Ultra-Thin Highly Absorbing Medium-Based Optical Nanocavity for Photonic and Optoelectronic Devices.

Optical cavities, which generally consist of an optically transparent medium with wavelength-scale thickness, have been widely used in various areas ranging from lasers and modulators to sensors and filters. A trivial optical absorption in the cavity allows incident light to constructively interfere with reflected light many times without serious loss, thus being able to create a resonance at a certain wavelength. However, a traditional optical cavity has faced challenges in achieving an angle-insensitive property, thereby dramatically limiting their applications in a wide variety of fields. In this dissertation, we present several demonstrations, all based on optical nanocavities featuring strong resonance behaviors in highly absorbing media with the ultra-thin cavity thickness (< 30 nm) as compared to the wavelength of incident light, which is distinctly different from the conventional optical cavity systems. We firstly demonstrate angle invariant (up to 70°) transmissive and reflective structural color filters with high-color-purity exploiting a concept described above. We also present a new photovoltaic (PV) scheme incorporating novel optical design (ultra-thin cavity) and electrical design (dopant-free amorphous silicon) to create colored semitransparent PV cells, which could be harmoniously integrated with interiors and exteriors of the buildings, such as facades, windows, ceilings, and walls. This enables large surfaces of architectures to be efficiently utilized to generate the electric power. ~3 (2)% of power conversion efficiency with desired reflective (transmissive) colors that are insensitive to the angle of incidence and the polarization state of incident light is achieved. To improve the power conversion efficiency of the colored PV cells, we propose and experimentally demonstrate a spectrum splitting method and microcavity-integrated PV scheme, both of which show ~4% of power conversion efficiency. Lastly, we describe how our strategy could be applied to other applications, such as perovskite PV cells, broadband visible absorbers, and low reflective wire grid polarizers. The presented approach could open door to numerous applications, such as energy-efficient ultra-thin colored display technologies and decorative building-integrated PV.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/113617/1/ktlee_1.pd

A numerically efficient output-only system-identification framework for stochastically forced self-sustained oscillators

Self-sustained oscillations are ubiquitous in nature and engineering. In this paper, we propose a novel output-only system-identification framework for identifying the system parameters of a self-sustained oscillator affected by Gaussian white noise. A Langevin model that characterizes the self-sustained oscillator is postulated, and the corresponding Fokker--Planck equation is derived from stochastic averaging. From the drift and diffusion terms of the Fokker--Planck equation, unknown parameters of the system are identified. We develop a numerically efficient algorithm for enhancing the accuracy of parameter identification. In particular, a modified Levenberg--Marquardt optimization algorithm tailored to output-only system identification is introduced. The proposed framework is demonstrated on both numerical and experimental oscillators with varying system parameters that develop into self-sustained oscillations. The results show that the computational cost required for performing the system identification is dramatically reduced by using the proposed framework. Also, system parameters that were difficult to be extracted with the existing method could be efficiently computed with the system identification method developed in this study. Pertaining to the robustness and computational efficiency of the presented framework, this study can contribute to an accurate and fast diagnosis of dynamical systems under stochastic forcing.Comment: 17 pages, 10 figure

Triclinic Na3.12Co2.44(P2O7)(2) as a High Redox Potential Cathode Material for Na-Ion Batteries

Two types of sodium cobalt pyrophosphates, triclinic Na3.12Co2.44(P2O7)(2) and orthorhombic Na2CoP2O7, are compared as high-voltage cathode materials for Na-ion batteries. Na2CoP2O7 shows no electrochemical activity, delivering negligible capacity. In contrast, Na3.12Co2.44(P2O7)(2) exhibits good electrochemical performance, such as high redox potential at ca. 4.3 V (vs. Na/Na+) and stable capacity retention over 50 cycles, although Na3.12Co2.44(P2O7)(2) delivered approximately 40 mA h g(-1). This is attributed to the fact that Na2CoP2O7 (similar to 3.1 angstrom) has smaller diffusion channel size than Na3.12Co2.44(P2O7)(2) (similar to 4.2 angstrom). Moreover, the electrochemical performance of Na3.12Co2.44(P2O7)(2) is examined using Na cells and Li cells. The overpotential of Na cells is smaller than that of Li cells. This is due to the fact that Na3.12Co2.44(P2O7)(2) has a smaller charge transfer resistance and higher diffusivity for Na+ ions than Li+ ions. This implies that the large channel size of Na3.12Co2.44(P2O7)(2) is more appropriate for Na+ ions than Li+ ions. Therefore, Na3.12Co2.44(P2O7)(2) is considered a promising high-voltage cathode material for Na-ion batteries, if new electrolytes, which are stable above 4.5 V vs. Na/Na+, are introduced.

Angular‐ and polarization‐independent structural colors based on 1D photonic crystals

Wide‐angle, polarization‐independent structural reflective colors from both directions based on a one‐dimensional photonic crystal are demonstrated. Our device produces a distinct and saturated color with high angular tolerant performance up to ±70° for any polarization state of an incident light wave, which is highly desirable for a broad range of research areas. Moreover, the purity of the color and luminous intensity of the proposed device are improved as compared to conventional colorant‐based color filters and colloidal glasses. The present approach may have the potential to replace existing color filters and pigments and pave the way for various applications, including color displays and image sensor technologies.A 1D photonic crystal‐based structural reflective color with angle‐invariant, polarization‐independent, and high‐purity characteristics is presented. Our proposed device is capable of creating a distinctive color that is insensitive with respect to the angle of incidence up to ±70° regardless of the polarization state of incident light. The presented approach can open the door to numerous applications, such as colored display technologies and imaging sensors.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111776/1/lpor201500029.pd

Development of a hybrid magnetic resonance/computed tomography-compatible phantom for magnetic resonance guided radiotherapy

The purpose of the present study was to develop a hybrid magnetic resonance/computed tomography (MR/CT)-compatible phantom and tissue-equivalent materials for each MR and CT image. Therefore, the essential requirements necessary for the development of a hybrid MR/CT-compatible phantom were determined and the development process is described. A total of 12 different tissue-equivalent materials for each MR and CT image were developed from chemical components. The uniformity of each sample was calculated. The developed phantom was designed to use 14 plugs that contained various tissue-equivalent materials. Measurement using the developed phantom was performed using a 3.0-T scanner with 32 channels and a Somatom Sensation 64. The maximum percentage difference of the signal intensity (SI) value on MR images after adding K2CO3 was 3.31%. Additionally, the uniformity of each tissue was evaluated by calculating the percent image uniformity (%PIU) of the MR image, which was 82.18 ±1.87% with 83% acceptance, and the average circular-shaped regions of interest (ROIs) on CT images for all samples were within ±5 Hounsfield units (HU). Also, dosimetric evaluation was performed. The percentage differences of each tissue-equivalent sample for average dose ranged from -0.76 to 0.21%. A hybrid MR/CT-compatible phantom for MR and CT was investigated as the first trial in this field of radiation oncology and medical physics

Multi-Color Luminescence Transition of Upconversion Nanocrystals via Crystal Phase Control with SiO2 for High Temperature Thermal Labels

Upconversion nanocrystals (UCNs)-embedded microarchitectures with luminescence color transition capability and enhanced luminescence intensity under extreme conditions are suitable for developing a robust labeling system in a high-temperature thermal industrial process. However, most UCNs based labeling systems are limited by the loss of luminescence owing to the destruction of the crystalline phase or by a predetermined luminescence color without color transition capability. Herein, an unusual crystal phase transition of UCNs to a hexagonal apatite phase in the presence of SiO2 nanoparticles is reported with the enhancements of 130-fold green luminescence and 52-fold luminance as compared to that of the SiO2-free counterpart. By rationally combining this strategy with an additive color mixing method using a mask-less flow lithography technique, single to multiple luminescence color transition, scalable labeling systems with hidden letters-, and multi-luminescence colored microparticles are demonstrated for a UCNs luminescence color change-based high temperature labeling system

Novel water filtration of saline water in the outermost layer of mangrove roots

The scarcity of fresh water is a global challenge faced at present. Several desalination methods have been suggested to secure fresh water from sea water. However, conventional methods suffer from technical limitations, such as high power consumption, expensive operating costs, and limited system durability. In this study, we examined the feasibility of using halophytes as a novel technology of desalinating high-concentration saline water for long periods. This study investigated the biophysical characteristics of sea water filtration in the roots of the mangrove Rhizophora stylosa from a plant hydrodynamic point of view. R. stylosa can grow even in saline water, and the salt level in its roots is regulated within a certain threshold value through filtration. The root possesses a hierarchical, triple layered pore structure in the epidermis, and most Na+ ions are filtered at the first sublayer of the outermost layer. The high blockage of Na+ ions is attributed to the high surface zeta potential of the first layer. The second layer, which is composed of macroporous structures, also facilitates Na+ ion filtration. This study provides insights into the mechanism underlying water filtration through halophyte roots and serves as a basis for the development of a novel bio-inspired desalination method.Creative Research Initiative (Diagnosis of Biofluid Flow Phenomena and Biomimic Research) of the Ministry of Science, ICT and Future Planning (MSIP) , National Research Foundation (NRF) of Korea , ICT R&D program of MSIP/IITP (Korea