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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/6

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    PublicationOpen Access
    Enhanced sinterability, thermal conductivity and dielectric constant of glass-ceramics with PVA and BN additions
    (Multidisciplinary Digital Publishing Institute (MDPI), 2022) Akkasoğlu, Ufuk; Çiçek, Buğra; N/A; Department of Chemistry; Arıbuğa, Dilara; Balcı, Özge; Researcher; Department of Chemistry; Koç University AKKİM Boron-Based Materials _ High-technology Chemicals Research _ Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); Graduate School of Sciences and Engineering; College of Sciences; N/A; 295531
    With the rapid development of the microelectronics industry, many efforts have been made to improve glass-ceramics' sinterability, thermal conductivity, and dielectric properties, which are essential components of electronic materials. In this study, low-alkali borosilicate glass-ceramics with PVA addition and glass-BN composites were prepared and successfully sintered at 770 degrees C. The phase composition, density, microstructure, thermal conductivity, and dielectric constant were investigated. It was shown that PVA addition contributes to the densification process of glass-ceramics (~88% relative density, with closed/open pores in the microstructure) and improves the thermal conductivity of glass material from 1.489 to 2.453 W/K.m. On the other hand, increasing BN addition improves microstructures by decreasing porosities and thus increasing relative densities. A glass-12 wt. % BN composite sample exhibited almost full densification after sintering and presented apparent and open pores of 2.6 and 0.08%, respectively. A high thermal conductivity value of 3.955 W/K.m and a low dielectric constant of 3.00 (at 5 MHz) were observed in this material. Overall, the resulting glass-ceramic samples showed dielectric constants in the range of 2.40-4.43, providing a potential candidate for various electronic applications.
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    PublicationOpen Access
    Impact of rehabilitation on fatigue in post-Covid-19 patients: a systematic review and meta-analysis
    (Multidisciplinary Digital Publishing Institute (MDPI), 2022) de Sire, Alessandro; Moggio, Lucrezia; Marotta, Nicola; Agostini, Francesco; Tasselli, Anna; Ferrante, Vera Drago; Curci, Claudio; Calafiore, Dario; Ferraro, Francesco; Bernetti, Andrea; Ammendolia, Antonio; Taşkıran, Özden Özyemişçi; Faculty Member; School of Medicine; 133091
    The post-COVID-19 syndrome may affect patients after the COVID-19 post-acute phase. In particular, the 69% of patients reported persistent fatigue at the discharge. To date, no clear data are available regarding the most effective rehabilitative approaches for the treatment of this condition. Thus, this systematic review aimed to evaluate the rehabilitation treatment's efficacy on fatigue in post-COVID-19 patients. We systematically searched PubMed, Scopus, and Web of Science databases to find longitudinal study designs presenting: post-COVID-19 patients as participants; a rehabilitative approach aimed to reduce post-COVID-19 syndrome as intervention; and fatigue intensity assessed through an evaluation tool that quantified the perceived exertion (i.e., fatigue severity scale, FSS; Borg Scale (BS); Borg Category Ratio 10, CR10; Checklist Individual Strength (CIS) fatigue scale; FACIT (Functional Assessment of Chronic Illness Therapy) fatigue scale). The present systematic review protocol was registered on PROSPERO (registration number CRD42021284058). Out of 704 articles, 6 studies were included. Nearly all patients showed COVID-19-related fatigue, and after the rehabilitation treatment, only 17% of subjects reported the persistency of symptoms. The overall effect size reported a -1.40 decrease in Borg Category Ratio 10 with a SE of 0.05 and a 95% CI between -1.50 and -1.30 (p < 0.001). The present systematic review and meta-analysis underlines the rehabilitation role in the fatigue reduction in patients affected by post-COVID-19 syndrome.
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    PublicationOpen Access
    Phase noise in oscillators as differential-algebraic systems with colored noise sources
    (Society of Photo-optical Instrumentation Engineers (SPIE), 2004) Department of Electrical and Electronics Engineering; Demir, Alper; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; 3756
    Oscillators are key components of many kinds of systems, particularly electronic and opto-electronic systems. Undesired perturbations, i.e. noise, in practical systems adversely affect the spectral and timing properties of the signals generated by oscillators resulting in phase noise and timing jitter, which are key performance limiting factors, being major contributors to bit-error-rate (BER) of RF and possibly optical communication systems, and creating synchronization problems in clocked and sampled-data electronic systems. In this paper. we review our work on the theory and numerical methods for nonlinear perturbation and noise analysis of oscillators described by a system of differential-algebraic equations (DAEs) with white and colored noise sources. The bulk of the work reviewed in this paper first appeared in [1], then in [2] and [3]. Prior to the work mentioned above, we developed a theory and numerical methods for nonlinear perturbation and noise analysis of oscillators described by a system of ordinary differential equations (ODEs) with white noise sources only [4, 5]. In this paper, we also discuss some open problems and issues in the modeling and analysis of phase noise both in free running oscillators and in phase/injection-locked ones.
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    PublicationOpen 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/A
    Nanoscale 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.
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    PublicationOpen Access
    Optimization of room-temperature continuous-wave cr (4+)-Doped solid-state lasers: experiment and modeling
    (Institute of Electrical and Electronics Engineers (IEEE), 2001) Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851
    A novel study to determine the optimum crystal and resonator parameters for Cr4+ doped lasers subjected to lifetime thermal loading was permormed. The comparison of the results of lasing threshold, pump absorption saturation, and power efficiency measurements was done with the predictions of a theoretical model to determine the laser cross sections. Using the best-fit cross-section values, numerical optimization studies were carried out to determine the optimum crystal absorption, resonator reflectivity, and crystal length which maximized the output power.
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    PublicationOpen Access
    Tunable integrated optical filters based on sapphire microspheres and liquid crystals
    (Society of Photo-optical Instrumentation Engineers (SPIE), 2010) Gilardi, Giovanni; Yılmaz, Hasan; Sharif, Murib Mohammed; Asquini, Rita; D'Alessandro, Antonio; Beccherelli, Romeo; Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851
    We present an integrated optical narrowband electrically tunable filter based on the whispering gallery modes of sapphire microspheres and double ion-exchanged channel BK7 glass waveguides. Tuning is provided by a liquid crystal infiltrated between the spheres and the glass substrate. By suitably choosing the radii of the spheres and of the circular apertures, upon which the spheres are positioned, arrays of different filters can be realized on the same substrate with a low cost industrial process. We evaluate the performance in terms of quality factor, mode spacing, and tuning range by comparing the numerical results obtained by the numerical finite element modeling approach and with the analytical approach of the Generalized Lorenz-Mie Theory for various design parameters. By reorienting the LC in an external electrical field, we demonstrate the tuning of the spectral response of the sapphire microsphere based filter. We find that the value of the mode spacing remains nearly unchanged for the different values of the applied electric field. An increase of the applied electric field strength, changes the refractive index of the liquid crystal, so that for a fixed geometry the mode spacing remains unchanged.
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    PublicationOpen Access
    Self-Q-Switched Cr:LiCAF laser near 800 nm
    (Society of Photo-optical Instrumentation Engineers (SPIE), 2013) Department of Physics; Beyatlı, Ersen; Sennaroğlu, Alphan; Demirbaş, Ümit; PhD Student; Faculty Member; Master Student; Department of Physics; College of Sciences; N/A; 23851; N/A
    Self-Q-switching (SQS) of lasers enable the generation of Q-switched pulses from simple laser cavities without using any additional saturable absorbers or active modulators. Earlier studies have reported SQS in ruby, Nd:YAG, and Cr:LiSAF lasers. However, these systems were mostly flashlamp pumped and required cooling of the crystal and/or misalignment of the laser cavity for the observation of SQS. In this presentation, for the first time to our knowledge, we report SQS operation of a Cr:LiCAF laser. SQS was achieved in an astigmatically compensated x-cavity containing only a Cr:LiCAF crystal that was end-pumped with a 130-mW continuous-wave (cw) diode at 660 nm. During regular cw operation, the laser produced a diffraction-limited beam with 50 mW of output power and had a spectral width of 0.5 nm near 795 nm. SQS operation of the Cr:LiCAF laser could be initiated by fine adjustment of the separation between the curved mirrors of the cavity and was observed at several mirror separations within the stability range of the resonator. During SQS operation, the laser generated pulses with about 5 microsecond duration in the 780-800 nm wavelength range, at repetition rates between 10 and 30 kHz. SQS operation was further accompanied with a decrease in the output power to the 30-45 mW range. In this regime, the output beam became multimode and spectral broadening up to 12.5 nm (FWHM) was observed.
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    PublicationOpen Access
    Dielectric resonators based on amorphous silicon
    (Society of Photo-optical Instrumentation Engineers (SPIE), 2001) Department of Physics; Serpengüzel, Ali; Faculty Member; Department of Physics; College of Sciences; 27855
    In this paper, we report on the design, modeling, fabrication, and characterization of dielectric microresonators based on hydrogenated amorphous silicon nitride and hydrogenated amorphous silicon oxide. The microresonators were modelled using the transfer matrix method (TMM). Quarter wavelength thick stacks of hydrogenated amorphous silicon nitride and hydrogenated amorphous silicon oxide were consecutively deposited using low temperature plasma enhanced chemical vapor deposition (PECVD). For the characterization of the dielectric microresonators the intrinsic photoluminescence of the amorphous silicon nitride is used. The photoluminescence is enhanced by at least an order of magnitude at the resonance wavelength of 710 nm. The minimum resonance linewidth is 6 nm, corresponding to a quality factor of 118. The maximum rejection bandwidth of the distributed Bragg reflector (DBR) is 150 nm. The enhancement and inhibition of the photoluminescence is understood by the modified photon density of states of the dielectric microresonator. The linewidth of the photoluminescence is also narrowed with respect to the linewidth of the bulk amorphous silicon nitride, again due to the presence of the electromagnetic modes of the microresonator.
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    PublicationOpen Access
    Microstructured head-up display screen for automotive applications
    (Society of Photo-optical Instrumentation Engineers (SPIE), 2012) Freeman, Mark O.; Department of Electrical and Electronics Engineering; Hedili, M. Kıvanç; Ürey, Hakan; Master Student; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 8579
    A novel see-through screen is developed for automobiles which reduces the size of the head-up display (HUD) unit considerably. The screen is illuminated by a laser scanning pico-projector and a real image is formed on the screen. The screen has thousands of hexagonally packed microlenses that are partially reflective and embedded in an index matched medium which provides very good see-through capability. Light reflected from the microlenses expand and form a hexagon shaped viewing window. This system is called a direct projection HUD system as the pico projector projects directly onto the screen and forms a real image on it. The system is very compact and does not require any space under the dashboard, which saves on space for the car manufacturers, or allows it to be used immediately as an aftermarket HUD installed in any car.
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    PublicationOpen Access
    Adaptive Q control for tapping-mode nanoscanning using a piezoactuated bimorph probe
    (American Institute of Physics (AIP) Publishing, 2007) Department of Mechanical Engineering; Günev, İhsan; Varol, Aydın; Karaman, Sertaç; Başdoğan, Çağatay; Master Student; Faculty Member; Department of Mechanical Engineering; College of Engineering; N/A; N/A; N/A; 125489
    A new approach, called adaptive Q control, for tapping-mode atomic force microscopy (AFM) is introduced and implemented on a homemade AFM setup utilizing a laser Doppler vibrometer and a piezoactuated bimorph probe. In standard Q control, the effective Q factor of the scanning probe is adjusted prior to the scanning depending on the application. However, there is a trade-off in setting the effective Q factor of an AFM probe. The Q factor is either increased to reduce the tapping forces or decreased to increase the maximum achievable scan speed. Realizing these two benefits simultaneously using standard Q control is not possible. In adaptive Q control, the Q factor of the probe is set to an initial value as in standard Q control, but then modified on the fly during scanning when necessary to achieve this goal. In this article, we present the basic theory behind adaptive Q control, the electronics enabling the online modification of the probe's effective Q factor, and the results of the experiments comparing three different methods: scanning (a) without Q control, (b) with standard Q control, and (c) with adaptive Q control. The results show that the performance of adaptive Q control is superior to the other two methods.