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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/6
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Publication Open Access Ultraslow optical modes in Bose-Einstein condensates(Society of Photo-optical Instrumentation Engineers (SPIE), 2007) Tarhan, Devrim; Postacıoğlu, Nazmi; Department of Physics; Müstecaplıoğlu, Özgür Esat; Faculty Member; Department of Physics; College of Sciences; 1674Light can be slowed down to ultraslow speeds via electromagnetically induced transparency in atomic Bose-Einstein condensates. This is thought to be useful for storage of quantum information for weak probe pulses. We investigate the effects of inhomogeneous density profile of-the condensate on propagation of such ultraslow pulses. We find that spatial density of an atomic condensate leads to a graded refractive index profile, for an off-resonant probe pulse when condensate parameters are suitably chosen. Within the window of negligible absorption, conditions for degenerate multiple waveguide modes are determined. Both analytical and numerical studies are presented to reveal the effects of experimentally controllable parameters, such as temperature and interatomic interaction strength on the number of modes. Group velocity dispersion and modal dispersion are discussed. The effect of waveguide dispersion, in addition to usual material dispersion, on ultraslow pulses is pointed out.Publication Open Access Database for CO2 separation performances of MOFs based on computational materials screening(American Chemical Society (ACS), 2018) Eruçar, İlknur; Department of Chemical and Biological Engineering; Altıntaş, Çiğdem; Avcı, Gökay; Harman, Hilal Dağlar; Azar, Ayda Nemati Vesali; Velioğlu, Sadiye; Keskin, Seda; Researcher; Post Doctorate Student; Department of Chemical and Biological Engineering; College of Engineering; N/A; N/A; N/A; N/A; N/A; 40548Metal-organic frameworks (MOFs) are potential adsorbents for CO2 capture. Because thousands of MOFs exist, computational studies become very useful in identifying the top performing materials for target applications in a time-effective manner. In this study, molecular simulations were performed to screen the MOF database to identify the best materials for CO2 separation from flue gas (CO2/N-2) and landfill gas (CO2/CH4) under realistic operating conditions. We validated the accuracy of our computational approach by comparing the simulation results for the CO2 uptakes, CO2/N-2 and CO2/CH4 selectivities of various types of MOFs with the available experimental data. Binary CO2/N-2 and CO2/CH4 mixture adsorption data were then calculated for the entire MOF database. These data were then used to predict selectivity, working capacity, regenerability, and separation potential of MOFs. The top performing MOF adsorbents that can separate CO2/N-2 and CO2/CH4 with high performance were identified. Molecular simulations for the adsorption of a ternary CO2/N-2/CH4 mixture were performed for these top materials to provide a more realistic performance assessment of MOF adsorbents. The structure-performance analysis showed that MOFs with Delta Q(st)(0) > 30 kJ/mol, 3.8 angstrom < pore-limiting diameter < 5 angstrom, 5 angstrom < largest cavity diameter < 7.5 angstrom, 0.5 < phi < 0.75, surface area < 1000 m(2)/g, and rho > 1 g/cm(3) are the best candidates for selective separation of CO2 from flue gas and landfill gas. This information will be very useful to design novel MOFs exhibiting high CO2 separation potentials. Finally, an online, freely accessible database https://cosmoserc.ku.edu.tr was established, for the first time in the literature, which reports all of the computed adsorbent metrics of 3816 MOFs for CO2/N-2, CO2/CH4, and CO2/N-2/CH4 separations in addition to various structural properties of MOFs.Publication Open Access In situ formation of copper phosphate on hydroxyapatite for wastewater treatment(Multidisciplinary Digital Publishing Institute (MDPI), 2022) Rahmani, Fatemeh; Ghadi, Arezoo; Khaksar, Samad; Doustkhah, Esmail; PhD Student; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM)Here, we control the surface activity of hydroxyapatite (HAp) in wastewater treatment which undergoes peroxodisulfate (PDS) activation. Loading the catalytically active Cu species on HAp forms a copper phosphate in the outer layer of HAp. This modification turns a low active HAp into a high catalytically active catalyst in the dye degradation process. The optimal operational conditions were established to be [Cu-THAp](0) = 1 g/L, [RhB](0) = 20 mg/L, [PDS](0) = 7.5 mmol/L, and pH = 3. The experiments indicate that the simultaneous presence of Cu-THAp and PDS synergistically affect the degradation process. Additionally, chemical and structural characterizations proved the stability and effectiveness of Cu-THAp. Therefore, this work introduces a simple approach to water purification through green and sustainable HAp-based materials.Publication Open Access Plasmon-coupled photocapacitor neuromodulators(American Chemical Society (ACS), 2020) Ülgüt, Burak; Çetin, Arif E.; N/A; N/A; Department of Molecular Biology and Genetics; Department of Electrical and Electronics Engineering; Department of Chemical and Biological Engineering; Melikov, Rustamzhon; Srivastava, Shashi Bhushan; Karatüm, Onuralp; Doğru-Yüksel, Itır Bakış; Jalali, Houman Bahmani; Sadeghi, Sadra; Dikbaş, Uğur Meriç; Kavaklı, İbrahim Halil; Nizamoğlu, Sedat; PhD Student; Researcher; PhD Student; PhD Student; Master Student; Faculty Member; Faculty Member; Department of Molecular Biology and Genetics; Department of Electrical and Electronics Engineering; Department of Chemical and Biological Engineering; Graduate School of Sciences and Engineering; College of Sciences; College of Engineering; N/A; N/A; N/A; N/A; N/A; N/A; N/A; 40319; 130295Efficient transduction of optical energy to bioelectrical stimuli is an important goal for effective communication with biological systems. For that, plasmonics has a significant potential via boosting the light-matter interactions. However, plasmonics has been primarily used for heat-induced cell stimulation due to membrane capacitance change (i.e., optocapacitance). Instead, here, we demonstrate that plasmonic coupling to photocapacitor biointerfaces improves safe and efficacious neuromodulating displacement charges for an average of 185% in the entire visible spectrum while maintaining the faradic currents below 1%. Hot-electron injection dominantly leads the enhancement of displacement current in the blue spectral window, and the nanoantenna effect is mainly responsible for the improvement in the red spectral region. The plasmonic photocapacitor facilitates wireless modulation of single cells at three orders of magnitude below the maximum retinal intensity levels, corresponding to one of the most sensitive optoelectronic neural interfaces. This study introduces a new way of using plasmonics for safe and effective photostimulation of neurons and paves the way toward ultrasensitive plasmon-assisted neurostimulation devices.Publication Open Access Silicon microsphere photonics - art. no. 65931J(Society of Photo-optical Instrumentation Engineers (SPIE), 2007) Department of Physics; Department of Electrical and Electronics Engineering; Serpengüzel, Ali; Kurt, Adnan; Ayaz, Ulaş Kemal; Faculty Member; Teaching Faculty; Department of Physics; Department of Electrical and Electronics Engineering; College of Sciences; 27855; 194455; N/AElectrophotonic integrated circuits (EPICs), or alternatively, optoelectronic integrated circuit (OEICs) are the natural evolution of the microelectronic integrated circuit (IC) with the added benefit of photonic capabilities. Traditionally, the microelectronics IC industry has been based on group IV silicon, whereas the microphotonics industry on group III-V semiconductors. However, silicon based photonic microdevices have been making strands in "siliconizing" photonics. Silicon microspheres with their high quality factor whispering gallery modes (WGMs), are ideal candidates for wavelength division multiplexing (WDM) in the standard near-infrared telecommunications bands. In our experiments, we are using silicon microspheres with a refractive index of 3.48 and a radius of 500 micrometers. The optical resonances of the silicon microspheres provide the necessary narrow linewidths, that are needed for high resolution WDM applications. Potential WDM applications include filters, modulators, switches, detectors, and possibly light sources.Publication Open Access Top-down technique for scaling to nano in silicon MEMS(American Vacuum Society (AVS), 2017) Wollschlaeger, Nicole; Oesterle, Werner; Leblebici, Yusuf; Department of Mechanical Engineering; Alaca, Burhanettin Erdem; Nadar, Gökhan; Yılmaz, Mustafa Akın; Kılınç, Yasin; Taşdemir, Zuhal; Faculty Member; PhD Student; PhD Student; Department of Mechanical Engineering; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; 115108; N/A; N/A; N/A; N/ANanoscale building blocks impart added functionalities to microelectromechanical systems (MEMS). The integration of silicon nanowires with MEMS-based sensors leading to miniaturization with improved sensitivity and higher noise immunity is one example highlighting the advantages of this multiscale approach. The accelerated pace of research in this area gives rise to an urgent need for batch-compatible solutions for scaling to nano. To address this challenge, a monolithic fabrication approach of silicon nanowires with 10-mu m-thick silicon-on-insulator (SOI) MEMS is developed in this work. A two-step Si etching approach is adopted, where the first step creates a shallow surface protrusion and the second step releases it in the form of a nanowire. It is during this second deep etching step that MEMS-with at least a 2-order-of-magnitude scale difference-is formed as well. The technique provides a pathway for preserving the lithographic resolution and transforming it into a very high mechanical precision in the assembly of micro-and nanoscales with an extreme topography. Validation of the success of integration is carried out via in situ actuation of MEMS inside an electron microscope loading the nanowire up to its fracture. The technique yields nanowires on the top surface of MEMS, thereby providing ease of access for the purposes of carrying out surface processes such as doping and contact formation as well as in situ observation. As the first study demonstrating such monolithic integration in thick SOI, the work presents a pathway for scaling down to nano for future MEMS combining multiple scales.Publication Open Access Temperature control in dissipative cavities by entangled dimers(American Chemical Society (ACS), 2019) Dağ, Ceren B.; Niedenzu, Wolfgang; Özaydın, Fatih; Kurizki, Gershon; Department of Physics; Müstecaplıoğlu, Özgür Esat; Faculty Member; Department of Physics; College of Sciences; 1674We show that the temperature of a cavity field can be drastically varied by its interaction with suitably entangled atom pairs (dimers) traversing the cavity under realistic atomic decoherence. To this end we resort to the hitherto untapped resource of naturally entangled dimers whose state can be simply controlled via molecular dissociation, collisions forming the dimer, or unstable dimers such as positronium. Depending on the chosen state of the dimer, the cavity-field mode can be driven to a steady-state temperature that is either much lower or much higher than the ambient temperature, despite adverse effects of cavity loss and atomic decoherence. Entangled dimers enable much broader range of cavity temperature control than single "phaseonium" atoms with coherently superposed levels. Such dimers are shown to constitute highly caloric fuel that can ensure high efficiency or power in photonic thermal engines. Alternatively, they can serve as controllable thermal baths for quantum simulation of energy exchange in photosynthesis or quantum annealing.Publication Open Access An information theoretical analysis of multi-terminal neuro-spike communication network in spinal cord(Association for Computing Machinery (ACM), 2018) Department of Electrical and Electronics Engineering; Akan, Özgür Barış; Civaş, Meltem; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; Graduate School of Sciences and Engineering; 6647; N/ACommunication theoretical understanding of healthy and diseased connections in the spinal cord motor system is crucial for realizing future information and communication technology (ICT) based diagnosis and treatment techniques for spinal cord injuries (SCI). A spinal cord motor nucleus associated with a particular muscle constitutes an ideal candidate for studying to have an understanding of SCI. Typical spinal cord motor nucleus system contains pool of lower motor neurons (MNs) controlling a muscle by integrating synaptic inputs from spinal interneurons (INs), upper motor neurons (DNs) and sensory neurons (SNs). In this study, we consider this system from ICT perspective. Our aim is to quantify the rate of information flow across a spinal cord motor nucleus. To this end, we model an equivalent single-hop multiterminal network, where multiple transmitting nodes representing heterogeneous population of DNs, INs and SNs sen information to multiple receiving nodes corresponding to MNs. To identify the outputs at receiving nodes, we define corresponding neurospike communication channel and then find the bound on total rates across this network. Based on the network model, we analyze achievable rates for a particular motor nucleus system called Tibialis Anterior (TA) motor nucleus in the spinal cord numerically and simulate several spinal cord dysfunction scenarios. The numerical results reveal that decrease in the maximum total rates with the lower motor neuron injury causes weakness in the affected muscle.Publication Open Access Reply to comment on "database for CO2 separation performances of MOFs based on computational materials screening"(American Chemical Society (ACS), 2019) Department of Chemical and Biological Engineering; Altıntaş, Çiğdem; Velioğlu, Sadiye; Keskin, Seda; Researcher; Department of Chemical and Biological Engineering; Graduate School of Sciences and Engineering; N/A; 200650; 40548Publication Open Access Liquid crystal eastomer actuated reconfigurable microscale kirigami metastructures(Wiley, 2021) Zhang, Mingchao; Shahsavan, Hamed; Guo, Yubing; Pena-Francesch, Abdon; Zhang, Yingying; Department of Mechanical Engineering; Sitti, Metin; Faculty Member; Department of Mechanical Engineering; College of Engineering; School of Medicine; 297104Programmable actuation of metastructures with predesigned geometrical configurations has recently drawn significant attention in many applications, such as smart structures, medical devices, soft robotics, prosthetics, and wearable devices. Despite remarkable progress in this field, achieving wireless miniaturized reconfigurable metastructures remains a challenge due to the difficult nature of the fabrication and actuation processes at the micrometer scale. Herein, microscale thermo-responsive reconfigurable metasurfaces using stimuli-responsive liquid crystal elastomers (LCEs) is fabricated as an artificial muscle for reconfiguring the 2D microscale kirigami structures. Such structures are fabricated via two-photon polymerization with sub-micrometer precision. Through rationally designed experiments guided by simulations, the optimal formulation of the LCE artificial muscle is explored and the relationship between shape transformation behaviors and geometrical parameters of the kirigami structures is build. As a proof of concept demonstration, the constructs for temperature-dependent switching and information encryption is applied. Such reconfigurable kirigami metastructures have significant potential for boosting the fundamental small-scale metastructure research and the design and fabrication of wireless functional devices, wearables, and soft robots at the microscale as well.