https://scholarworks.unist.ac.kr/handle/201301/78 Wed, 08 Apr 2026 00:31:43 GMT 2026-04-08T00:31:43Z https://scholarworks.unist.ac.kr/handle/201301/91042 Title: Research on the Magnetocaloric Effect of Lanthanum Iron Silicon Alloy and Heusler Alloy Materials Author(s): WANG, ZHIHAO Abstract: In today's advanced technological era, many technical fields cannot do without refrigeration technology. Refrigeration technology plays a crucial role in the development of human society. However, the widely used gas compression refrigeration technology in traditional refrigeration still has serious drawbacks, among which the most prominent ones are environmental pollution, low efficiency and high energy consumption. Therefore, a new type of refrigeration technology to replace the traditional gas compression refrigeration technology is needed. Currently, magnetic refrigeration technology is such a highly anticipated new technology. Magnetic refrigeration c ompared with traditional gas compression refrigeration, the magnetic refrigeration technology has many advantages, such as high efficiency, low energy consumption, low noise and environmental friendliness. In the past half century, significant breakthroughs have been made in the research of magnetic refrigeration technology. Many magnetic refrigeration materials with huge magnetocaloric effects have been discovered. Magnetic refrigeration has been widely applied in many research fields, and it is gradually entering people's lives in room-temperature refrigerators. Among the widely studied magnetocaloric effect materials, the NaZn13-type La(FeSi)13 alloy with a cubic structure is highly favored due to its low production cost, controllable Curie temperature TC , large MS , relatively stable chemical properties, and high saturation magnetization. Particularly, La(FeCoSi)13Bx alloy, as a new type of cubic NaZn13-type La(FeSi)13 series magnetic refrigeration working material, has a TC close to room temperature. It not only has a large MS but is also easy to be industrially synthesized, and is expected to become the standard working material for future room- temperature magnetic refrigerators. In the meanwhile, in Ni-Mn-(In, Sn, Ga, Sb) alloys, the magnetic- induced structural phase transition (martensite to austenite) is closely coupled with the magnetic order transition. At this time, the increase in structural order leads to a decrease in lattice entropy, resulting in adiabatic heat absorption. Moreover, since the contribution of lattice entropy at this time is dominant, the adiabatic heat absorption response caused by it is larger than that of traditional magnetic thermal materials. Among them, nickel-manganese-indium alloys are extensively studied magnetic thermal materials. By adjusting the content of indium, the martensite phase transition temperature (TM), Curie temperature (TC), and martensite Curie temperature (TM C) can be effectively controlled, thereby optimizing its magnetic heat performance. This paper starts from the practical application perspective, selects low-cost industrial purity raw materials, and improves the composition ratio of the existing magnetic refrigeration working material La(FeCoSi)13 alloy. That is, by partially replacing the La and Fe elements in the alloys system, the magnetocaloric effect of La(FeCoSi)13B0.25 alloys is studied. In the heusler alloy system research, the Ni50Mn50-xInx (x=14.5-16) range is selected to systematically explore the regulation laws of In content on the phase transition behavior, magnetic properties, and magnetic heat performance of the alloy, providing a theoretical basis for further component design and performance optimization of the alloy in this system. Major: Department of Physics Sat, 31 Jan 2026 15:00:00 GMT https://scholarworks.unist.ac.kr/handle/201301/91042 2026-01-31T15:00:00Z https://scholarworks.unist.ac.kr/handle/201301/91041 Title: Theoretical Study of Plasma Photonics with Particle-In-Cell Simulation: Plasma Dipole Oscillator, Radial Plasma Oscillator and Gradient Plasma Photonic Crystal Author(s): Lee, Jaeho Abstract: Laser–plasma physics has advanced significantly since the pioneering work of T. Tajima and J. M. Daw- son in 1979, which first demonstrated electron acceleration via laser wakefields [1]. The progress in understanding fundamental laser–plasma phenomena has opened new avenues for diverse applications. The high damage threshold (∼TV/m) and strong electrostatic fields (∼GV/m) inherent to plasmas, when driven by high-power lasers, offer considerable potential for applications such as terahertz (THz) radia- tion sources, attosecond and X-ray pulse generation, and laser wakefield acceleration (LWFA). In recent years, the field of plasma photonics—which explores plasmas as optical media due to their density- dependent refractive index—has attracted renewed interest. With appropriate density modulations, plas- mas can function as optical components such as mirrors [2, 3], waveplates [4], compressors [5–7], grat- ings [8–10], and even amplifiers [11, 12], making them particularly well suited for manipulating ultra- intense laser pulses. This dissertation focuses on three primary categories of plasma photonics: (1) plasma dipole oscillator (PDO), (2) radial plasma oscillator (RPO), and (3) gradient plasma photonic crystal (GPPC). Each of these topics will be carefully discussed based on particle-in-cell (PIC) simula- tions and theoretical analysis. PDO is a type of active plasma photonic structure that can be generated by colliding two detuned laser pulses in plasma. Depending on the laser intensity, the excitation mech- anism can be categorized into a nonlinear current-driven regime or a particle trapping-driven regime, and PDO is primarily utilized as a source of narrowband THz radiation. It can also be considered as a theoretical model for the Langmuir wave collapse occurring in the coronal plasma. RPO is simi- lar to PDO, but it arises in a linearly density-gradient plasma when two co-propagating detuned laser pulses satisfy a resonance condition (∆ω ≈ ωp). Specifically, after the laser pulses pass through the resonance point, the plasma wave number varies over time due to the density gradient, and the plasma wave number approaches zero (kpe ≈ 0). After that, all plasma electrons oscillate in phase. At this point, RPO—resembling a radial plane antenna—is formed, generating a radially polarized THz beam. GPPC is a plasma photonic crystal formed in a density gradient plasma. Such plasma gratings are mainly gen- erated by the beat wave produced through the overlap of incident and reflected laser pulses, and their dynamics are predominantly governed by ponderomotive potential. By tuning parameters such as the grating gap, density amplitude, and density profile, the dispersive properties of the plasma grating can be controlled, making it highly useful for plasma-based pulse compression. Major: Department of Physics Sat, 31 Jan 2026 15:00:00 GMT https://scholarworks.unist.ac.kr/handle/201301/91041 2026-01-31T15:00:00Z https://scholarworks.unist.ac.kr/handle/201301/91039 Title: A Deep Learning Framework for Tracking Motile Bacteria Leveraging Semi-Synthetic Image Augmentation and Quantitative Analysis of Run-and-Tumble Dynamics Author(s): Son, Joowang Abstract: Swimming bacteria are widely studied as a representative model organism in active matter, providing a key experimental system for investigating single-cell self-propulsion and stochastic motility dynamics. In this thesis, I extend this line of research through two complementary studies. First, to quantitatively analyze swimming dynamics of moderately dense populations of rod-shaped bacteria, I develop a tracking pipeline that distinguishes partially overlapped bacterial cells using embedding-based instance segmentation trained solely on semi-synthetically augmented images, eliminating the need for manual labeling. By generating semi-synthetic training images that combine real cell image patches with diverse background conditions, I train an embedding-based instance segmentation model capable of reliably resolving individual cells even under partial overlaps and in complex imaging environments. The complete pipeline, released as an open-source software package named SynEmbTrack, is publicly available on GitHub. Second, using the reconstructed single-cell trajectories, I extract key motility features—including transition rates of run to tumble and tumble to run, run speed, and noises for speed and angular displacements— and analyze their temperature dependence across seven conditions (20◦C–50◦C). To infer the underlying motility parameters, I employ a sequential optimization strategy that evaluates a carefully defined loss function comparing experimental distributions and autocorrelations of the speed and orientation with stochastic run-and-tumble simulations. This approach yields consistent and robust estimates of the run-and-tumble parameters across temperatures, revealing clear temperature dependent trends in motility behavior. Together, these two studies provide an integrated framework that features image-based single-cell tracking and stochastic model–based parameter inference. The results establish a generalizable analytical tool for studying bacterial motility and offer insights into the microscopic mechanisms underlying temperature-dependent swimming dynamics. This framework also provides a basis for extending the analysis to more complex environmental conditions, including spatio-temporal gradients that induce taxis, as well as interactions with the surrounding medium or boundary geometries. Major: Department of Physics Sat, 31 Jan 2026 15:00:00 GMT https://scholarworks.unist.ac.kr/handle/201301/91039 2026-01-31T15:00:00Z https://scholarworks.unist.ac.kr/handle/201301/91040 Title: Sub-10-nm Plasmonic Nanogaps for Raman and Photoluminescence Modulation Author(s): CHEN, CHENG Abstract: With the rapid progress of nanophotonics and nanofabrication technologies, surface plasmons have demonstrated unique capabilities in enhancing light–matter interactions, enabling energy confinement and spectral control at subwavelength scales. Among the various plasmonic phenomena, surface-enhanced Raman scattering (SERS) stands out as a prototypical example, capable of achieving high sensitivity and molecular specificity down to the single-molecule level. Owing to these features, SERS has found extensive applications in diverse areas including bioanalysis, environmental monitoring, molecular analysis, and spectroscopic studies of two-dimensional (2D) materials. The performance of SERS substrates, however, is critically determined by the intensity and spatial distribution of localized electromagnetic fields, particularly the strong coupling that occurs within metallic nanogaps. Both theoretical and experimental studies have confirmed that when the nanogap width is reduced below 10 nm, the electromagnetic field can be tightly confined, leading to orders-of-magnitude enhancement of the Raman signal. Nevertheless, the precise and reproducible fabrication of such sub-10-nm gaps remains a formidable challenge due to intrinsic limitations in the resolution of conventional lithography, resist morphology stability, and proximity effect control. This technical bottleneck not only restricts the large-scale realization of high-performance SERS substrates but also hinders the broader implementation of plasmonic structures for multispectral optical modulation and on- chip photonic integration. To address these challenges, this dissertation systematically investigates the feasibility and underlying mechanisms of a top-down three-step nanofabrication approach—comprising metal deposition, electron-beam lithography (EBL) patterning, and argon ion milling—for constructing metallic nanogap structures and achieving enhanced optical responses. The research is divided into two parts. In the first part, we developed a reproducible fabrication process for square grid nanogap arrays with tunable gap widths ranging from 7 nm to 60 nm and array periods of 150 nm, 200 nm, and 300 nm. The fabricated structures exhibited high uniformity and reproducibility across large areas. Combined experimental measurements and finite-element simulations revealed that the electromagnetic field enhancement and SERS intensity increase exponentially with decreasing gap width, yielding an overall Raman enhancement factor of approximately 10³ under 633 nm excitation. Furthermore, systematic comparison of different array periods revealed a trade-off between hotspot density and proximity effects: while shorter periods increase the number of gaps per unit area, excessive proximity-induced pattern distortion can reduce local field intensity. In addition, few-layer molybdenum disulfide (MoS₂) was transferred onto the nanogap arrays to explore the interplay between plasmonic enhancement and mechanical strain. Raman spectra showed progressive redshifts of characteristic peaks with increasing gap width, indicating that the suspended MoS₂ regions experience tunable tensile strain induced by the underlying nanogap geometry. These results demonstrate that geometric engineering of nanogaps not only optimizes SERS performance but also provides a route to strain control in 2D materials. In the second part, we propose and experimentally verify vertically stacked metal– insulator–metal–insulator–metal (MIMIM) nanogap metasurfaces that support dual Fabry–Pérot (FP) gap-plasmon resonances, enabling tunable multispectral photoluminescence (PL) enhancement. The structures were fabricated by alternately depositing Au and Al₂O₃ layers, defining nanoscale line masks via EBL, and performing directional ion milling to form trapezoidal stacked arrays. Finite-element simulations and reflection spectroscopy confirmed that the upper and lower nanogaps exhibit independent FP resonances at distinct spectral ranges—the visible and near-infrared, respectively—due to their different lateral lengths. PL measurements using Rhodamine 6G (R6G) and IR-820 dyes further demonstrated that each resonance selectively modulates emission within its respective wavelength region. The R6G emission was enhanced and spectrally tuned near the visible FP resonance, whereas the IR-820 emission was modified by the lower-gap resonance in the near-infrared. By systematically varying the EBL exposure dose, we precisely tuned the lateral dimensions of the stacked nanogaps and observed predictable redshifts of both reflection dips and PL peaks, in excellent agreement with simulation results. These findings establish stacked nanogaps as an effective platform for multiwavelength emission control within a single nanostructure. Overall, this dissertation constructs a unified framework that bridges the design principles of planar nanogap arrays for SERS optimization and vertically stacked nanogaps for multimodal PL modulation. Through rigorous integration of deterministic nanofabrication, optical characterization, and electromagnetic modeling, the research elucidates the quantitative relationships among gap geometry, localized field enhancement, and spectral response. The outcomes not only provide reproducible fabrication strategies for sub-10-nm plasmonic cavities but also expand their functional versatility from Raman enhancement and strain engineering in 2D materials to multispectral light-emitting and sensing devices. The results presented herein offer new design concepts and practical methodologies for high-sensitivity molecular detection, tunable photonic interfaces, and multi-band plasmonic integration at the nanoscale. Keywords: Surface plasmon, Nanogap array, SERS; Electron-beam lithography, Strain engineering, Photoluminescence, MIMIM nanogap metasurface. Major: Department of Physics Sat, 31 Jan 2026 15:00:00 GMT https://scholarworks.unist.ac.kr/handle/201301/91040 2026-01-31T15:00:00Z