 nm ytterbium-doped all-fiber laser pumped by tunable pump sources around 915 nm reported in detail. Item Open Access[2+2] cycloadditions of sorbyl tosylate with imines/1-azadienes: a one-pot domino approach for α-alkylidene-β-lactams and their computational studies and antimicrobial evaluation(Wiley-Blackwell, 2018) Kumar, Y.; Bedi, P. M. S.; Singh, P.; Adeniyi, A. A.; Singh-Pillay, A.; Singh, P.; Bhargava, G.The manuscript describes a straightforward and atom-efficient method for the synthesis of α-alkylidene-β-lactams using sorbyl tosylate and imines/1-azadienes at high temperature (80 °C). The Density functional theory calculations have shown the prevalence of the first order kinetics in these [2+2] cycloadditions to produce mixture of 3-butadienyl-azetidin-2ones and 3-but-2-enylidene-azetidin-2-ones in good yields. The 3-but-2-enylidene-azetidin-2-ones have also shown antimicrobial activity against the E. coli, S. aureus, P. aeruginosa, B. cereus and B. subtilis. Item Open Access21.2 mV/K high-performance Ni(50 nm)-Au(100 nm)/Ga2O3/p-Si vertical MOS type diode and the temperature sensing characteristics with a novel drive mode(Institute of Electrical and Electronics Engineers, 2022-11-09) Çiçek, O.; Arslan, E.; Altındal, Ş.; Badali, Y.; Özbay, Ekmel; Özbay, EkmelSensitivity ( S ) and drive mode are crucial issues for the vertical metal-oxide-semiconductor (MOS) type diode applied in temperature sensing. In this study, experimentally, we indicated that the S values of the Ni(50 nm) - Au(100 nm) /Ga2O3/ p -Si vertical MOS type diode, using the measured capacitance–voltage ( Cm – V ) outputs, are obtained with a novel drive mode. We applied the constant capacitance mode to drive the silicon thermo-diodes as well as constant current mode, and constant voltage mode, which are known as two different methods in the literature. Meanwhile, the S value is 21.2 mV/K at 1 nF. This value is the highest value proven in the literature excepting the cryogenic temperature region, and near room temperature. This study provided an original structure for the silicon thermo-diodes and a novel way to drive them. Item Open Access2D anisotropic photonic crystals of hollow semiconductor nanorod with liquid crystals(2013) Karaomerlioglu F.; Şimsek, Şevket; Mamedov, Amirullah M.; Özbay, EkmelPhotonic crystals (PCs) have many applications in order to control light-wave propagation. A novel type of two-dimensional anisotropic PC is investigated band gap and optical properties as a hollow semiconductor nanorod with nematicliquid crystals (LC). The PC structure composed of an anisotropic nematicLC in semiconductor square hollow nanorod is designed using the plane wave expansion (PWE) method and finite-difference time-domain (FDTD) method. It has been used 5CB (4-pentyl-4'-cyanobiphenyl) as LC core, and Tellurium (Te) as square hollow nanorod material.The PC with hollow Tenanorod with nematicLC is compared with the PC with solid Tenanorodand the PC with hollow Tenanorod. © (2013) Trans Tech Publications, Switzerland. Item Open AccessA Rotaxane-Based photosensitizer for photodynamic therapy(WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim, 2019) Özkan, Melis; Keser, Yağmur; Hadi, Seyed Ehsan; Tuncel, Dönüş; Özkan, Melis; Keser, Yağmur; Hadi, Seyed Ehsan; Tuncel, DönüşA rotaxane was synthesized through a catalytically self‐threading reaction in which CB6 serves as a macrocycle and acts as a catalyst for the 1,3‐dipolar cycloaddition reaction between the alkyne substituted porphyrin core and azide functionalized stopper groups by forming triazole. Application of this rotaxane as a photosensitizer in photodynamic therapy against cancer cells and in bacteria inactivation have also been demonstrated. This photosensitizer has an excellent water solubility and remains stable in biological media at physiological pH (7.4) for prolonged times. It has the ability to generate singlet oxygen efficiently; while it shows no dark cytotoxicity up to 300 µm to the MCF7 cancer cell line, it is photocytotoxic even at 2 µm and reduces the cell viability to around 70 % when exposed to white light. It also displays light‐triggered biocidal activity both against gram‐negative bacteria (Escherichia coli, E. coli) and gram‐positive bacteria (Bacillus subtilis). Upon white light irradiation for 1 min with a flux of 22 mW/cm2 of E. coli suspension incubated with rotaxane (3.5 µm), a killing efficiency of 96 % is achieved, whereas in the dark the effect is recorded as only around 9 %. Item Open AccessA 6-18 GHz GaN power amplifier MMIC with high gain and high output power density(IEEE, 2019) Sütbaş, Batuhan; Özipek, Ulaş; Gürdal, A.; Özbay, Ekmel; Sütbaş, Batuhan; Özipek, Ulaş; Özbay, EkmelA three-stage reactively-matched 6-18 GHz power amplifier MMIC design is presented. The design effort is focused on obtaining a low-loss output matching network for a high output power density. Active unit cells consist of an 8×125 μm transistor stabilized with a symmetrical parallel RC circuit. The wideband amplifier is fabricated using our in-house 0.25 μm GaN on SiC HEMT process. The fabrication technology details and overall device performance are reported. Experimental results show that the MMIC has a minimum gain of 22 dB and a maximum gain of 26.5 dB across the operation band. An average output power density higher than 3.3W/mm with an associated average power-added efficiency of 22.5% is achieved. The MMIC demonstrates output power greater than 9.5 W at the center frequency. This design is distinguished from recent studies with its low-ripple high gain and high output power density. Item Open Access6th International Conference on Competitive Materials and Technology Processes and 2nd European Conference on Silicon and Silica Based Materials(Institute of Physics, 2022) Whittingham, M. Stanley; Kulkov, Sergey N.; Beck, Uwe; Sekino, Tohru; Eshtiaghi, Nicky; Fiore, Saverio; Sadowski, Tomasz; Yang, Jun; Nagasawa, Shigeru; Contreras Garcia, M.E.; Srdic, Vladimir V.; Mamedov, Amirullah M.; Itatani, Kiyoshi; Stupina, Alena; Lecomte-Nana, Gizéle; Kotova, Olga; Rauwel, Erwan; Chen, Soo; Urakov, Aleksandr; Gömze, Ludmila; Kovács, A. Tünde; Jeon, Jae-Ho; Avetissov, Igor; Toyama, Takeshi; Panesar, Daman; Mori, Takao; Chemam, Abdelbaki; Ewais, Emad; Nguyen, Quang Chinh; Kocserha, A. István; Khramchenkov, Maxim; Kurovics, Emese; Knyazeva, Anna; Arinstein, Arkadi; Bursikova, Vilma; Shevchenko, Vladimir; Gömze, László A.; Iwayama, Tsutomu; Mahnicka-Goremikina, Ludmila; Chang, Jeong-Ho; Pytel, Zdzislaw; Hussainova, Irina; Kocserha, István; Amato, Rosaria D.; Ibrahim, Jamal Eldin F.M.; Mezinskis, Gundars; Sedlacik, Michal; Lederer, Gr. Martin; Krenek, Tomas; Mamedov, Amirullah M.The aims of the 2nd European Conference on Silicon and Silica Based Materials (ec-siliconf2) and the 6th International Conference on Competitive Materials and Technology Processes (ic-cmtp6) are the followings: • Promote new methods and results of scientific research in the fields of material, chemical, physical, biological, environmental, healts as well as processing and technology sciences. • Change information between the theoretical and applied sciences and promote their technical and technological implantations. • Improve the communication between the scientist of different nations, countries, and continents. Among the major fields of interest are traditional and advanced materials with increased physical, chemical, biological, medical, thermal and mechanical properties, including their crystalline and nano-structures, phase transformations as well as methods of their technological processes, tests and measurements. Multidisciplinary applications of material science and technological problems encountered in sectors like metallic and non-metallic materials and composites, including metal-alloys, ceramics, polymers, glasses, thin films, etc. Mashinery, IT tools, aerospace, automotive and marine industry, electronics, energy, construction materials, medicine, biosciences, and environmental sciences are of particular interest. List of The International Scientific Advisory Board (ISAB), The International Organizing committee (IOC), The International Scientific Advisory Board (ISAB), The International Organizing committee (IOC), The Session of ic-cmtp6, The Session of ec-siliconf2 and Acknowledgement are available in this pdf. Item Open Access915 nm pumped 1018 nm Yb-doped all-fiber high power fiber laser system(Institute of Electrical and Electronics Engineers Inc., 2019) Midilli, Yakup; Efunbajo, O. Benjamin; Şimşek, Bartu; Ortaç, Bülend; Midilli, Yakup; Efunbajo, O. Benjamin; Şimşek, Bartu; Ortaç, BülendFibers lasers have attracted great attention in the last decades and the power scaling has reached tens of kW levels. Especially with the tandem pump configuration, pumping the active medium with a pump laser light instead of a diode laser, has made a breakthrough and so many research has been conducted about 1018 nm fiber laser systems . Decreasing the quantum defect, the slope efficiency could be increased up to 90 % levels; on the other hand, due to the emission cross section of the Ytterbium (Yb), to operate the laser in the 1018 nm wavelength region is very challenging because of the presence of the ASE about the 1030 nm region. However, in the literature by using 976 nm pump diodes multi-hundred watts level 1018 nm fiber lasers could be demonstrated [2, 3]. Item Open Access94.8 km-range direct detection fiber optic distributed acoustic sensor(Optical Society of America(OSA), 2019) Uyar, Faruk; Uyar, Talha; Ünal, Canberk; Kartaloğlu, Tolga; Özdur, İbrahim; Özbay, Ekmel; Uyar, Faruk; Uyar, Talha; Ünal, Canberk; Kartaloğlu, Tolga; Özdur, İbrahim; Özbay, EkmelThis work demonstrates an ultra-long range direct detection fiber optic distributed acoustic sensor which can detect vibrations at a distance of 94.8 km with 10 m resolution along the sensing fiber. Item Open Access97 percent light absorption in an ultrabroadband frequency range utilizing an ultrathin metal layer: randomly oriented, densely packed dielectric nanowires as an excellent light trapping scaffold(Royal Society of Chemistry, 2017) Ghobadi, A.; Dereshgi, S. A.; Hajian, H.; Birant, G.; Butun, B.; Bek, A.; Özbay, Ekmel; Özbay, EkmelIn this paper, we propose a facile and large scale compatible design to obtain perfect ultrabroadband light absorption using metal-dielectric core-shell nanowires. The design consists of atomic layer deposited (ALD) Pt metal uniformly wrapped around hydrothermally grown titanium dioxide (TiO2) nanowires. It is found that the randomly oriented dense TiO2 nanowires can impose excellent light trapping properties where the existence of an ultrathin Pt layer (with a thickness of 10 nm) can absorb the light in an ultrabroadband frequency range with an amount near unity. Throughout this study, we first investigate the formation of resonant modes in the metallic nanowires. Our findings prove that a nanowire structure can support multiple longitudinal localized surface plasmons (LSPs) along its axis together with transverse resonance modes. Our investigations showed that the spectral position of these resonance peaks can be tuned with the length, radius, and orientation of the nanowire. Therefore, TiO2 random nanowires can contain all of these features simultaneously in which the superposition of responses for these different geometries leads to a flat perfect light absorption. The obtained results demonstrate that taking unique advantages of the ALD method, together with excellent light trapping of chemically synthesized nanowires, a perfect, bifacial, wide angle, and large scale compatible absorber can be made where an excellent performance is achieved while using less materials. Item Open AccessAb initio modeling of elastic and optical properties of Sb and Bi sesquioxides(Institute of Physics Publishing, 2018) Koc, H.; Akhundov, C. G.; Mamedov, Amirullah M.; Özbay, EkmelFirst-principle calculations performed the structural, mechanical, electronic, and optical properties of Sb2O3 and Bi2O3 compounds in monoclinic (claudetite and α-Bi2O3) and orthorhombic (valentinite) structures. Local density approximation has been used for modeling exchange-correlation effects. The lattice parameters, bulk modulus, and the first derivate of bulk modulus (to fit to the Murnaghan's equation of state) of considered compounds have been calculated. The second-order elastic constants have been calculated, and the other related quantities have also been estimated in the present work. The electronic bands structures and the partial densities of states corresponding to the band structures are presented and analyzed. The real and imaginary parts of dielectric functions and energy-loss function are calculated. Our structural estimation and some other results are in agreement with the available experimental and theoretical data. Item Open AccessAb initio study of electronic properties of armchair graphene nanoribbons passivated with heavy metal elements(Elsevier, 2019) Narin, P.; Abbas, J. M.; Atmaca, G.; Kutlu, E.; Lisesivdin, S. B.; Özbay, Ekmel; Özbay, EkmelIn this study, electronic properties of graphene nanoribbons with armchair edges (AGNRs) have been investigated with Density Functional Theory (DFT). Effects of heavy metal (HM) elements, including Zinc (Zn), Cadmium (Cd) and Mercury (Hg) atoms on electronic behavior of AGNRs have been calculated by passivating for both one and two edges of AGNRs in detail. To explain the electronic behavior of investigated AGNRs, the electronic band structure, the density of states (DOS), total energy have been calculated. Energetically favorable structures have been determined using calculated binding energy values. The obtained bandgap values of investigated structures changes between 0.30 and 0.64 eV. Increasing atomic number of passivation atoms have led to an increment in the bandgap of AGNRs. Item Open AccessAb initio study of Ru-terminated and Ru-doped armchair graphene nanoribbons(Taylor and Francis, 2012) Sarikavak-Lisesivdin, B.; Lisesivdin, S. B.; Özbay, Ekmel; Özbay, EkmelWe investigate the effects of ruthenium (Ru) termination and Ru doping on the electronic properties of armchair graphene nanoribbons (AGNRs) using first-principles methods. The electronic band structures, geometries, density of states, binding energies, band gap information, and formation energies of related structures are calculated. It is well founded that the electronic properties of the investigated AGNRs are highly influenced by Ru termination and Ru doping. With Ru termination, metallic band structures with quasi-zero-dimensional, one-dimensional and quasi-one-dimensional density of states (DOS) behavior are obtained in addition to dominant one-dimensional behavior. In contrast to Ru termination, Ru doping introduces small but measurable (12.4 to 89.6meV) direct or indirect band gaps. These results may present an additional way to produce tunable band gaps in AGNRs. Item Open AccessAbsorption enhancement in InGaN-based photonic crystal-implemented solar cells(SPIE, 2012-07-26) Gundogdu, T. F.; Gokkavas, M.; Özbay, Ekmel; Özbay, EkmelWe investigate the absorption characteristics of InGaN solar cells with high indium (0.8) content and a one-dimensional periodic nano-scale pattern (implemented) in the InGaN layer theoretically. The short-circuit current of our InGaN-based solar cell structure is calculated for different lattice constant, etch depth, and fill factor values. A substantial increase in the absorption (17.5% increase in short-circuit current) is achieved when the photonic crystal pattern is thoroughly optimized. (c) 2012 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.JNP.6.061603] Item Open AccessAccurate and process-tolerant resistive load(IEEE, 2020) Sütbaş, Batuhan; Özbay, Ekmel; Atalar, Abdullah; Sütbaş, Batuhan; Özbay, Ekmel; Atalar, AbdullahResistive terminations cannot preserve high-quality matching at high frequencies due to the parasitic effects of the nonideal resistor. Moreover, resistance values of the termination resistors in integrated circuits are subject to process variations. Therefore, it is difficult to obtain accurate and process-tolerant terminations that are crucial for high performance in microwave circuits. We propose a new resistive network that compensates for the high-frequency parasitic effects of the resistors to improve the bandwidth of the termination. In addition to maintaining accuracy, the presented network provides tolerance to variation in the resistor values. The accuracy and tolerance of the proposed structure is analytically shown and experimentally verified by three test structures at the X-band fabricated on a GaN technology. The experimental results show that a small size and wideband 50-Ω load with a return loss better than 25 dB can be obtained, while the resistor value changes ±30%. Item Open AccessAccurate isolation networks in quadrature couplers and power dividers(IEEE, 2021) Sütbaş, Batuhan; Özbay, Ekmel; Atalar, Abdullah; Sütbaş, Batuhan; Özbay, Ekmel; Atalar, AbdullahWhen quadrature couplers and power dividers are implemented in integrated circuits, accurate isolation networks can not be realized due to the nonideal resistors and the process variations. We present an isolation network design technique which cancels the resistor parasitic effects and also increases the tolerance to variations in the resistance values. A Lange coupler and a power divider are designed at Ka-band using the proposed accurate and process-tolerant isolation networks. The improvement is analytically shown and empirically verified with our in-house GaN-based microstrip MMIC process. For the coupler, the measured return losses and isolation are better than 20 dB from DC to 40 GHz. The power divider achieves 20 dB return losses and isolation in a fractional bandwidth of 50%. Both devices maintain 20 dB performance even when the variation in sheet resistance is as high as 30%. Item Open AccessActive metamaterial nearly perfect light absorbers: A review [Invited](Optical Society of America, 2019) Hajian, Hodjat; Ghobadi, Amir; Bütün, Bayram; Özbay, Ekmel; Hajian, Hodjat; Bütün, Bayram; Özbay, EkmelAchieving nearly perfect light absorption from the microwave to optical region utilizing metamaterials has begun to play a significant role in photonics and optoelectronics due to their vital applications in thermal emitters, thermal photovoltaics, photovoltaics, sensing, filtering, and photodetection. However, employing passive components in designing perfect absorbers based on metamaterials and photonic crystals imposes some limits on their spectral operation. In order to overcome those limits, extensive research has been conducted on utilizing different materials and mechanisms to obtain active metamaterial light absorbers. In this review paper, we investigate the recent progress in tunable and reconfigurable metamaterial light absorbers through reviewing different active materials and mechanisms, and we provide a perspective for their future development and applications. Item Open AccessActive Nanophotonics(IEEE, 2020) Alu, A.; Demir, Hilmi Volkan; Jagadish, c; Demir, Hilmi VolkanBeing able to manipulate and control light flows at small scales holds the promise to open groundbreaking opportunities for a variety of technologies. Consider, for instance, the challenges currently faced in the world of computing: as data rates and processing demands increase worldwide at an exponential rate, we are facing unsustainable increases in energy consumption associated with data centers and streaming providers. To address these challenges, optical computing and communications offer an interesting alternative to electronic-based systems. Using light for these purposes, however, is hindered by the fact that photons are not easily squeezed to volumes beyond the diffraction limit, i.e., below the wavelength scale, which would be required both to enable lowenergy computation at sufficiently high speeds and to match the degree of integration density available in electronic systems. Today, the field of optoelectronics, which combines highdensity electronic devices to process the data and low-energy data transport enabled by light, is growing at a very fast pace. Item Open AccessActively tunable thin films for visible light by thermo-optic modulation of ZnO(Wiley-VCH Verlag, 2016) Battal, E.; Okyay, Ali Kemal; Okyay, Ali KemalApplications of active control of light matter interactions within integrated photonics, hyper-spectral imaging, reconfigurable lasers, and selective bio-surfaces have enormously increased the demand for realization of optical modulation covering the spectrum from ultraviolet (UV) up to infrared (IR) wavelength range. In this study, we demonstrate ZnO-based actively tunable perfect absorber operating within UV and visible spectrum with more than 5 nm shift in the resonant absorption wavelength. Using spectroscopic ellipsometry technique, we extract temperature-dependent optical constants of atomic layer-deposited ZnO within 0.3-1.6 and 4-40 μm spectra. We also observe bandgap narrowing of ZnO at elevated temperatures due to lattice relaxation verified by the red-shift of phonon-modes. At around its bandgap, refractive index variations up to 0.2 is obtained and ZnO is shown to exhibit thermo-optic coefficient as high as 9.17 Ã— 10-4 K-1 around the bandgap which is the largest among well-known large bandgap materials. Â© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Item Open AccessAdaptive thermal camouflage using sub-wavelength phase-change metasurfaces(Institute of Physics Publishing Ltd., 2022-12-09) Omam, Zahra Rahimian; Ghobadi, Amir; Özbay, Ekmel; Khalichi, Bahram; Omam, Zahra Rahimian; Ghobadi, Amir; Özbay, Ekmel; Khalichi, BahramSub-wavelength metasurface designs can be used to artificially engineer the spectral thermal signature of an object. The real-time control of this emission can provide the opportunity to switch between radiative cooling (RC) and thermal camouflage functionalities. This performance could be achieved by using phase-change materials (PCMs). This paper presents a sub-wavelength dynamic metasurface design with the adaptive property. The proposed metasurface is made of vanadium dioxide (VO2) nanogratings on a silver (Ag) substrate. The design geometries are optimized in a way that both narrowband and broadband mid-infrared (MIR) emitters can be realized. At low temperatures, insulating VO2 nanogratings trigger the excitation of Fabry–Perot mode inside the grating and surface plasmon polaritons at the metal–dielectric interface with an emission peak located in the MIR region to maximize the RC performance of the design. As temperature rises, the PCM transforms into a metallic phase material and supports excitation of Wood's anomaly and localized surface plasmon resonance modes. Accordingly, the thermal signature is adaptively suppressed.