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Open Access
1018 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.
Open Access
130 nJ 77 fs dissipative soliton fiber laser
(OSA, 2010) Baumgartl, M.; Ortaç, Bülend; Lecaplain, C.; Hideur, A.; Limpert J.; Tünnermann, A.
We report on ultrashort high-energy pulse generation from an all-normal-dispersion fiber oscillator. The watt-level laser directly emits chirped pulses with a duration of 1ps and 163nJ of pulse energy. These can be compressed to 77fs. © 2010 Optical Society of America.
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.
Open Access
21.2 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
Sensitivity ( 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.
Open Access
2D anisotropic photonic crystals of hollow semiconductor nanorod with liquid crystals
(2013) Karaomerlioglu F.; Şimsek, Şevket; Mamedov, Amirullah M.; Özbay, Ekmel
Photonic 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.
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, Ferdi
3-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).
Open Access
2D RF pulse design for optimized reduced field-of-view imaging at 1.5T and 3T
(Elsevier Inc., 2021-10-22) Eren, Orhun Caner; Barlas, Bahadır Alp; Sarıtaş, Emine Ülkü
Two-dimensional spatially selective radiofrequency (2DRF) excitation pulses are widely used for reduced field-of-view (FOV) targeted high-resolution diffusion weighted imaging (DWI), especially for anatomically small regions such as the spinal cord and prostate. The reduction in FOV achieved by 2DRF pulses significantly improve the in-plane off-resonance artifacts in single-shot echo planar imaging (ss-EPI). However, long durations of 2DRF pulses create a sensitivity to through-plane off-resonance effects, especially at 3 T where the off-resonance field doubles with respect to 1.5 T. This work proposes a parameter-based optimization approach to design 2DRF pulses with blips along the slice-select axis, with the goal of maximizing slab sharpness while minimizing off-resonance effects on 1.5 T and 3 T MRI scanners, separately. Extensive Bloch simulations are performed to evaluate the off-resonance robustness of 2DRF pulses. Three different metrics are proposed to quantify the similarity between the actual and ideal 2D excitation profiles, based on the signals within and outside the targeted reduced-FOV region. In addition, simulations on a digital brain phantom are performed for visual comparison purposes. The results show that maintaining a sharp profile is the primary design requirement at 1.5 T, necessitating the usage of relatively high slab sharpness with a time-bandwidth product (TBW) around 8–10. In contrast, off-resonance robustness is the primary design requirement at 3 T, requiring the usage of a moderate slap sharpness with TBW around 5–7.
Open Access
2D vibrational properties of epitaxial silicene on Ag(111)
(IOP Publishing, 2016-10) Solonenko, D.; Gordan, O. D.; Lay, G. L.; Sahin, H.; Cahangirov, S.; Zahn, D. R. T.; Vogt, P.
The two-dimensional silicon allotrope, silicene, could spur the development of new and original concepts in Si-based nanotechnology. Up to now silicene can only be epitaxially synthesized on a supporting substrate such as Ag(111). Even though the structural and electronic properties of these epitaxial silicene layers have been intensively studied, very little is known about its vibrational characteristics. Here, we present a detailed study of epitaxial silicene on Ag(111) using in situ Raman spectroscopy, which is one of the most extensively employed experimental techniques to characterize 2D materials, such as graphene, transition metal dichalcogenides, and black phosphorous. The vibrational fingerprint of epitaxial silicene, in contrast to all previous interpretations, is characterized by three distinct phonon modes with A and E symmetries. Both, energies and symmetries of theses modes are confirmed by ab initio theory calculations. The temperature dependent spectral evolution of these modes demonstrates unique thermal properties of epitaxial silicene and a significant electron-phonon coupling. These results unambiguously support the purely two-dimensional character of epitaxial silicene up to about 300°C, whereupon a 2D-to-3D phase transition takes place. The detailed fingerprint of epitaxial silicene will allow us to identify it in different environments or to study its modifications.
Open Access
3 boyutlu kartezyen olmayan paralel görüntülemede değişken görüntü alanına dayalı geriçatım
(IEEE, 2018) Şenel, Celal Furkan; Çukur, Tolga
MRG’de yaygın olarak kullanılan geriçatım yöntemleri değişken yoğunluklu Kartezyen olmayan taramalara da uygulanabilmektedir; fakat bu durumda bu yöntemlerin genellikle yüksek hesaplama karmaşıklığı içeren çok sayıda yinelemeye ihtiyaç duyması özellikle 3 boyutlu geriçatımlara uygulanabilirliklerini sınırlamaktadır. 2 boyutlu Kartezyen olmayan veri için daha hızlı geriçatım almak amacıyla, PILS’e dayalı, değişken görüntü alanlarını kullanan bir teknik yakın zamanda teklif edilmiştir. Bu çalışmada bu teknik 3 boyutlu değişken yoğunluklu veriye uygulanmış, ek olarak elde edilen görüntüler dalgacık regülarizasyonu kullanılarak kalan artifaktlardan temizlenmiştir. Önce regülarizasyon için farklı parametrelerin başarımları karşılaştırılmış, sonra regülarizasyon da dâhil olmak üzere değişken görüntü alanı yönteminin, karelerin toplanması, PILS ve ESPIRiT geriçatımları ile başarımları karşılaştırılmıştır. Elde edilen sonuçlar teklif edilen yöntemin başarısının karşılaştırılan diğer geriçatımlardan üstün olduğunu göstermektedir.
Open Access
∼3-nm ZnO nanoislands deposition and application in charge trapping memory grown by single ALD step
(Nature Publishing Group, 2016) El-Atab, N.; Chowdhury F.; Ulusoy, T. G.; Ghobadi, A.; Nazirzadeh A.; Okyay, Ali Kemal; Nayfeh, A.
Low-dimensional semiconductor nanostructures are of great interest in high performance electronic and photonic devices. ZnO is considered to be a multifunctional material due to its unique properties with potential in various applications. In this work, 3-nm ZnO nanoislands are deposited by Atomic Layer Deposition (ALD) and the electronic properties are characterized by UV-Vis-NIR Spectrophotometer and X-ray Photoelectron Spectroscopy. The results show that the nanostructures show quantum confinement effects in 1D. Moreover, Metal-Oxide-Semiconductor Capacitor (MOSCAP) charge trapping memory devices with ZnO nanoislands charge storage layer are fabricated by a single ALD step and their performances are analyzed. The devices showed a large memory window at low operating voltages with excellent retention and endurance characteristics due to the additional oxygen vacancies in the nanoislands and the deep barrier for the trapped holes due to the reduction in ZnO electron affinity. The results show that the ZnO nanoislands are promising in future low power memory applications. © The Author(s) 2016.
Open Access
3D-printed, implantable alginate/CuS nanoparticle scaffolds for local tumor treatment via synergistic photothermal, photodynamic, and chemodynamic therapy
(American Chemical Society, 2023-09-08) Çolak, B.; Cihan, M. C.; Ertaş, Yavuz Nuri
A promising method for treating cancer is localized therapy, which mainly employs hydrogel-based delivery systems. Recently, the capability of 3D printing techniques has been revealed as a promising tool to tackle cancer. In this work, alginate (Alg)-based 3D-printed implantable scaffolds containing bovine serum albumin (BSA)-coated copper sulfide (CuS) nanoparticles, Alg-CuS/BSA, were fabricated for local breast cancer therapy and applied to inhibit tumor development through utilizing synergistic photothermal, photodynamic, and chemodynamic effects. The Alg-CuS/BSA scaffolds were flexible; however, their modulus was significantly lower than that of human breast tissues. Under 808 nm irradiation, the scaffolds demonstrated efficient photothermal, photodynamic, and chemodynamic effects both in vitro and in vivo via improving photothermal transduction and singlet-oxygen formation, and also as Fenton catalysts, the scaffolds produced hydroxyl radicals in the presence of H2O2 within the tumor microenvironment. Without the use of conventional anticancer drugs, the promising tumor treatment of implanted scaffolds can offer potential applications in local cancer treatment and prevent metastasis after surgical removal of tumors.
Open Access
4,5-dianilinophtalimide protects neuroendocrine cells against serum deprivation-induced stress and apoptosis
(2013) Ergin V.; Erdogan, M.; Karasu Ç.; Menevşe, A.
OBJECTIVE: The aim of this study was to reveal the effects of 4,5-dianilinophthalimide (DAPH), which inhibits amyloid β fibrillization, against serum deprivation (SD)-induced apoptosis and the possible mechanisms in differentiated PC12 neuron cells. METHODS: Firstly, we evaluated whether DAPH protects cell viability exposed to SD by MTT assay. Next, we examined the changes of phospho-p38 MAPK (Thr180/Tyr182), phospho-HSP27 (Ser82), phospho-c-JUN (Ser73) and cleaved-CASP3 (Asp175) profiles by immunoblotting, in PC12 cells exposed to SD. Intracellular reactive oxygen species (ROS) level was also measured. RESULTS: SD induced apoptosis accompanied by up-regulation of phospho-p38 MAPK (Thr180/Tyr182), phospho-HSP27 (Ser82), phospho-c-JUN (Ser73), cleaved-CASP3 (Asp175) and intracellular ROS content. Co-treatment with nontoxic doses of DAPH prevented apoptosis by the attenuation of activated proteins and reduction of ROS level. These results suggest that serum deprivation-induced apoptosis inhibited by DAPH administration. CONCLUSION: We have provided for the first evidence that DAPH has a neuroprotective effect on SD-caused stress, probably via contributing the reestablishment of redox homeostasis. © 2013 Neuroendocrinology Letters.
Open Access
4,8 T/m manyetik parçacık görüntüleme tarayıcı tasarımı ve yapımı
(IEEE, 2018) Ütkür, Mustafa; Muslu, Yavuz; Sarıtaş, Emine Ülkü
Manyetik Parçacık Görüntüleme (MPG), ilk yayımlandığı 2005 yılından bu yana hızla gelişerek anjiyografi, kök hücre takibi ve kanser görüntüleme gibi uygulama alanlarında ciddi ilerlemeler kaydetmiştir. İyonlaştırıcı ışıma kullanmaması ve kullandığı izleyici maddelerin sağlığa zararlı olmayan demir oksit nanoparçacıkları olması sayesinde güvenli bir görüntüleme yöntemi olarak umut vaad etmektedir. Henüz insan boyutunda bir MPG tarayıcı yapılmamıs¸ olsa da yapılan preklinik araştırmalar MPG’nin milimetre altı çözünürlüğe sahip olduğunu göstermektedir. Bu çalışmada Bilkent Ulusal Manyetik Rezonans Araştırma Merkezi (UMRAM) bünyesinde geliştirdiğimiz ilk MPG tarayıcının tasarım ve yapım aşamaları sunulmaktadır. Bu MPG tarayıcı x-yönünde 4,8 T/m seçme alanı gradyanına sahiptir, ve 9,7 kHz eksitasyon alan frekansı kullanmaktadır.
Open Access
48 W continuous-wave output from a high- efficiency single emitter laser diode at 915 nm
(Institute of Electrical and Electronics Engineers, 2022-09-19) Liu, Y.; Yang, G.; Zhao, Y.; Tang, S.; Lan, Y.; Zhao, Y.; Demir, Abdullah
Improving the power and efficiency of 9xx-nm broad-area laser diodes has a great help in reducing the cost of laser systems and expanding applications. This letter presents an optimized epitaxial structure with high power and conversion efficiency. Laser diodes with 230 μm emitter width and 5 mm cavity length deliver continuous-wave output power up to 48.5 W at 48 A, 30 °C, the highest power reported for 9xx-nm single emitter lasers so far. The slope efficiency is as high as 1.23 W/A due to a low internal optical loss of 0.31 cm−1 and a high internal efficiency of 96%. The maximum power conversion efficiency reaches 72.6% at 15.3 W and 67.3% at the operating power of 30 W under a heatsink temperature of 25 °C. Life test results show no failure in 1000 hours for 55 laser diodes.
Open Access
48 W continuous-wave output power with high efficiency from a single emitter laser diode at 915 nm
(SPIE - International Society for Optical Engineering, 2023-03-14) Yang, G.; Liu, Y.; Zhao, Yongming; Tang, S.; Zhao, Yuliang; Lan, Y.; Bai, L.; Li, Y.; Wang, X.; Demir, Abdullah; Zediker, Mark S.; Zucker, Erik P.
Improving the power and efficiency of 9xx-nm broad-area laser diodes reduces the cost of laser systems and expands applications. LDs with more than 25 W output power combined with power conversion efficiency (PCE) above 65% can provide a cost-effective high-power laser module. We report a high output power and high conversion efficiency laser diode operating at 915 nm by investigating the influence of the laser internal parameters on its output. The asymmetric epitaxial structure is optimized to achieve low optical loss while considering high internal efficiency, low series resistance, and modest optical confinement factor. Experimental results show an internal optical loss of 0.31 cm-1 and internal efficiency of 96%, in agreement with our simulation results. Laser diodes with 230 μm emitter width and 5 mm cavity length have T0 and T1 characteristic temperatures of 152 and 567 K, respectively. The maximum power conversion efficiency reaches 74.2% at 5 °C and 72.6% at 25 °C, and the maximum output power is 48.5 W at 48 A (at 30 ℃), the highest reported for a 9xx-nm single emitter laser diode. At 25 oC, a high PCE of 67.5% is achieved for the operating power of 30 W at 27.5 A, and the lateral far-field angle with 95% power content is around 8°. Life test results show no failure in 1200 hours for 55 laser diodes. In addition, 55.5 W output was achieved at 55 A from a laser diode with 400 μm emitter width and 5.5 mm cavity length. A high PCE of 64.3% is obtained at 50 W with 47 A. © 2023 SPIE.
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.
Open Access
A [5]Rotaxane-Based photosensitizer for photodynamic therapy
(WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim, 2019) Özkan, Melis; Keser, Yağmur; Hadi, Seyed Ehsan; Tuncel, Dönüş
A [5]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 [5]rotaxane (3.5 µm), a killing efficiency of 96 % is achieved, whereas in the dark the effect is recorded as only around 9 %.
Open Access
A 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
A 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.
Open Access
A 60 GHz beam-steering reconfigurable antenna
(IEEE, 2016) Khalat, A.; Towfiq, Md. A.; Cetiner, B. A.; Ceylan, Ö.; Bıyıklı, Necmi
We present the design, microfabrication, and characterization of a multifunctional reconfigurable antenna (MRA) with beam steering capability operating at 60 GHz band (59-66 GHz). The MRA provides 3 different beam directions pertaining to: θ {-30°,0°,30°}; φ = 90° based on reconfigurable parasitic layer approach. The structure consists of three layers namely, feed, driven antenna and reconfigurable parasitic layers. The first two layers use RF and microfabrication process compatible quartz (ϵr = 3.9, tanδ = 0.0002) substrate while parasitic layer is formed on a low-cost pyrex (ϵr = 4.9, tanδ = 0.01) material with air cavities formed underneath. The upper surface of pyrex has 3×3 rectangular shaped metallic pixels, four of which are interconnected by means of switching. By judiciously controlling the switch status the beam-steering is accomplished. The simulated impedance and gain characteristics show ∼ 15% bandwidth over which the maximum realized gain remains relatively flat around ∼ 7.2 dB for all modes of operation. © 2016 IEEE.
Open Access
60W stacked-HEMT based asymmetric X-band GaN SPDT switch for single chip T/R modules
(IEEE - Institute of Electrical and Electronics Engineers, 2023-10-25) Ertürk, Volkan; Gürdal, Armağan; Çankaya Akoğlu, Büşra; Özbay, Ekmel
This paper presents a high-power, asymmetric single-pole double-throw (SPDT) monolithic microwave integrated circuit (MMIC) switch using high electron mobility transistors (HEMT) with AlGaN/GaN technology for single chip X-band T/R modules. The SPDT switch is designed in series-shunt topology for high-power handling and low-loss performance. For high-power handling, shunt-stacked HEMTs on the transmit (Tx) path and series-stacked HEMTs on the receive (Rx) path are used. In its Tx mode, the switch has achieved an insertion loss better than 0.75 dB throughout the 6-13 GHz bandwidth with a return loss of 14 dB and an isolation of 28 dB. It can handle more than 60 W RF input power at 0.1 dB compression. In its Rx mode, the switch can receive signals with an insertion loss lower than 1.15 dB with 14 dB return loss and 19 dB isolation. With its low insertion and high-power handling capacity from C-band to Ku-band, this switch shows state-of-the-art performance for communication systems.

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