arXiv subjects

Jiyun Kim

Publications and source records attributed to Jiyun Kim.

9 recordsLinked to original sources

arXiv2026

Twist-Reconfigurable van der Waals Moir\'e Photonic Crystals

Moir\'e photonics has emerged as a fascinating concept to design and in situ control of the optical bands. Moir\'e enabled light localisation arises from the relative twist between periodic layers, rather than from fixed, pre-fabricated cavity features. So far, however, the realisation of practical moir\'e photonic crystals in the visible range has been elusive, due to challenges in engineering nanoscale structures and twisting them dynamically post fabrication. Here, we realise a mechanically reconfigurable moir\'e photonic crystal, comprising from two patterned van der Waals crystals (tungsten di sulphide, WS$_2$) slabs separated by an optically active hexagonal boron nitride (hBN) spacer. We reconfigured the same pair of WS$_2$ slabs from a twist angle of 3.8{\deg} to 8.4{\deg} and reconstructed their three-dimensional dispersion using momentum-resolved reflectivity spectroscopy. Further, by reducing the twist angle between the slabs, we observe a denser manifold of folded and hybridised resonances that coincides with a 30-fold enhancement of emission from embedded colour centres. Our results open exciting opportunities for in-situ dispersion engineering and programmable light matter interactions employing van der Waals nanostructures.

physics.optics↗

arXiv2026

On chip, multifunctional quantum sensing using single spins in a van der Waals crystal

Nanoscale thermometry and magnetometry are in high demand across a wide range of scientific and technological applications. In this context, optically addressable spins in solids have emerged at the forefront of on-chip quantum sensing. However, simultaneous quantum sensing of multiple parameters (e.g., temperature and magnetic field) using the same spin sensor remains challenging due to cross-sensitivity to multiple physical quantities. Here, we demonstrate independent dual sensing of temperature and magnetic field using single quantum emitters in hexagonal boron nitride (hBN). We experimentally verify the independent response of the zero-phonon line (ZPL) position to temperature and of optically detected magnetic resonance (ODMR) to magnetic fields. Furthermore, we demonstrate local temperature sensing of a microcircuit while simultaneously measuring an external magnetic field. Our results establish quantum emitters in hBN as a robust platform for multifunctional quantum sensing under realistic operating conditions.

quant-ph↗

arXiv2026

In-situ Silicon Doped hBN by High-Temperature Molecular Beam Epitaxy Enables Single Photon Emission

Hexagonal boron nitride (hBN) has emerged as a leading host for optically active quantum defects. Yet introduction of specific impurity species other than carbon remains unexplored. Here, we demonstrate an in-situ silicon doping of hBN grown by high-temperature molecular beam epitaxy (HT-MBE). By systematically varying the growth temperature from 900 to 1390 {\deg}C under a constant silicon flux, we establish an optimal window for Si incorporation to host a diverse range of emitters from 430-750 nm at room temperature. By transferring silicon-doped hBN film on SiO$_2$ substrate, we verified that single photon emitter activity was sustained in the hBN, demonstrating compatibility with device integration. The plausible origins of the observed optical transitions were discussed, and several potential candidates were proposed. Our results demonstrate a step toward a comprehensive understanding of in-situ doping of hBN and its utilization for quantum photonic applications.

physics.optics↗

arXiv2026

Near-UV Single-Pixel Imaging with All-Inorganic Lead-Free Perovskite

Single-pixel imaging (SPI) is a powerful computational imaging technology that reconstructs spatial information from sequentially encoded optoelectrical signals without pixelated detector arrays. Solution-processible metal halide perovskites are promising photoactive candidates for SPI, but the toxicity of lead-based compositions remains a critical barrier to practical development. Here, we demonstrate one-step fabrication of low-dimensional, lead-free K$_2$CuBr$_3$ thin film as near-UV photoactive channels for single-pixel imaging. By systematic antisolvent engineering, compact and uniform K2CuBr3 films are obtained and integrated into planar photoconductors devices. The resulting photodetectors exhibit stale photoswitching under 405 nm illumination, low dark current on the order of $10^{-10}$ A, with fast response and recovery time 38.82 and 61.94 $\mu$s, respectively. Integrated into an SPI configuration, the K2CuBr3 photoconductor successfully reconstructs near-UV images, with the signal-to-noise ratio improving from 16.4 to 31.7 dB as the illumination irradiance increases. This work highlights solution-processed lead-free copper halides as promising photoactive materials for compact, non-toxic and cost-effective UV computational imaging systems.

physics.optics↗

arXiv2026

Raon-Speech Technical Report

We present Raon-Speech, a top-performing 9B-parameter speech language model (SpeechLM) for English and Korean speech understanding, answering, and generation, and Raon-SpeechChat, a high-performing full-duplex extension for natural real-time conversation. Raon-Speech successfully transforms a pre-trained LLM into a SpeechLM that both understands and generates speech while preserving strong text capabilities. It trains on 1.38M hours of highly curated English and Korean speech and text datasets with the following training stages: (1) speech modules alignment, (2) end-to-end SpeechLM pre-training with knowledge distillation, and (3) multi-task preference optimization-based post-training. Across 42 English and Korean speech and text benchmarks, Raon-Speech establishes the strongest overall profile on speech-centric tasks in our comparison against eight similarly sized recent audio foundation models, including Qwen2.5-Omni and Fun-Audio-Chat, while preserving strong text question answering performance. Building upon it, Raon-SpeechChat enables natural full-duplex conversation by continual training on 119K hours of time-aligned real and synthetic dialogue data. It proceeds through three complementary training stages: (1) causal encoder adaptation, (2) full-duplex pre-training, (3) full-duplex fine-tuning for voice and role-control. On multiple full-duplex benchmarks, Raon-SpeechChat shows its clearest strengths on the turn-taking and interruption-sensitive behaviors covered by FDB v1.0, and remains competitive across the broader full-duplex evaluation suite. We open-source all model checkpoints, the training and inference pipeline, and an interactive demo.

cs.CL↗

arXiv2026

Harnessing Non-Boltzmann Steady States in Lanthanide Nanocrystals for Mid-Infrared Optoelectronics

Converting mid-infrared (MIR) radiation to visible or near-infrared wavelengths is essential for imaging and sensing, yet achieving sensitive, low-power, and scalable detection remains challenging. Lanthanide nanocrystals provide an alternative through ratiometric luminescence but are typically constrained by Boltzmann statistics, which tie population distributions to lattice temperature and limit signal contrast. Here we show that MIR irradiation rebalances dissipative relaxation pathways, driving lanthanide emitters into a non-Boltzmann steady state that enables non-thermal control of population distributions. This allows emission behaviors inaccessible under thermal equilibrium. We exploit this regime to achieve linear MIR detection with respect to MIR power across 6.8 to 8.6 micrometers. The ratiometric response is intrinsically independent of the pump power, enabling operation at an ultralow excitation power of 10 uW, several orders of magnitude lower than conventional approaches. Using standard silicon photodetectors, we then demonstrate room-temperature MIR imaging with detection limits approaching 4 nW um-2. Our results establish lanthanide nanoparticles as an efficient platform for MIR conversion and sensing in nanophotonic systems.

physics.optics↗

arXiv2025

WaggleNet: A LoRa and MQTT-Based Monitoring System for Internal and External Beehive Conditions

Bee populations are declining globally due to habitat loss, pesticide exposure, and climate change, threatening agricultural productivity and food security. While existing smart beehive systems monitor internal conditions, they typically overlook external environmental factors that significantly influence colony health, and are constrained by high cost, limited scalability, and inadequate contextual analysis. We present WaggleNet, a novel dual-scope monitoring system that simultaneously captures both internal hive conditions and external environmental parameters using a cost-effective LoRa-MQTT architecture. Our system deploys modular worker nodes ($\sim$\$15 each) equipped with temperature, humidity, light, and GPS sensors both inside and around beehives. A master node functions as a LoRa-MQTT gateway, forwarding data to a cloud server with a mobile application interface. Field experiments confirmed reliable operation with 100\% packet delivery over 110 meters in line-of-sight conditions and 95 meters in obstructed environments, including successful deployment inside wooden hive structures. Our system demonstrated stable end-to-end latency under 5 seconds and continuous operation over a two-month period across diverse environmental conditions. By bridging the gap between internal and external monitoring, WaggleNet enables contextual anomaly detection and supports data-driven precision beekeeping in resource-constrained settings.

cs.NI↗

arXiv2023

Spin-Orbital Coupling in All-Inorganic Metal-Halide Perovskites: the Hidden Force that Matters

Highlighted with improved long-term thermal and environmental stability, all-inorganic metal halide perovskites exhibit tunable physical properties, cost-effective synthesis, and satisfactory optoelectronic performance, attracting increasing research interests worldwide. However, a less explored feature of these materials is their strong spin-orbit coupling (SOC), which is the hidden force influencing not only band structure but also properties including magnetoresistance, spin lifetime and singlet-triplet splitting. This review provides an overview of the fundamental aspects and the latest progress of the SOC and debate regarding Rashba effects in all-inorganic metal halide perovskites, providing critical insights into the physical phenomena and potential applications. Meanwhile, crystal structures and photophysics of all-inorganic perovskite are discussed in the context of SOC, along with the related experimental and characterization techniques. Furthermore, a recent understanding of the band topology in the all-inorganic halide perovskites is introduced to push the boundary even further for the novel applications of all-inorganic halide perovskites. Finally, an outlook is given on the potential directions of breakthroughs via leveraging the SOC in halide perovskites.

cond-mat.mtrl-sci↗

arXiv2022

Why Is It Hate Speech? Masked Rationale Prediction for Explainable Hate Speech Detection

In a hate speech detection model, we should consider two critical aspects in addition to detection performance-bias and explainability. Hate speech cannot be identified based solely on the presence of specific words: the model should be able to reason like humans and be explainable. To improve the performance concerning the two aspects, we propose Masked Rationale Prediction (MRP) as an intermediate task. MRP is a task to predict the masked human rationales-snippets of a sentence that are grounds for human judgment-by referring to surrounding tokens combined with their unmasked rationales. As the model learns its reasoning ability based on rationales by MRP, it performs hate speech detection robustly in terms of bias and explainability. The proposed method generally achieves state-of-the-art performance in various metrics, demonstrating its effectiveness for hate speech detection.

cs.CL↗