Scopus İndeksli Yayınlar Koleksiyonu
Permanent URI for this collectionhttps://hdl.handle.net/20.500.12573/395
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Article Tapered Curved-Beam Hinges for Electret-Based Vibration Energy Harvesting Devices(IOP Publishing Ltd, 2024-12-01) Hah, DooyoungInterest in vibration energy harvesting have been growing recently for various applications. One of the major development goals for vibration energy harvesters has been improvement in energy conversion efficiency. To pursue that goal, one of the main approaches has been to broaden the spectra of harvesters. Employment of nonlinear springs, such as curved-beam hinges, has proven to be effective for that purpose. The main contribution of the current study is to introduce a lateral taper to the curved beam so as to further optimize the harvester performances. Via numerical analysis by using stochastic differential equations, the study shows that at 0.05g of vibration strength, tapered curved-beam hinges can result in higher electric power output than the non-tapered ones. Deformation-induced stress was taken into consideration as well, in reference to the fracture strength of the material (single-crystal silicon). At lower vibration strength (0.02g), spring nonlinearity becomes weaker, and as a result, the narrowest curved-beam hinge produces the highest output power. Overall, the current study demonstrates that tapering of the curved beam can be a useful addition in the vibration energy harvester design.Conference Object Symmetric Electret-Based Vibration Energy Harvesters With Curved-Beam Hinges(IEEE, 2023-05-28) Hah, DooyoungBroadband power spectral characteristics are desirable in vibration energy harvesters, and it can be achieved by employing curved-beam hinges, which exhibit force-displacement nonlinearity. Via numerical analysis by using stochastic differential equations and colored-noise inputs, this study shows that a symmetric configuration of the curved-beam hinges in electret-based harvesters can produce higher (up to 8% more) power outputs than an asymmetric one. It also presents that the harvesters with curved-beam hinges can produce higher (up to 4.4 times) power outputs than those with straight hinges when the vibration magnitude is 0.05g.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: 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.Conference Object Citation - WoS: 1Citation - Scopus: 2Effects of Curved-Beam Heights to Harvested Energy in a Blanaced Comb-Drive Configuration(IEEE, 2021-08-25) Hah, DooyoungEnergy harvesting devices have been gaining increasing interests, especially in the areas of internet of things (IoTs) and sensor networks. Due to the broadband and random nature of typical vibration energy sources available in the environment, significant amount of research efforts have been put into the bandwidth broadening of the energy harvesters. Utilization of spring nonlinearity has been one of the most studied subject in that regard. In this work, response of an energy harvesting device with curved-beam springs to colorednoise vibration is studied numerically, based on stochastic differential equations. The harvester considered in this study is an electrostatic type with electrets and a balanced comb-drive configuration. The study mainly focuses on the effect of the beam height to the harvested power. The results show that curved-beam springs can increase the harvested electric power by 52% (2.69 mW versus 1.77 mW) in comparison to straight-beam springs of the same dimensions. Buckling-induced rapid snapping of the curved beams is attributed to such a power increase.Conference Object Citation - Scopus: 2Design of Wide-Band Tunable Optical Filters With Cascaded Microring Resonators and Shaped-Finger Comb-Drive Actuators(Institute of Electrical and Electronics Engineers Inc., 2016-05) Hah, DooyoungUtilizing the Vernier effect, series coupling of multiple microring resonators with different sizes is used to design a wide-band (free spectral range: 36 nm) tunable filter. For index modulators that shift the filter spectrum by changing effective indices through evanescent coupling, silicon waveguides are considered, which make the fabrication simpler. Effects of the index modulator width to the filter characteristics are studied. A narrower modulator (width: 50 nm) does not incur much loss to the resonator, but requires hopping among several bands since its tuning effect is moderate. On the other hand, a wider modulator (width: 100 nm) can cover the full free spectral range without band hopping, but induces severe loss when it is close to the resonator. The shaped-finger comb-drive actuator design method is applied to obtain linear drop channel control. © 2017 Elsevier B.V., All rights reserved.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.