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Publication Metadata only A planar metamaterial with dual-band double-negative response at EHF(2010) Gündoğdu, T. Funda; Gökkavaş, Mutlu; Soukoulis, Costas M.; Özbay, Ekmel; Department of Physics; Güven, Kaan; Faculty Member; Department of Physics; College of Sciences; 52290We report the fabrication and electromagnetic characterization of a planar composite metamaterial (CMM) that is designed to achieve dual-frequency double-negative response at the lower end of the extremely high-frequency (EHF) band. The CMM is based on cut wire pairs and continuous wire elements. Dual-frequency operation is obtained by employing cut wire pairs of two different lengths within the unit cell of the CMM. The magnetic response of the cut wire pairs and the left-handed transmission band of the CMM are demonstrated by experiment and numerical simulations. It is found that the combined electric response of the dual-band CMM is complicated and imposes certain restrictions to the structure design in achieving true left-handed response at both designated frequencies.Publication Metadata only An aerogel-based photocatalytic microreactor driven by light guiding for degradation of toxic pollutants(Elsevier Science Sa, 2021) Jonas, Alexandr; N/A; Department of Chemical and Biological Engineering; Department of Physics; Özbakır, Yaprak; Erkey, Can; Kiraz, Alper; PhD Student; Faculty Member; Faculty Member; Department of Chemical and Biological Engineering; Department of Physics; Graduate School of Sciences and Engineering; College of Engineering; College of Sciences; N/A; 29633; 22542Efficient utilization of light in photocatalytic chemical processes requires careful optimization of the photocatalytic reactor layout to maximize the interaction between the incident light, photocatalyst and reactant molecules. Herein, we report a new type of photocatalytic flow microreactor with an integrated light guide, formed by a channel fabricated inside a hydrophobic composite aerogel monolith made of silica and titania (TiO2). The liquid-filled channel simultaneously acts as a reaction vessel and as a liquid-core optofluidic waveguide, distributing the incident light over the whole reaction volume. Anatase TiO2 nanoparticles embedded in the channel walls then serve as a photocatalyst that can efficiently interact with both the guided light and the reactant solution along the channel length. Composite aerogels were synthesized with TiO2 content between 1 and 50 wt %, retaining their interconnected mesoporous network, low refractive index, and waveguide propagation losses below -3.9 dB/cm over this range of compositions. Using photocatalytic degradation of phenol - an organic compound with harmful environmental effects - as a model chemical reaction, the performance of the microreactor was systematically investigated. Reactant conversion was observed to increase with increasing incident light power, decreasing reactant flow rate and increasing mass fraction of TiO2 in the composite. An analytical model of the reactor/light guide system was developed that predicted successfully the scaling of the reactant conversion with the incident light power and reactant flow rate. The presented concept of aerogel-based optofluidic photocatalytic microreactors is readily scalable and possesses great potential for carrying out other photocatalytic reactions in both polar and non-polar solvents.Publication Metadata only Analysis of thermal fields in orthogonal machining with infrared imaging(Elsevier Science Sa, 2008) Department of Mechanical Engineering; Department of Mechanical Engineering; Department of Physics; Lazoğlu, İsmail; Serpengüzel, Ali; N/A; Faculty Member; Faculty Member; Department of Mechanical Engineering; Department of Physics; Manufacturing and Automation Research Center (MARC); Manufacturing and Automation Research Center (MARC); N/A; College of Engineering; College of Engineering; College of Sciences; N/A; 179391; 27855The validation of a previously developed finite difference temperature prediction model is carried out for orthogonal machining process with a high precision infrared camera set-up, considering the temperature distribution in the tool. the thermal experiments are conducted with two different materials; al 7075, AISI 1050, with two different tool geometries; inserts having a rake angle of 6 degrees and 18 degrees, for different cutting velocities and feedrates. an infrared camera set-up is utilized for the thermal experiments. the results of the high precision infrared thermal measurements are compared with the outputs of the finite difference temperature model, considering the maximum and the mean temperatures in the tool-chip interface zone and the temperature distributions on the tool take face. the maximum tool-chip interface temperature increases with increasing cutting velocity and feedrate. the relationship between the maximum tool-chip interface temperature and the rake angle of the tool is not distinctive. the experimental results show good agreement with the simulations. (c) 2007 Elsevier B.V. all rights reserved.Publication Metadata only Biological lasing in liquid microdroplets deposited on a superhydrophobic surface(IEEE, 2014) Jonas, A.; McGloin, D.; N/A; N/A; Department of Chemistry; Department of Physics; Department of Physics; Aas, Mehdi; Karadağ, Yasin; Bayraktar, Halil; Anand, Suman; Kiraz, Alper; PhD Student; PhD Student; Faculty Member; Researcher; Faculty Member; Department of Chemistry; Department of Physics; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; College of Sciences; N/A; N/A; 201764; N/A; 22542Publication Metadata only Characterization of fluid mixtures at high pressures using frequency response of microcantilevers(2017) Bozkurt, Asuman Aşıkoğlu; Jonas, Alexandr; Department of Chemical and Biological Engineering; N/A; Department of Physics; Department of Mechanical Engineering; Department of Chemical and Biological Engineering; Eris, Gamze; Baloch, Shadi Khan; Kiraz, Alper; Alaca, Burhanettin Erdem; Erkey, Can; Researcher; PhD Student; Faculty Memeber; Faculty Member; Faculty Member; Department of Physics; Department of Mechanical Engineering; Department of Chemical and Biological Engineering; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Engineering; Graduate School of Sciences and Engineering; College of Sciences; College of Engineering; College of Engineering; N/A; N/A; 22542; 115108; 29633The frequency response of ferromagnetic nickel microcantilevers immersed in binary mixtures of carbon dioxide (CO2) and nitrogen (N-2) at 308 K and pressures up to 23 MPa was investigated. Experimental data were analyzed using the model developed by Sader for a clamped oscillatory beam immersed in a fluid and a very good agreement between the measured resonant frequencies and quality factors (Q factors) and the theoretical predictions of the model with cantilever characteristic parameters regressed from experimental data was observed. This suggested that the data could be used to simultaneously measure the density and the viscosity of fluid mixtures over a wide range of pressures. Subsequently, density and viscosity of binary mixtures of CO2 and N-2 were determined using N-2 as the reference fluid and compared to the predictions of Gerg equation of state and Chung equation, respectively. For the studied fluids with different compositions, the average relative difference between the experimental density values and the values predicted using Gerg equation of state and NIST database ranged from 1.0 to 13%. The average relative difference between the experimental viscosity values and the values obtained using Chung equation and NIST database ranged from 2.4 to 15%. Since the resonant frequency and Q factor were found to vary with composition at a fixed temperature and pressure, the technique can in principle also be used to measure the composition of a binary mixture at a fixed temperature and pressure. The study represents the first systematic attempt to use microcantilevers for the characterization of high-pressure fluid mixtures and paves the way for devising portable sensors for in-line monitoring of thermophysical properties and composition of fluid mixtures under a wide range of environmental conditions. (C) 2017 Elsevier B.V. All rights reserved.Publication Metadata only Continuous-wave and graphene mode-locked operation of a Tm3+:KY3F10 laser at 2.3 μm(Optica Publishing Group (formerly OSA), 2019) Tonelli, Mauro; Bae, Ji Eun; Rotermund, Fabian; Petrov, Valentin; N/A; Department of Physics; Muti, Abdullah; Sennaroğlu, Alphan; PhD Student; Faculty Member; Department of Physics; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Sciences; N/A; 23851We report, for the first time to our knowledge, continuous-wave, broadly tunable laser operation of the Tm3+:KY3F10 gain medium between 2260 and 2385 nm and graphene mode locking, yielding 976-fs pulses near 2340 nm.Publication Metadata only Design and instrumentation of an opto-digital confocal microscope(Institute of Electrical and Electronics Engineers (IEEE), 2018) Kurt, Adnan; Kiraz, Berna; N/A; Department of Physics; Zengin, Berk; Kiraz, Alper; Master Student; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; N/A; 22542Confocal microscopy has become a vital technique for life sciences due to higher lateral and axial resolution it provides compared to standard epifluorescence microscopy. On the contrary, accessibility to confocal microscopes did not escalate in proportion to its usage around the globe. Therefore, it was aimed to build an opto-digital confocal microscope, eventually leading to a product which will be affordable for research groups, institutes and hospitals. In this work, we present a home-built confocal microscopy setlp using commercially available equipment. The design of the setup was realized using a 488 nm laser, an inverted microscope and optical/optomechanical parts including mirrors, lenses, beam splitter, scan lens. In addition, X-Y galvo scanner was controlled by using a custom-built control electronics. Instrumentation was made using a National Instruments DAQ card and LabVIEW based software. For characterization purposes reference samples were successfully imaged, following imaging of biological specimen. Gained know-how during development and prototyping will contribute vastly to process of producing a robust, desktop, easy to use and affordable confocal microsocope for researchers and organizations at a diverse scale.Publication Metadata only Determination of composition of ethanol-CO2 mixtures at high pressures using frequency response of microcantilevers(Elsevier Science Bv, 2018) Jonas, Alexandr; Department of Physics; Department of Mechanical Engineering; Department of Chemical and Biological Engineering; N/A; Kiraz, Alper; Alaca, Burhanettin Erdem; Erkey, Can; Baloch, Shadi Khan; Faculty Member; Faculty Member; Faculty Member; PhD Student; Department of Physics; Department of Mechanical Engineering; Department of Chemical and Biological Engineering; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Sciences; College of Engineering; College of Engineering; Graduate School of Sciences and Engineering; 22542; 115108; 29633; N/AThe measurement of the composition of ethanol-CO2 mixtures at high pressures is important in many applications involving supercritical fluids such as drying of alcogels or release of MEMs. Resonant frequency and quality factor (Q-factor) of microcantilevers immersed in ethanol-CO2 mixtures were measured at a temperature of 308 K and pressure range from 8 MPa to 22 MPa. The measurements were carried out for different mixture compositions ranging from 0.91 to 6.16 wt% of ethanol in CO2. At a given pressure and temperature, the resonant frequencies were found to decrease linearly with the increasing ethanol weight percent in the mixture. The sensitivity of the resonant frequency to changes in composition was found to increase with decreasing pressure. The experimental results show that ethanol-CO2 mixture composition can be determined with good accuracy using mainly the measured resonant frequency of microcantilevers.Publication Metadata only Determination of viscosity and density of fluids using frequency response of microcantilevers(Elsevier Science Bv, 2015) Jonas, Alexandr; Department of Chemical and Biological Engineering; Department of Mechanical Engineering; Department of Physics; Department of Chemical and Biological Engineering; Eris, Gamze; Alaca, Burhanettin Erdem; Kiraz, Alper; Erkey, Can; Researcher; Faculty Member; Faculty Member; Faculty Member; Department of Mechanical Engineering; Department of Physics; Department of Chemical and Biological Engineering; College of Engineering; College of Engineering; College of Sciences; College of Engineering; N/A; 115108; 22542; 29633We report on the simultaneous measurement of density and viscosity of nitrogen in gas and supercritical phases at 308.15 K and pressures up to 24 MPa. The density and viscosity were extracted from the measured frequency responses of an oscillated microcantilever immersed in N-2. To this end, a model of oscillatory motion of immersed cantilevers incorporating the effects of hydrodynamic forces was employed. Using argon as a reference fluid of known density and viscosity, cantilever calibration parameters were obtained from nonlinear regression of cantilever resonant frequencies and quality factors recorded in argon. Subsequently, these calibration parameters were used in the model equations to determine the density and viscosity of nitrogen at the given experimental pressure and temperature. In the studied pressure range, the root-mean-square deviations of the measured density and viscosity of nitrogen from the reference values obtained from NIST database were 2.5% and 5.2%, respectively. (C) 2015 Elsevier B.V. All rights reserved.Publication Metadata only Development of a thulium (tm:yap) laser system for brain tissue ablation(Springer, 2011) Bilici, Temel; Mutlu, Sevinc; Kalaycioglu, Hamit; Kurt, Adnan; Gulsoy, Murat; Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851In this study, a thulium (Tm:YAP) laser system was developed for brain surgery applications. As the Tm:YAP laser is a continuous-wave laser delivered via silica fibers, it would have great potential for stereotaxic neurosurgery with highest local absorption in the IR region. The laser system developed in this study allowed the user to set the power level, exposure time, and modulation parameters (pulse width and on-off cycles). The Tm:YAP laser beam (200-600 mW, 69-208 W/cm(2)) was delivered from a distance of 2 mm to cortical and subcortical regions of ex-vivo Wistar rat brain tissue samples via a 200-mu m-core optical fiber. The system performance, dosimetry study, and ablation characteristics of the Tm:YAP laser were tested at different power levels by maximizing the therapeutic effects and minimizing unwanted thermal side-effects. The coagulation and ablation diameters were measured under microscope. The maximum ablation efficiency (100 x ablation diameter/coagulation diameter) was obtained when the Tm:YAP laser system was operated at 200 mW for 10 s. At this laser dose, the ablation efficiency was found to be 71.4% and 58.7% for cortical and subcortical regions, respectively. The fiber-coupled Tm:YAP laser system in hence proposed for the delivery of photothermal therapies in medical applications.