WoS İndeksli Yayınlar Koleksiyonu
Permanent URI for this collectionhttps://hdl.handle.net/20.500.12573/394
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Article Citation - WoS: 2Citation - Scopus: 1Shell-Shaped Active Layers for Omnidirectional Organic Photovoltaic Cells(SPIE - Society of Photo-Optical Instrumentation Engineers, 2022-10-27) Hah, DooyoungFor the employment of organic photovoltaic cells in wearable electronic systems, improvements in energy conversion efficiency and omnidirectionality (angular coverage) are highly appreciated. This study aims at those improvements by introducing shell-shaped active layers. The proposed device structures enhance light absorption and angular range through light coupling to guided modes in the active layer. Two shapes, i.e., a triangle and a semicircle, are examined for the shell cross-sections. Numerical simulation using finite-element analysis and finite-difference time-domain methods demonstrates that the devices with the triangular-shell-shaped active layers exhibit an average absorption enhancement of up to 63% for transverse electric (TE)-polarization and up to 32% for transverse magnetic (TM)-polarization when compared with the flat active layers of the same thicknesses. The average enhancements of the semicircular-shell-shaped active layers are found to be slightly lower than those values, with 60% for TE and 28% for TM. The examined structures also show good omnidirectionality with decent absorption up to an 81 deg incidence angle for the triangular-shell-shaped device and up to a 76 deg angle for the semicircular one when TM polarization is considered. These absorption enhancements and improved angular coverages make the proposed structures highly attractive for wearable electronic system applications. (c) 2022 Society of Photo-Optical InstrumentationEngineers (SPIE)Article Citation - WoS: 3Citation - Scopus: 3RF MEMS Variable Attenuators With Improved dB-Linearity(Springer Heidelberg, 2023-02-22) Hah, DooyoungA variable attenuator is one of the essential components in radio frequency (RF) systems, such as automatic gain control amplifiers and full-duplex systems. Variable attenuators based on microelectromechanical systems (MEMS) technology have several advantages over the semiconductor counterparts, including low power consumption and suppressed harmonics. Attenuation can be realized by disruption of signal propagation, which is induced by moving electrodes placed next to a signal line. In this work, the effect of the moving electrodes on the RF characteristics of the variable attenuators is studied via numerical simulation. It is observed that 10 lm of moving electrode displacement can result in 18 dB of attenuation dynamic range at 20 GHz. The similar type of RF MEMS variable attenuators reported previously showed substantial nonlinearity in attenuation-voltage characteristics, which becomes a serious drawback for applications where high-precision attenuation management is required. The main objective of the current study is, therefore, to achieve high dB-linearity, by employing shaped-finger comb-drive actuators in the moving electrode displacement control. In addition, a nonlinear relationship between force and displacement in a clamped-clamped beam spring is taken into account for more accurate device modelling. Through finite element analysis, it is shown that an improvement by a factor of twelve can be obtained in dB-linearity by using a single-comb shaped-finger actuator, compared to standard straight-finger comb-drives. The study also shows that the dB-linearity can be further (2.2 times additionally) improved by utilizing dual-comb shaped finger actuators.Article Citation - WoS: 5Citation - Scopus: 5Linear Variable Optical Attenuators With Shaped-Finger Comb-Drive Actuators(Optical Soc Amer, 2020-01-06) Hah, DooyoungA design method is proposed for variable optical attenuators (VOAs), aiming at linear attenuation-voltage characteristics, and verified by finite element analysis. Devices of interest are planar VOAs based on microelectromechanical systems technology, with either a knife-edge shutter or a reflector. The proposed method calculates the shape of the fingers of the comb-drive actuators that are used to move the optical component (shutter or reflector) to change the attenuation level. The calculation is, in effect, tantamount to solving a differential equation that encompasses the optical model of the device, electromechanical behavior of the actuators, and the objective of the design, i.e., linear attenuation-voltage characteristics. The design method is almost all-analytical with minimum usage of numerical analysis. The obtained designs are further examined by three-dimensional finite element analysis to understand their effectiveness and to probe the validity of the approximations used. The best linearity factor (defined as % deviation from the ideal case) obtained is 1.34% for both shutter- and reflection-type devices when the conditions are set as 1-dB insertion loss and 50-dB maximum attenuation. (C) 2020 Optical Society of AmericaArticle Citation - WoS: 4Citation - Scopus: 3Hemispherical-Shell Organic Photovoltaic Cells for Absorption Enhancement and Improved Angular Coverage(SPIE - Society of Photo-Optical Instrumentation Engineers, 2024-02-14) Hah, DooyoungA hemispherical shell shape is proposed for an organic photovoltaic cell structure, aiming at enhancing both light absorption and angular coverage. Three-dimensional finite element analysis method is used to study the absorption spectra within the hemispherical-shell-shaped active layer. The study reveals that the proposed structure can result in 66% and 36% of absorption improvements compared to a flat-structured device when the incoming light is transverse electric (TE)- and transverse magnetic (TM)-polarized, respectively. It is also learned that the proposed hemispherical shell structure has absorption improvement as high as 13% (TE) and 21% (TM) when compared to the previously reported semicylindrical shell structure. The angular coverage of the proposed structure is improved as well, reaching 81 deg (TE) and 82 deg (TM), which becomes quite useful for the wearable electronics applications where the incidence angle can vary in a random manner. These improvements can be attributed to better light coupling and guiding through the active layer made possible by the hemispherical shell shape of the device. (c) 2024 Society of Photo-Optical Instrumentation Engineers (SPIE)Article Citation - WoS: 1Citation - Scopus: 1Electret Vibration Energy Harvesters With Symmetrically Configured Curved-Beam Hinges(Springer Heidelberg, 2024-03) Hah, DooyoungFor vibration energy harvesters, broadband power spectral characteristics are often desirable. One way of achieving broadband spectrum is to employ curved-beam hinges, utilizing their nonlinear spring characteristics. In our previous study, electret-based vibration energy harvesters employing curved-beam hinges were investigated via numerical analysis based on stochastic differential equations and colored-noise inputs. It showed that the harvesters with curved-beam hinges can produce higher power than the ones with ordinary straight beams when the external acceleration is between 0.02g and 0.05g. It was also learned that the straight-beam device, a Duffing oscillator, performs better than the curved-beam device, a Duffing-Holmes oscillator, at higher acceleration (>= 0.1g\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\ge 0.1g$$\end{document}). Since the energy harvesting efficiency is one of the most important performance indicators, continuous search for novel configurations with improved efficiency is needed. For that purpose, a symmetric configuration of curved-beam hinges (a Duffing oscillator) is proposed in this work, in contrast to the previously reported one with an asymmetric configuration (a Duffing-Holmes oscillator). This study shows, via numerical analysis, that the symmetric configuration can produce higher (up to 7.3% more) power outputs for the external acceleration magnitude higher than 0.1g, when compared to an asymmetric configuration. The study results also show that it can produce higher power outputs (up to 4.5 times) than the harvesters employing ordinary straight-beam hinges.Article Citation - WoS: 6Citation - Scopus: 6C-Band Optical Filters With Micromechanical Tuning(Springer, 2017-10-13) Hah, DooyoungTunable optical filters that cover the entire range of the C-band (1530-565 nm) are designed by utilizing the Vernier effect, i.e. series coupling of microring resonators of different sizes, and the micromechanical tuning method. The micromechanical tuning method employs lateral comb-drive actuators to control evanescent coupling between the resonators and index modulators. Single crystalline silicon is used as the material for all of the main components including bus waveguide cores, resonators, index modulators, and comb-drive actuators. A finite-difference time-domain method is used for optical analysis of the filter. The simulation results show good agreement with those by analytical methods, previously reported. The width of the index modulator is found to play an important role to the filter characteristics. A wider modulator (e.g., width: 100 nm) can cover the full tuning range of 35 nm without switching among different bands owing to stronger effective index change effect, but induces significant loss to the filter, especially when it is brought close to the resonator. While a narrow modulator (e.g., width: 50 nm), on the other hand, induces moderate loss to the filter, it requires hopping among multiple bands to cover the full range since the effective index change incurred is again moderate. In order to achieve linear tuning characteristics in the cascaded-resonator filters, the shaped-finger comb-drive actuator design method is applied. The design method based on two-dimensional slice approximation is further examined by three-dimensional finite element analysis for verification. It is shown that the design method can also work for the cascaded-resonator filters, even for the one that requires band hopping. Effects of fabrication imperfections to the designed device characteristics are studied as well.Conference Object Citation - WoS: 3Citation - Scopus: 3Analytical Design of MEMS Variable Capacitors Based on Shaped-Finger Comb-Drives(Springer Heidelberg, 2019-02-23) Hah, DooyoungA variable capacitor is one of the widely used components in radio frequency (RF) circuits. Variable capacitors can benefit from the microelectromechanical systems (MEMS) technology, to be equipped with attractive characteristics such as high quality factor and wide tuning range. One of the design goals for MEMS varactors has been to realize linear capacitance-voltage (C-V) characteristics, for which a design method is proposed in this paper, based on shaped-finger comb-drive actuators. The shaped-finger design method, originally developed for a tunable optical filter application by the author, is redeveloped in this work for a linear C-V varactor. Moreover, the conformal mapping method is employed in calculation of capacitances, making the whole design process more time-efficient, being almost all-analytical with the minimum usage of numerical analysis methods. Effects of sense capacitor finger shapes to the optimized drive capacitor finger shapes and the corresponding C-V characteristics are investigated as well. Variable capacitors with the shaped-finger design show linearity factor (LF)-defined as the maximum deviation from the perfect linear relationship-as good as 0.4%, enormously improved from that of the conventional constant-finger-gap devices (LF: 49.9%). Further probed by 3-D numerical analysis, the C-V characteristics of the designed variable capacitor show LF better than 2.62% in the case of constant-gap sense capacitors, and as good as 0.77% in the case of shaped-finger sense capacitors. Versatility of the design method is further demonstrated by presenting a varactor for linear resonant frequency-voltage (f-V) characteristics in voltage-controlled oscillator (VCO) applications. Finally, effects of etch bias, one of common fabrication imperfections, to the linearity of C-V characteristics are studied. The developed analytical design method with shaped fingers can find a wide range of applications where comb-drive actuators are used.Article Citation - WoS: 2Citation - Scopus: 2Analysis of Optical Gyroscopes With Vertically Stacked Ring Resonators(Tubitak Scientific & Technological Research Council Turkey, 2021-05-31) Hah, DooyoungWithout any moving part, optical gyroscopes exhibit superior reliability and accuracy in comparison to mechanical sensors. Microring-resonator-based optical gyroscopes emerged as alternatives for bulky conventional Sagnac interferometer sensors, especially attractive for applications with limited footprints. Previously, it has been reported that planar incorporation of multiple resonators does not bring about improvement in sensitivity for a given area because the increase in Sagnac phase accumulation does not outrun the increase of area. Therefore, it was naturally suggested to consider vertical stacking of ring resonators because then, the resonators can share the same footprint. In this work, sensitivity performances of such configurations with vertically stacked microring resonators are analyzed and compared to that of a basic (single-resonator) configuration. Through comprehensive study, it is learned that the sensitivity performance of the devices with vertically-stacked resonators (either with a single bus waveguide or with two bus waveguides) does not exceed that of the basic sensor device (single resonator with one bus waveguide), i.e. the basic structure is yet to be remained as the most efficient configuration.Article Citation - WoS: 5Citation - Scopus: 5Analysis of Electret-Based Vibration Energy Harvesting Devices With Curved-Beam Hinges(Sage Publications Ltd, 2023-01-26) Hah, DooyoungRecently, vibration energy harvesting devices have gained growing interests. One of the main requirements for them is a broad bandwidth owing to stochastic spectral characteristics of the general vibration sources. Among various approaches for wide bandwidth, curved-beam hinges are quite attractive due to their simple structures. Although there have been several reports on curved beams, a more detailed study is needed. The device under study is an electret-based one with balanced comb-drive configuration. The whole system is modeled by using nonlinear stochastic differential equations. The numerical analysis results show that there is an optimum curve height for maximum power output, which depends on various conditions, such as external vibration strength, comb-drive dimensions, and initial electret charges. At the external acceleration magnitude of 0.02g and 0.05g, the device with curved beams can produce up to 2.9 times and 4.8 times higher power output, respectively, than one with straight beams for given device geometries. To the contrary, at lower and higher vibration magnitudes, straight-beam devices harvest more energy than curved-beam ones. Therefore, it can be concluded that the curved beam height needs to be carefully determined based on the conditions of the application, especially on the characteristics of the external vibration sources.Article Citation - WoS: 4Citation - Scopus: 4All-Polymer Ultrasonic Transducer Design for an Intravascular Ultrasonography Application(Tubitak Scientific & Technological Research Council Turkey, 2019-07-26) Hah, DooyoungIntravascular ultrasonography (IVUS), a medical imaging modality, is used to obtain cross-sectional views of blood vessels from inside. In IVUS, transducers are brought to the proximity of the imaging targets so that high-resolution images can be obtained at high frequency without much concern of signal attenuation. To eliminate mechanical rotation rendered in conventional IVUS, it is proposed to manufacture a transducer array on a flexible substrate and wrap it around a cylindrical frame. The transducer of consideration is a capacitive micromachined ultrasonic transducer (CMUT). The whole device needs to be made out of polymers to be able to endure a high degree of bending (radius: 1 mm) Bending of the devices leads to considerable changes in the device characteristics, including resonant frequency and pull-in voltage due to geometrical dimension changes and stress induced. The main purpose of this work is to understand the effect of bending on the device characteristics by means of finite element analysis. Another objective of the work is to understand the relationships between such an effect and the device geometries. It is learned that the bending-induced stress depends strongly on anchor width, membrane thickness, and substrate thickness. It is also learned that resonant frequency and pull-in voltage become lower in most cases because of using a flexible substrate in comparison to those of the device on a rigid substrate. Bending-induced stress increases the spring constant and hence increases resonant frequency and pull-in voltage, although this effect is relatively weaker. For most of the device geometries, pull-in voltage is too high for the polymer material to endure. This is the main drawback of the all-polymer CMUT. In order to meet the design goal of 20 MHz resonant frequency, the membrane radius has to be smaller than 7.7 mu m for a thickness of 3 mu m.