Browsing by Author "Hah, Dooyoung"

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Citation - WoS: 6
Citation - Scopus: 6
Absorption Enhancement by Semi-Cylindrical Structures for an Organic Solar Cell Application
(Optical Soc Amer, 2020) Hah, Dooyoung
Organic solar cells are attractive for various applications with their flexibility and low-cost manufacturability. In order to increase their attractiveness in practice, it is essential to improve their energy conversion efficiency. In this work, semi-cylindrical-shell-shaped structures are proposed as one of the approaches, aiming at absorption enhancement in an organic solar cell. Poly(3-hexylthiophene-2,5-diyl) blended with indene-C60 bisadduct (P3HT:ICBA) is considered as the active layer. Light coupling to the guided modes and a geometrical advantage are attributed to this absorption enhancement. Finite-difference time-domain methods and finite element analysis are used to examine the absorption spectra for two types of devices, i.e., a debossed type and an embossed type. It is shown that absorption enhancement increases as the radius of the cylinder increases, but reaches a saturation at about 4-mu m radius. The average absorption enhancement with an active layer thickness of 200 nm and radius of 4 mu m, and for incidence angles between 0 degrees and 70 degrees, is found as 51%-52% for TE-polarized input and as 30%-33% for TM-polarized input when compared to a flat structure. Another merit of the proposed structures is that the range of incidence angles where the integrated absorption is at the level of the normal incidence is significantly broadened, reaching 70 degrees-80 degrees. This feature can be highly useful especially when organic solar cells are to be placed around a round object. The study results also exhibit that the proposed devices bear broadband absorption characteristics. (C) 2020 Optical Society of America
Citation - WoS: 4
Citation - Scopus: 4
All-Polymer Ultrasonic Transducer Design for an Intravascular Ultrasonography Application
(Tubitak Scientific & Technological Research Council Turkey, 2019) Hah, Dooyoung
Intravascular 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.
AlN Piezoelectric Quad-Actuators for 2D Optical Micro Scanning
(Taylor & Francis Ltd, 2022) Hah, Dooyoung
Piezoelectric actuation has been one of the frequent choices for optical micro scanning. In most of the cases, lead zirconate titanate (PZT) has been used as the piezoelectric material. However, PZT has a potential issue in biomedical applications due to the content of lead. For this, AlN can be used as an alternative. The main drawback of AlN is its low piezoelectric coefficients. In order to overcome such a drawback, this paper presents a novel actuator configuration, designed for a quasi-static operation mode. Quad-actuators and meander-shaped hinges are the essence of the proposed actuator configuration. Numerical simulation study is carried out to prove the concept of the device. The study also shows that the proposed scanner can have the optical scan angle of 9 degree at a quasi-static mode. Two different scan modes, a raster-like mode and a Lissajous mode are tested, demonstrating the two-dimensional scanning capability of the device.
Citation - WoS: 5
Citation - Scopus: 5
Analysis of Electret-Based Vibration Energy Harvesting Devices With Curved-Beam Hinges
(Sage Publications Ltd, 2023) Hah, Dooyoung
Recently, 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.
Citation - WoS: 2
Citation - Scopus: 2
Analysis of Optical Gyroscopes With Vertically Stacked Ring Resonators
(Tubitak Scientific & Technological Research Council Turkey, 2021) Hah, Dooyoung
Without 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.
Citation - WoS: 3
Citation - Scopus: 4
Analytical Design of Linear Variable Capacitors With Shaped-Finger Comb-Drive Actuators
(Institute of Electrical and Electronics Engineers Inc., 2018) Hah, Dooyoung
Variable capacitors have a broad usage in radio frequency (RF) circuits. Microelectromechanical systems (MEMS) technology can provide variable capacitors with high quality factor and wide tuning range characteristics. One of the design goals for MEMS varactors has been linear capacitance- voltage (C-V) characteristics. To design a linear C-V varactor, a shaped-finger comb-drive actuator is proposed in this paper. The shaped-finger design method, originally developed to obtain linear wavelength-voltage relationships in a tunable optical filter, is modified in this work for a linear C-V varactor, which involves development of a new governing equation. Moreover, conformal mapping is employed in calculation of capacitances, making the whole design process almost all-analytical with the minimum usage of numerical analysis methods. Variable capacitors with the shaped-finger design show linearity factor (LF) - defined as the maximum deviation from the perfect linear relationship - as low as 0.4%, tremendously improved from that of the conventional constant-finger-gap devices (LF: 49.9%). The characteristics of the designed variable capacitor are further investigated through 3-D numerical analysis, and show LF better than 11.5% for the finger thickness in the range between 1 and 10 micrometers. Versatility of the design method is further demonstrated by design of a varactor with linear resonant frequency-voltage (f-V) characteristics for voltage-controlled oscillator (VCO) applications. The developed analytical design method with shaped fingers can find a wide range of applications where comb-drive actuators are used. © 2018 Elsevier B.V., All rights reserved.
Citation - WoS: 3
Citation - Scopus: 3
Analytical Design of MEMS Variable Capacitors Based on Shaped-Finger Comb-Drives
(Springer Heidelberg, 2022) Hah, Dooyoung
A 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.
Citation - WoS: 6
Citation - Scopus: 6
C-Band Optical Filters With Micromechanical Tuning
(Springer, 2018) Hah, Dooyoung
Tunable 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.
Citation - WoS: 3
Citation - Scopus: 4
Clinical Probe Utilizing Surface Enhanced Raman Scattering
(A V S Amer Inst Physics, 2014) Kim, Jeonghwan; Hah, Dooyoung; Daniels-Race, Theda; Feldman, Martin
Conventional Raman scattering is a well-known technique for detecting and identifying complex molecular samples. In surface enhanced Raman scattering, a nanorough metallic surface close to the sample enormously enhances the Raman signal. In previous work, the metallic surface was a thin layer of gold deposited on a rough transparent epoxy substrate. The advantage of the clear substrate was that the Raman signal could be obtained by passing light through the substrate, on to opaque samples simply placed against its surface. In this work, a commercially available Raman spectrometer was coupled to a distant probe. Raman signals were obtained from the surface, and from the interior, of a solid specimen located more than 1 m away from the spectrometer. The practical advantage of this arrangement is that it opens up surface enhanced Raman spectrometry to a clinical environment, with a patient simply sitting or lying near the spectrometer. (C) 2014 American Vacuum Society.
Citation - WoS: 9
Citation - Scopus: 9
A Design Method of Comb-Drive Actuators for Linear Tuning Characteristics in Mechanically Tunable Optical Filters
(Springer, 2017) Hah, Dooyoung
A new method is proposed in design of comb-drive actuators for specific voltage-displacement characteristics with finger gaps as the design parameters. The design method proposed by the author previously is further refined by adopting a more accurate model which considers fringe electric fields. The proposed method is applied to design comb-drive actuators with an aim to achieve linear tuning characteristics in mechanically tunable optical add-drop filters with microring resonators. To make an assessment of the accuracy of the proposed design method, three-dimensional electrostatic numerical analysis is conducted to obtain capacitances of the designed comb-drive actuators as functions of the moving finger displacement. Obtained capacitances are used to find the tuning characteristics (resonant wavelength vs. voltage) of the filter, in combination with the results from the author's other work where a relationship between the resonant wavelength and the displacement of an index modulator was studied. It is found that by employing the actuators designed by the proposed method, the maximum deviation from linearity (MDL) can be reduced by 17.2 % points (from 25.7 % of the conventional design to 8.5 % of the new design). MDL is further reduced to 4.4 % by making a few modifications in the design.
Design of Capacitive Micromachined Ultrasonic Transducers (CMUTS) on a Flexible Substrate for Intravascular Ultrasonography (IVUS) Applications
(IEEE, 2017) Hah, Dooyoung; Je, Chang Han; Lee, Sung-Q
Effects of substrate bending to the characteristics of capacitive miniaturized ultrasonic transducers (CMUTs) on a flexible substrate are studied through finite element analysis (FEA) for the design purpose. The target application of the devices is intravascular ultrasonography (IVUS) where transducers are brought to the proximity of the imaging targets so that high resolution images can be obtained without much concern of signal attenuation. In order to eliminate mechanical rotation used in the conventional IVUS, the transducer array can be manufactured on a flexible substrate and to wrap it around a cylindrical frame. It can be anticipated that the characteristics of the transducers will be altered by such bending of the substrate through geometrical dimension changes and stress induced. Pullin voltages and resonant frequencies of CMUTs were studied via FEA for various bending radii and membrane thicknesses. It was found that both pull-in voltages and resonant frequencies become smaller for the transducers on a bent substrate compared to the ones on a flat substrate. It was also found that pull-in voltages decrease as the substrate bending radius is reduced.
Citation - Scopus: 7
Design of Capacitive Micromachined Ultrasonic Transducers (Cmuts) on a Flexible Substrate for Intravascular Ultrasonography (Ivus) Applications
(Institute of Electrical and Electronics Engineers Inc., 2017) Hah, Dooyoung; Je, Changhan; Lee, Sung Q.
Effects of substrate bending to the characteristics of capacitive miniaturized ultrasonic transducers (CMUTs) on a flexible substrate are studied through finite element analysis (FEA) for the design purpose. The target application of the devices is intravascular ultrasonography (IVUS) where transducers are brought to the proximity of the imaging targets so that high resolution images can be obtained without much concern of signal attenuation. In order to eliminate mechanical rotation used in the conventional IVUS, the transducer array can be manufactured on a flexible substrate and to wrap it around a cylindrical frame. It can be anticipated that the characteristics of the transducers will be altered by such bending of the substrate through geometrical dimension changes and stress induced. Pull-in voltages and resonant frequencies of CMUTs were studied via FEA for various bending radii and membrane thicknesses. It was found that both pull-in voltages and resonant frequencies become smaller for the transducers on a bent substrate compared to the ones on a flat substrate. It was also found that pull-in voltages decrease as the substrate bending radius is reduced. © 2017 Elsevier B.V., All rights reserved.
Citation - Scopus: 3
Design of Mechanically Tunable Optical Filters With Microring Resonators
(Institute of Electrical and Electronics Engineers Inc., 2015) Hah, Dooyoung; Bordelon, John
Design strategies for the mechanically tunable optical filters with microring resonators and comb-drive actuators are discussed. Electromechanical simulation results are combined with the electromagnetic analysis of the device. A method to design comb-drive actuators to achieve linear resonant wavelength-voltage characteristics is presented. © 2015 Elsevier B.V., All rights reserved.
Citation - Scopus: 2
Design of Wide-Band Tunable Optical Filters With Cascaded Microring Resonators and Shaped-Finger Comb-Drive Actuators
(Institute of Electrical and Electronics Engineers Inc., 2016) Hah, Dooyoung
Utilizing 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.
Citation - WoS: 1
Citation - Scopus: 2
Effects of Curved-Beam Heights to Harvested Energy in a Blanaced Comb-Drive Configuration
(IEEE, 2021) Hah, Dooyoung
Energy 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.
Citation - WoS: 1
Citation - Scopus: 1
Electret Vibration Energy Harvesters With Symmetrically Configured Curved-Beam Hinges
(Springer Heidelberg, 2024) Hah, Dooyoung
For 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.
Esnek Kağıt Tabanlı Kapasitif Sensör Kullanarak Solunum İzleme
(Abdullah Gül Üniversitesi, Fen Bilimleri Enstitüsü, 2022) Solak, İrfan; İçöz, Kutay; Hah, Dooyoung
Respiration is an action known to be essential and crucial for life. Unfortunately, in some cases such as illnesses and accidents various respiratory problems can be experienced. It might be difficult to maintain normal respiration for the people who have respiratory diseases. It is known that respiration monitoring of people who have respiratory problems, albeit for different reasons, is important in terms of their treatment and maintaining their life quality. Current respiration monitoring systems are expensive and bulky. Many of these systems are only available at hospitals or in laboratories. Low-cost, easy to use and portable respiratory monitoring devices are needed. Having these motivations, we aimed to monitor respiration by designing and producing a paper-based sensor that is easy to manufacture, low-cost, and highly responsive. The sensor, which is the subject of this thesis project, has potential to be used for different purposes such as measuring the humidity in the environment. In this project, we focused on designing a system for people who have respiratory problems by providing respiration monitoring data. In addition, according to the data obtained, we are able to analyze the health status of the users. Therefore, this sensor can be used both for the detection of respiration diseases and monitor the status of the patients. In this way, respiration related unhealthy situation can be detected and treated immediately.
An FEM Study of die Attach Packaging Effect on Nanomechanical Si Optical Filters
(IEEE, 2017) Seok, Seonho; Hah, Dooyoung
This paper presents a finite element analysis of die attach packaging stress effect on emerging nanomechanical silicon optical filters. The proposed silicon optical filter is composed of Si waveguides and a microring resonator having a few hundred nm in thickness and a few tens of mu m in length. Photonic integrated circuit is typically implemented by attaching a new component to a common ceramic interposer with other components. Such an attachment process would be a cause of unwanted performance deviation of MEMS or NEMS devices due to the packaging stress. Therefore, an FEM model has been used to evaluate deflection and stress of NEMS waveguides and microring resonators which are main elements for the proposed optical filter.
An Fem Study of Die Attach Packaging Effect on Nanomechanical Si Optical Filters
(Institute of Electrical and Electronics Engineers Inc., 2017) Seok, Seonho; Hah, Dooyoung
This paper presents a finite element analysis of die attach packaging stress effect on emerging nanomechanical silicon optical filters. The proposed silicon optical filter is composed of Si waveguides and a microring resonator having a few hundred nm in thickness and a few tens of μm in length. Photonic integrated circuit is typically implemented by attaching a new component to a common ceramic interposer with other components. Such an attachment process would be a cause of unwanted performance deviation of MEMS or NEMS devices due to the packaging stress. Therefore, an FEM model has been used to evaluate deflection and stress of NEMS waveguides and microring resonators which are main elements for the proposed optical filter. © 2017 Elsevier B.V., All rights reserved.
Citation - Scopus: 1
Flexible and Ecofriendly Keypad Based on Paper and Pencil
(IEEE-Inst Electrical Electronics Engineers Inc, 2023) Er, Sahane Firdevs; Kalabey, Nurefsan; Hah, Dooyoung
Pencil-on-paper electronics has gained growing interests as cost-effective, ecofriendly, flexible, and light-weight devices for various applications. In this letter, a flexible keypad employing pencil-on-paper capacitive touch sensors is presented. Spiral-type interdigitated sensors are interconnected via steel threads as well as graphite paths to constitute a ten-button keypad. Passivation of the sensors by means of acrylic paint coating works as protection against deterioration from bending, folding, and extended usage. Backing of the keypad with silicone provides extra durability. Sensor reading is administered by Arduino boards. The same manufacturing method is used to scale up the system to a 44-button keyboard. Functionalities of both systems are successfully demonstrated, and effectiveness of the passivation is verified.