Myung Lae Lee

Circuit-Model-Based Analysis of a Wireless Energy-Transfer System via Coupled Magnetic Resonances

A simple equivalent-circuit model is developed for a wireless energy-transfer system via coupled magnetic resonances, and a practical design method is also provided. Node equations for the resonance system are built with the method, expanding on the equations for a transformer, and the optimum distances of the coils in the system are derived analytically for optimum coupling coefficients for high transfer efficiency. In order to calculate the frequency characteristics for a lossy system, the equivalent model is established at an electric-design automation tool. The model parameters of the actual system are extracted, and the modeling results are compared with measurements. Through the developed model, it is seen that the system can transfer power over a midrange of a few meters and that impedance matching is important to achieve high efficiency.

Fabrication of MEMS devices by using anhydrous HF gas-phase etching with alcoholic vapor

In silicon surface micromachining, anhydrous HF GPE process was verified as a very effective method for the dry release of microstructures. The developed gas-phase etching (GPE) process with anhydrous hydrogen fluoride (HF) gas and alcoholic vapor such as methanol, isopropyl alcohol (IPA) was characterized and its selective etching properties were discussed. The structural layers are P-doped multi-stacked polysilicon and silicon-on-insulator (SOI) substrates and sacrificial layers are tetraethylorthosilicate (TEOS), low-temperature oxide (LTO), plasma enhanced chemical vapor deposition (PECVD) oxide, phosphosilicate glass (PSG) and thermal oxides on silicon nitride or polysilicon substrates. We successfully fabricated and characterized micro electro mechanical system (MEMS) devices with no virtually process-induced stiction and no residues. The characteristics of the MEMS devices for microsensor and microactuator, microfluidic elements and optical MEMS application were evaluated by experiment.

Development and application of a laterally driven electromagnetic microactuator

A laterally driven electromagnetic microactuator (LaDEM) is introduced, and a micro-optical switch is designed and fabricated as an application. LaDEM offers parallel movement of the microactuator to the silicon substrate surface (in-plane mode). Polysilicon-on-insulator wafers and a reactive ion etching process were used to fabricate high-aspect-ratio vertical microstructures, which allowed the equipping of vertical micromirrors. A fabricated single leaf spring had a width of 1.2 μm, thickness of 16 μm, and length of 920 μm. The resistance of the fabricated leaf spring for the optical switch was 5 Ω. The deflection of the leaf spring started to profoundly increase at about 400 mA, and it showed snap-through phenomenon over that current value. Owing to the snap-through phenomenon, a large deflection of 60 μm was detected at 566 mA.

A polarimetric current sensor using an orthogonally polarized dual-frequency fibre laser

We demonstrate a novel -doped fibre laser operating in orthogonally polarized dual-frequency modes and its application to electric current sensing with frequency read-out. A Faraday rotating mirror and spatial hole burning effects in a gain medium and in a saturable absorber are utilized to control the lasing mode and polarization. The polarization mode beat frequency changes linearly in response to the non-reciprocal circular birefringence induced by an external magnetic field. When the laser was applied to sensing an alternating electric current with a simple phase-locked loop signal processing scheme, the slope coefficient of 8.0 kHz per turn and the noise equivalent current of 460 per turn were obtained. The new current sensor is immune to perturbations in the lead fibres.

Frequency-division-multiplexed polarimetric fiber laser current-sensor array

We demonstrate a novel frequency-domain-multiplexing technique for implementing polarimetric fiber laser current sensors. Each sensor operates at a different polarization-mode beat frequency that is modulated in response to applied electric current. A bank of bandpass filters can be used to separate signals from different sensors. A simple frequency-demodulation technique based on a phase-locked loop is used for signal processing.

Efficiency enhancement of semi-transparent sandwich type CH3NH3PbI3 perovskite solar cells with island morphology perovskite film by introduction of polystyrene passivation layer

Highly semi-transparent sandwich type CH3NH3PbI3 (MAPbI3) island perovskite solar cells with high efficiency were constructed by introduction of a polystyrene (PS) passivation interlayer. The PS insulator can prevent direct contact between the TiO2 electron conductor and the polytriarylamine hole conductor. The PS passivation interlayer could be selectively deposited on bare TiO2 by a consecutive spin-coating and spin-washing process. The average visible transmittance of semi-transparent sandwich type MAPbI3 island perovskite solar cells with and without PS passivation layer was ∼20.9% and ∼18.6%, respectively; its power conversion efficiency was 10.2% for forward scan condition and 10.6% for reverse scan condition (average = 6.17% ± 2.32% for 40 samples) and 5.9% for forward scan condition and 6.6% for reverse scan condition (average = 2.93% ± 1.57% for 40 samples); and its efficiency degradation for 30 days was ∼5% and ∼9%, respectively.

Monolithic Fabry–Perot Wavelength Tunable Filter with Electrothermal Actuation

We report on a micromachined monolithic Fabry–Perot wavelength tunable filter with a thick moving structure operated by an electrothermal actuation. The monolithic structure simplifies the fabrication process and the electrothermal actuation mechanism reduces the required operation voltage. For the wet etching of the AlGaAs sacrificial layer, an HCl-based solution rather than a HF-based one was used because it results in a larger selectivity between the AlxGa1-xAs layers and less damage to the suspended structure. The wavelength tuning range of the 7.64-µm-thick structure was 47 nm for the power consumption of 5 mW, which results in the high tuning efficiency of ~9.9 nm/mW. The wide tuning range of 81.2 nm for the 5.2-µm-thick structure, that is not possible with an electrostatic actuation mechanism due to the occurrence of breakdown, is achieved at the driving voltage below 5.7 V. Due to the simplicity of fabrication and the ease of integration, this structure is advantageous for use in wavelength tunable light sources and photodetectors.

Highly flexible, high-performance perovskite solar cells with adhesion promoted AuCl3-doped graphene electrodes

Super flexible TCO-free FAPbI3−xBrx planar type inverted perovskite solar cells with a 17.9% power conversion efficiency under 1 sun conditions were demonstrated by introducing an APTES (3-aminopropyl triethoxysilane) adhesion promoter between a PET flexible substrate and a AuCl3-doped single-layer graphene transparent electrode (TCE). Due to the formation of covalent bonds by the APTES inter-layer, the AuCl3-GR/APTES/PET substrate had excellent flexibility, whereas the AuCl3-GR/PET substrate and the ITO/PET substrate had significant degradation of the sheet resistance after a bending test due to the break-off or delamination of AuCl3-GR from the PET substrate and the cracking of ITO. Accordingly, the perovskite solar cells constructed on the AuCl3-GR/APTES/PET TCE substrate exhibited excellent bending stability and they maintained their PCE at over 90% of the initial value after 100 bending cycles at R ≥ 4 mm.

Planar latch-up microactuator driven by thermoelastic force

We designed and fabricated a planar-type thermoelastic microactuator with a latch-up operation for optical switching. Latch-up actuation is prerequisite to implement an optical switch with low power consumption and high reliability. The proposed microactuator consists of four cantilever-shaped thermal actuators, four displacement linkages, two shallow arch-shaped leaf springs, a mobile shuttle mass with a micromirror, and four elastic boundaries. The planar microactuator consists of phosphorous-doped 12 micrometers -thick polysilicon as a structural layer and LTO (Low Temperature Oxide) of 3 micrometers thickness as a sacrificial layer on polysilicon substrate. The experimental displacement of the microactuator was more than 21 micrometers at 10V input voltage for the prototype of a thermoelastic microactuator. The frequency response for square wave input was measured up to 50Hz, which was the highest frequency we can detect using optical microscope for now. The proposed microactuators have advantages of easy assembly with other optical component by way of fiber alignment in the substrate plane, and its fabrication process features simplicity while retaining batch-fabrication economy.

Fabrication of surface-micromachined thermally driven micropump by anhydrous HF gas-phase etching with 2-propanol

In silicon surface micromachining, the HF GPE process was verified as a very effective method for the dry release of microstructures. The developed GPE system with anhydrous HF gas and 2-propanol vapor was characterized and its selective etching properties were discussed. The polysilicon membrane was used as a structural layer and LTO and PECVD oxide as a sacrificial layer. We successfully fabricated the surface micromachined microstructures of a thermally driven micropump with no virtually process-induced stiction and no residues after the GPE of sacrificial oxides on polysilicon substrates.