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Item Open Access 1 mJ pulse bursts from a Yb-doped fiber amplifier(Optical Society of America, 2012-07-01) Kalaycıoğlu, Hamit; Eldeniz, Y. B.; Akçalan, Önder; Yavaş, Seydi; Efe, M.; İlday, Fatih ÖmerWe demonstrate burst-mode operation of a polarization-maintaining Yb-doped fiber amplifier capable of generating 60 μJ pulses within bursts of 11 pulses with extremely uniform energy distribution facilitated by a novel feedback mechanism shaping the seed of the burst-mode amplifier. The burst energy can be scaled up to 1 mJ, comprising 25 pulses with 40 μJ average individual energy. The amplifier is synchronously pulse pumped to minimize amplified spontaneous emission between the bursts. Pulse propagation is entirely in fiber and fiber-integrated components until the grating compressor, which allows for highly robust operation. The burst repetition rate is set to 1 kHz and spacing between individual pulses is 10 ns. The 40 μJ pulses are externally compressible to a full width at half-maximum of 600 fs. However, due to the substantial pedestal of the compressed pulses, the effective pulse duration is longer, estimated to be 1.2 ps.Item Open Access 1.3 μm GaAs based resonant cavity enhanced Schottky barrier internal photoemission photodetector(IEEE, Piscataway, NJ, United States, 2000) Necmi, B.; Kimukin, I.; Özbay, Ekmel; Tuttle, G.GaAs based photodetectors operating at 1.3 μm that depend on internal photoemission as the absorption mechanism were fabricated. Quantum efficiency (QE) was increased using resonant cavity enhancement (RCE) effect.Item Open Access 100-GHz resonant cavity enhanced Schottky photodiodes(Institute of Electrical and Electronics Engineers, 1998) Onat, B. M.; Gökkavas, M.; Özbay, Ekmel; Ata, E. P.; Towe, E.; Ünlü, M. S.Resonant cavity enhanced (RCE) photodiodes are promising candidates for applications in optical communications and interconnects where ultrafast high-efficiency detection is desirable. We have designed and fabricated RCE Schottky photodiodes in the (Al, In) GaAs material system for 900-nm wavelength. The observed temporal response with 10-ps pulsewidth was limited by the measurement setup and a conservative estimation of the bandwidth corresponds to more than 100 GHz. A direct comparison of RCE versus conventional detector performance was performed by high speed measurements under optical excitation at resonant wavelength (895 nm) and at 840 nm where the device functions as a single-pass conventional photodiode. A more than two-fold bandwidth enhancement with the RCE detection scheme was demonstrated.Item Open Access 175 fs-long pulses from a high-power single-mode er-doped fiber laser at 1550 nm(Elsevier, 2017) Elahi, P.; Kalaycıoğlu, H.; Li, H.; Akçaalan, Ö.; Ilday, F. Ö.Development of Er-doped ultrafast lasers have lagged behind the corresponding developments in Yb- and Tm-doped lasers, in particular, fiber lasers. Various applications benefit from operation at a central wavelength of 1.5 μm and its second harmonic, including emerging applications such as 3D processing of silicon and 3D printing based on two-photon polymerization. We report a simple, robust fiber master oscillator power amplifier operating at 1.55 μm, implementing chirp pulse amplification using single-mode fibers for diffraction-limited beam quality. The laser generates 80 nJ pulses at a repetition rate of 43 MHz, corresponding to an average power of 3.5 W, which can be compressed down to 175 fs. The generation of short pulses was achieved using a design which is guided by numerical simulations of pulse propagation and amplification and manages to overturn gain narrowing with self-phase modulation, without invoking excessive Raman scattering processes. The seed source for the two-stage amplifier is a dispersion-managed passively mode-locked oscillator, which generates a ∼40 nm-wide spectrum and 1.7-ps linearly chirped pulses.Item Open Access 2D Network overtakes 3D for photocatalytic hydrogen evolution(Royal Society of Chemistry, 2022-07-18) Ahmad, Aliyu Aremu; Türkan Gamze Ulusoy, Ghobadi; Özbay, Ekmel; Karadaş, Ferdi3-Dimensional (3D) cyanide coordination polymers, typically known as Prussian blue Analogues (PBAs), have received great attention in catalysis due to their stability, easily tuned metal sites, and porosity. However, their high crystallinities and relatively low number of surface-active sites significantly hamper their intrinsic catalytic activities. Herein, we report the utilization of a 2-dimensional (2D) layered cobalt tetracyanonickelate, [Co–Ni], for the reduction of protons to H2. Relying on its exposed facets, layered morphology, and abundant surface-active sites, [Co–Ni] can efficiently convert water and sunlight to H2 in the presence of a ruthenium photosensitizer (Ru PS) with an optimal evolution rate of 30 029 ± 590 μmol g−1 h−1, greatly exceeding that of 3D Co–Fe PBA [Co–Fe] and Co–Co PBA [Co–Co]. Furthermore, [Co–Ni] retains its structural integrity throughout a 6 hour photocatalytic cycle, which is confirmed by XPS, PXRD, and Infrared analysis. This recent work reveals the excellent morphologic properties that promote [Co–Ni] as an attractive catalyst for the hydrogen evolution reaction (HER).Item Open Access 3.5-GHz Intra-Burst Repetition Rate Ultrafast Yb-Doped Fiber Laser(Elsevier, 2016) Kerse, C.; Kalaycioʇlu, H.; Elahi, P.; Akçaalan, Ö.; Ilday, F. Ö.We report on an all-fiber Yb laser amplifier system with an intra-burst repetition rate of 3.5 GHz. The system is able to produce minimum of 15-ns long bursts containing approximately 50 pulses with a total energy of 215μJ at a burst repetition rate of 1 kHz. The individual pulses are compressed down to the subpicosecond level. The seed signal from a 108 MHz fiber oscillator is converted to approximately 3.5 GHz by a multiplier consisting of six cascaded 50/50 couplers, and then amplified in ten stages. The highly cascaded amplification suppresses amplified spontaneous emission at low repetition rates. Nonlinear interactions between overlapping pulses within a burst is also discussed.Item Open Access 45-GHz bandwidth-efficiency resonant-cavity-enhanced ITO-Schottky photodiodes(IEEE, 2001) Bıyıklı, Necmi; Kimukin, I.; Aytür, O.; Gökkavas, M.; Ünlü, M. S.; Özbay, EkmelHigh-speed Schottky photodiodes suffer from low efficiency mainly due to the thin absorption layers and the semitransparent Schottky-contact metals. We have designed, fabricated and characterized high-speed and high-efficiency AlGaAs-GaAs-based Schottky photodiodes using transparent indium-tin-oxide Schottky contact material and resonant cavity enhanced detector structure. The measured devices displayed resonance peaks around 820 nm with 75% maximum peak efficiency and an experimental setup limited temporal response of 11 ps pulsewidth. The resulting 45-GHz bandwidth-efficiency product obtained from these devices corresponds to the best performance reported to date for vertically illuminated Schottky photodiodes.Item Open Access 50 nm Hall Sensors for Room Temperature Scanning Hall Probe Microscopy(Institute of Physics Publishing, 2004) Sandhu, A.; Kurosawa, K.; Dede, M.; Oral, A.Bismuth nano-Hall sensors with dimensions ∼50nm × 50 nm were fabricated using a combination of optical lithography and focused ion beam milling. The Hall coefficient, series resistance and optimum magnetic field sensitivity of the sensors were 4 × 10-4 Ω/G, 9.1kΩ and 0.8G/√Hz, respectively. A 50nm nano-Bi Hall sensor was installed into a room temperature scanning Hall probe microscope and successfully used for directly imaging ferromagnetic domains of low coercivity garnet thin films.Item Open Access A 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.Operation 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 Access 83 W, 3.1 MHz, square-shaped, 1 ns-pulsed all-fiber-integrated laser for micromachining(Optical Society of America, 2011-08-29) Özgören, Kıvanç; Öktem, Bülent; Yılmaz, Sinem; İlday, F. Ömer; Eken, K.We demonstrate an all-fiber-integrated laser based on off-the-shelf components producing square-shaped, 1 ns-long pulses at 1.03 mu m wavelength with 3.1 MHz repetition rate and 83 W of average power. The master-oscillator power-amplifier system is seeded by a fiber oscillator utilizing a nonlinear optical loop mirror and producing incompressible pulses. A simple technique is employed to demonstrate that the pulses indeed have a random chirp. We propose that the long pulse duration should result in more efficient material removal relative to picosecond pulses, while being short enough to minimize heat effects, relative to nanosecond pulses commonly used in micromachining. Micromachining of Ti surfaces using 0.1 ns, 1 ns and 100 ns pulses supports these expectations. (C) 2011 Optical Society of AmericaItem Open Access 97 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, 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 Access A high-power and broadband gan spdt mmic switch using gate-optimized hemts(Institute of Electrical and Electronics Engineers, 2022-05-22) Erturk, V.; Gurdal, A.; Özbay, EkmelA high-power, broadband monolithic microwave integrated circuit (MMIC) single-pole double-throw (SPDT) switch is designed with gate-optimized high-electron-mobility transistors (HEMTs) using AlGaN/Gallium nitride (GaN) technology. The foot length of the gate is varied from 200 to 250 nm, and the head length is varied from 500 to 750 nm in the T-gate structure to optimize the radio frequency (RF) performance. The SPDT switch is designed in a series-shunt-shunt topology using gate topology as a design parameter. The switch has achieved an insertion loss better than 0.75 dB throughout the 3.5-13.5-GHz bandwidth. It can transmit 30-W output power at 0.1-dB compression point and handle 47.5-dBm input power at P-1dB. The isolation is above 25 dB, and the return loss is better than 11 dB. With its low insertion and high power-handling capacity in broadband, the SPDT switch shows state-of-the-art performance for high-power communication systems and radar applications.Item Open Access Ab 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, 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 Access Ab initio study of neutral ( TiO2 ) n clusters and their interactions with water and transition metal atoms(IOP Publishing, 2012-07-04) Çakır, D.; Gülseren, O.We have systematically investigated the growth behavior and stability of small stoichiometric (TiO2)n (n = 1–10) clusters as well as their structural, electronic and magnetic properties by using the first-principles plane wave pseudopotential method within density functional theory. In order to find out the ground state geometries, a large number of initial cluster structures for each n has been searched via total energy calculations. Generally, the ground state structures for the case of n = 1–9 clusters have at least one monovalent O atom, which only binds to a single Ti atom. However, the most stable structure of the n = 10 cluster does not have any monovalent O atom. On the other hand, Ti atoms are at least fourfold coordinated for the ground state structures for n ≥ 4 clusters. Our calculations have revealed that clusters prefer to form three-dimensional structures. Furthermore, all these stoichiometric clusters have nonmagnetic ground state. The formation energy and the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap for the most stable structure of (TiO2)n clusters for each n have also been calculated. The formation energy and hence the stability increases as the cluster size grows. In addition, the interactions between the ground state structure of the (TiO2)n cluster and a single water molecule have been studied. The binding energy (Eb) of the H2O molecule exhibits an oscillatory behavior with the size of the clusters. A single water molecule preferably binds to the cluster Ti atom through its oxygen atom, resulting an average binding energy of 1.1 eV. We have also reported the interaction of the selected clusters (n = 3, 4, 10) with multiple water molecules. We have found that additional water molecules lead to a decrease in the binding energy of these molecules to the (TiO2)n clusters. Finally, the adsorption of transition metal (TM) atoms (V, Co and Pt) on the n = 10 cluster has been investigated for possible functionalization. All these elements interact strongly with this cluster, and a permanent magnetic moment is induced upon adsorption of Co and V atoms. We have observed gap localized TM states leading to significant HOMO–LUMO gap narrowing, which is essential to achieve visible light response for the efficient use of TiO2 based materials. In this way, electronic and optical as well as magnetic properties of TiO2 materials can be modulated by using the appropriate adsorbate atomsItem Open Access Ab initio study of Ru-terminated and Ru-doped armchair graphene nanoribbons(Taylor and Francis, 2012) Sarikavak-Lisesivdin, B.; Lisesivdin, S. B.; 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 Access Ab initio temperature dependent studies of the homoepitaxial growth on Si(0 0 1) surface(2001) Dağ, S.; Çıracı, Salim; Kılıç, Ç.; Fong, C. Y.We performed ab initio zero temperature and finite temperature molecular dynamics calculations to investigate the homoepitaxial growth on the Si(0 0 1) surface. How do the deposited atoms (adatoms) form addimers and how do the addimers reach their favorable positions at the nucleation site of the growth process are presented. Once two epitaxial addimers, one over the dimer row and oriented perpendicular to the surface dimer bonds and the other over the adjacent trough, are aligned at high temperature, the nucleation site of the growth process is formed. The concerted bond exchange between these addimers and the reconstructed surface dimers is found to be the atomistic mechanism that leads to the homoepitaxial growth. © 2001 Elsevier Science B.V.Item Open Access Ab-initio electron transport calculations of carbon based string structures(American Physical Society, 2004) Tongay, S.; Senger, R. T.; Dag, S.; Çıracı, SalimThe new stable structures of carbon-based strings and their unusual electronic transport properties were discussed. Total energy and electronic structure calculations using first principles pseudopotential plane wave method within density functional theory (DFT) and supercell geometries were also carried out. It was found that carbon chains were suitable for structural and chemical functionalizations because of their flexibility. These carbon chains also form stable ring, helix, grid and network structures. The results show that the double covalent bonding of carbon atoms underlies their unusual chemical, mechanical and transport properties and carbon chains can form stable string structures with impressive physical properties.Item Open Access Aberrant cerebral network topology and mild cognitive impairment in early Parkinson’s disease(John Wiley & Sons, Inc., 2015-05-06) Pereira, J. B.; Aarsland, D.; Ginestet, C. E.; Lebedev, A. V.; Wahlund, L. O.; Simmons, A.; Volpe, G.; Westman, E.The aim of this study was to assess whether mild cognitive impairment (MCI) is associatedwith disruption in large-scale structural networks in newly diagnosed, drug-na€ıve patients with Parkin-son’s disease (PD). Graph theoretical analyses were applied to 3T MRI data from 123 PD patients and 56controls from the Parkinson’s progression markers initiative (PPMI). Thirty-three patients were classifiedas having Parkinson’s disease with mild cognitive impairment (PD-MCI) using the Movement DisordersSociety Task Force criteria, while the remaining 90 PD patients were classified as cognitively normal (PD-CN). Global measures (clustering coefficient, characteristic path length, global efficiency, small-world-ness) and regional measures (regional clustering coefficient, regional efficiency, hubs) were assessed inthe structural networks that were constructed based on cortical thickness and subcortical volume data.PD-MCI patients showed a marked reduction in the average correlation strength between cortical andsubcortical regions compared with controls. These patients had a larger characteristic path length andreduced global efficiency in addition to a lower regional efficiency in frontal and parietal regions com-pared with PD-CN patients and controls. A reorganization of the highly connected regions in the networkwas observed in both groups of patients. This study shows that the earliest stages of cognitive decline inPD are associated with a disruption in the large-scale coordination of the brain network and with adecrease of the efficiency of parallel information processing.Item Open Access Ablation-cooled material removal with ultrafast bursts of pulses(Nature Publishing Group, 2016) Kerse C.; Kalaycıoğlu, H.; Elahi, P.; Çetin B.; Kesim, D. K.; Akçaalan, Ö.; Yavaş S.; Aşık, M. D.; Öktem B.; Hoogland H.; Holzwarth, R.; Ilday, F. Ö.The use of femtosecond laser pulses allows precise and thermal-damage-free removal of material (ablation) with wide-ranging scientific, medical and industrial applications. However, its potential is limited by the low speeds at which material can be removed and the complexity of the associated laser technology. The complexity of the laser design arises from the need to overcome the high pulse energy threshold for efficient ablation. However, the use of more powerful lasers to increase the ablation rate results in unwanted effects such as shielding, saturation and collateral damage from heat accumulation at higher laser powers. Here we circumvent this limitation by exploiting ablation cooling, in analogy to a technique routinely used in aerospace engineering. We apply ultrafast successions (bursts) of laser pulses to ablate the target material before the residual heat deposited by previous pulses diffuses away from the processing region. Proof-of-principle experiments on various substrates demonstrate that extremely high repetition rates, which make ablation cooling possible, reduce the laser pulse energies needed for ablation and increase the efficiency of the removal process by an order of magnitude over previously used laser parameters. We also demonstrate the removal of brain tissue at two cubic millimetres per minute and dentine at three cubic millimetres per minute without any thermal damage to the bulk.Item Open Access Above room temperature ferromagnetism in Gd2B2 monolayer with high magnetic anisotropy(American Chemical Society, 2020) Görkan, T.; Vatansever, E.; Akıncı, Ü.; Gökoğlu, G.; Aktürk, E.; Çıracı, SalimThe realization of 2D ultrathin crystals with a ferromagnetic ground state that is sustainable at room temperature has been a real challenge now. By combining ab initio density functional theory with Monte Carlo simulations, we predicted a new 2D structure, Gd2B2 monolayer, which maintains its mechanical stability at elevated temperatures. More remarkably, it has a ferromagnetic ground state with high permanent magnetic moment, which persists far above room temperature. It exhibits high magnetocrystalline anisotropy along particular directions. We find also that both its magnetic anisotropy and Curie temperature can largely be altered by applied strain providing an excellent magnetoelastic tunability. This novel 2D crystal with high magnetic moment and Curie temperature combined with high structural and thermal stability can offer critical applications in magnetoelectronics.