Scopus İndeksli Yayınlar Koleksiyonu

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Now showing 1 - 6 of 6
  • Article
    Citation - WoS: 2
    Citation - Scopus: 1
    Shell-Shaped Active Layers for Omnidirectional Organic Photovoltaic Cells
    (SPIE - Society of Photo-Optical Instrumentation Engineers, 2022-10-27) Hah, Dooyoung
    For 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)
  • Conference Object
    Citation - Scopus: 2
    Planar Mems Variable Optical Attenuators (VOAs) With Linear Attenuation-Voltage Characteristics
    (Institute of Electrical and Electronics Engineers Inc., 2019-05) Hah, Dooyoung
    Variable optical attenuators (VOAs) are essential components in wavelength division multiplexing (WDM) networks, light waveform generators, and optical fiber test equipment. Among various types of planar MEMS VOAs, a shutter type and a reflective type have been most frequently studied so far. In a shutter type, a knife-edge-like beam blocker is inserted in between the butt-coupled input and output fibers, partially obstructing the coupling between the fibers. In a reflective type, a mirror that is placed in the optical path controls the alignment between the fibers to result in attenuation. The movement of the shutter or the mirror is controlled by MEMS actuators, such as comb-drive actuators. In most of the planar MEMS VOAs reported, the relationship between the attenuation and the control voltage has been highly nonlinear. This nonlinearity results in uneven resolution throughout the attenuation range. Although this nonlinearity can be addressed by employing a control system, a structure-based solution is preferred, which can curtail the requirement of power consumption, and prevent control instability issues. In this study, shaped-finger comb-drive actuators are used to obtain a linear relationship between the control voltage and the attenuation in planar MEMS VOAs. Two types, i.e. shutter-type and reflective-type, of VOAs are examined. First, the objective differential equation is established based on attenuation-displacement relationships, electrostatic/mechanical force balance equation, and the design objective (linearity) equation. Then, the differential equation (in terms of 2-D comb capacitance) is solved by using the Euler's method, and the finger gaps are calculated by using a conformal mapping method. When a single comb-drive actuator is used, an excluded zone needs to be introduced around the region of small displacement. Effects of the width of the excluded zone to the device characteristics are studied. The issue of zone exclusion can be addressed by adopting dual (control and bias) combs. The effects of design parameters to the VOA performances are studied. It is shown that the planar MEMS VOAs with linear attenuation-voltage relationships can be designed successfully by using the proposed method. © 2020 Elsevier B.V., All rights reserved.
  • Conference Object
    Citation - Scopus: 2
    Planar MEMS Variable Optical Attenuators (VOAs) With Linear Attenuation-Voltage Characteristics
    (IEEE, 2019-05) Hah, Dooyoung
    Variable optical attenuators (VOAs) are essential components in wavelength division multiplexing (WDM) networks, light waveform generators, and optical fiber test equipment. Among various types of planar MEMS VOAs, a shutter type and a reflective type have been most frequently studied so far. In a shutter type, a knife-edge-like beam blocker is inserted in between the butt-coupled input and output fibers, partially obstructing the coupling between the fibers. In a reflective type, a mirror that is placed in the optical path controls the alignment between the fibers to result in attenuation. The movement of the shutter or the mirror is controlled by MEMS actuators, such as comb-drive actuators. In most of the planar MEMS VOAs reported, the relationship between the attenuation and the control voltage has been highly nonlinear. This nonlinearity results in uneven resolution throughout the attenuation range. Although this nonlinearity can be addressed by employing a control system, a structure-based solution is preferred, which can curtail the requirement of power consumption, and prevent control instability issues. In this study, shaped-finger comb-drive actuators are used to obtain a linear relationship between the control voltage and the attenuation in planar MEMS VOAs. Two types, i.e. shutter-type and reflective-type, of VOAs are examined. First, the objective differential equation is established based on attenuation-displacement relationships, electrostatic/mechanical force balance equation, and the design objective (linearity) equation. Then, the differential equation (in terms of 2-D comb capacitance) is solved by using the Euler's method, and the finger gaps are calculated by using a conformal mapping method. When a single comb-drive actuator is used, an excluded zone needs to be introduced around the region of small displacement. Effects of the width of the excluded zone to the device characteristics are studied. The issue of zone exclusion can be addressed by adopting dual (control and bias) combs. The effects of design parameters to the VOA performances are studied. It is shown that the planar MEMS VOAs with linear attenuation-voltage relationships can be designed successfully by using the proposed method.
  • Article
    Citation - WoS: 5
    Citation - Scopus: 5
    Linear Variable Optical Attenuators With Shaped-Finger Comb-Drive Actuators
    (Optical Soc Amer, 2020-01-06) Hah, Dooyoung
    A 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 America
  • Conference Object
    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., 2018-05) 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.
  • Article
    Citation - WoS: 7
    Citation - Scopus: 6
    Absorption Enhancement by Semi-Cylindrical Structures for an Organic Solar Cell Application
    (Optical Soc Amer, 2020-09-25) 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