arXiv subjects

Bongjun Choi

Publications and source records attributed to Bongjun Choi.

12 recordsLinked to original sources

arXiv2026

Unveiling charge dynamics on the generation of high extinction wide pulse generation on thin-film lithium tantalate

We experimentally demonstrate high-extinction optical pulse generation in thin-film lithium tantalate (TFLT), achieving a wide range of pulse widths with sharp rise and fall edges, without measurable distortion or long temporal tails. We further provide a direct one-to-one comparison with thin-film lithium niobate (TFLN) modulators under identical measurement conditions. Our results reveal that the distorted pulse response observed in TFLN originates from charge activation and transport, whereas the distortion-free response in TFLT is enabled by the substantially larger activation energy of defect-related charge carriers. This larger activation barrier suppresses leakage-current-induced charging dynamics, thereby enabling stable and distortion-free pulse generation. The successful generation of high-extinction, distortion-free optical pulses in TFLT can play a significant role in integrated quantum photonic technologies, particularly for qubit preparation, measurement, and fast feedback circuits.

physics.optics↗

arXiv2026

Compositional gradient engineering for enhanced ferroelectricity in ultrathin AlScN

Ferroelectric AlScN is promising for CMOS-compatible non-volatile memory, but thickness scaling is limited by leakage, premature breakdown, and defect-mediated failure. Here we show that compositional grading within a continuous wurtzite AlN-AlScN lattice mitigates these limitations by distributing structural and polarization discontinuities across the film thickness, reducing defect formation and local field concentration. In a 20 nm graded heterostructure, monotonic Sc incorporation and AlN-rich boundaries produce reversible ferroelectric switching, an as-grown metal-polar state, a 21% higher breakdown field, 10% enhanced remanent polarization, and 40x higher resistivity relative to homogeneous AlScN. Time-domain PUND measurements reveal strongly suppressed post-switching leakage, consistent with reduced defect-assisted and polarization-coupled conduction. This improved dielectric robustness enables ferroelectric functionality in 5 nm graded stacks containing only a 2 nm $\mathrm{Al}_{0.64}\mathrm{Sc}_{0.36}\mathrm{N}$ region, with measurable switching near 1 V. These results establish compositional grading as a defect- and field-management strategy for scalable ultrathin wurtzite ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2026

Ultraviolet Exciton-Polaritons in Silver Phenylthiolate

Ultraviolet (UV) exciton-polaritons (EPs) enable nonlinear optics, polaritonic lasing, and polariton-mediated photochemistry in the short-wavelength regime, yet progress has been limited due to the scarcity of materials that combine large oscillator strength with stable and narrow UV excitons. Here, we demonstrate UV EPs in silver phenylthiolate (Thiorene, AgSPh), a van der Waals (vdW) layered metal-organic chalcogenolate (MOC) that forms a natural multi-quantum-well (MQW) architecture showing strong excitonic features. Imaging spectroscopic ellipsometry reveals a pronounced in-plane excitonic resonance at 3.46 eV with a narrow linewidth of ~ 60 meV, strong UV birefringence ({\Delta}n ~ 0.3), and a high refractive index (n ~ 2.1). Temperature-dependent photoluminescence (PL) shows excitonic emission with a large Stokes shift and substantial exciton-phonon coupling. In both open (self-cavity) and closed cavities, thickness-dependent and angle-resolved reflectance spectra exhibit clear anticrossing, yielding large Rabi splittings of approximately 500 meV. These values are among the largest reported in the UV, positioning thiorene as a promising platform for UV polariton lasers and polariton-enabled photochemistry.

physics.optics↗

arXiv2026

Controlled non-volatile modulation of optical dispersion in monolayer tungsten disulfide via ferroelectric polarization patterning

The manipulation of optical properties, including reflection, refraction, polarization, phase, and frequency, has long been central to advancing photonic and optoelectronic technologies. However, existing electro-optical approaches rely on volatile mechanisms that require continuous power consumption. Here, we demonstrate strong, nonvolatile modulation of optical dispersion in monolayer tungsten disulfide (ML WS2) using patterned ferroelectric domains in aluminum scandium nitride (AlScN). By locally poling ferroelectric domains into opposite states, we achieve substantial manipulation of the complex refractive index (Delta n > 0.7, Delta k > 0.4) and excitonic energy shifts (~50 meV) in ML WS2, comparable to previous gate-tuning approaches while eliminating continuous power consumption. We introduce an asymmetric screening model that reveals how ferroelectric polarization induces carrier-density-dependent Coulomb screening, leading to distinct excitonic behaviors between electron- and hole-doped regions. Furthermore, we demonstrate a gate-free lateral p-n homojunction with a rectification ratio of 6e10^3, formed through spatial carrier redistribution. These findings establish ferroelectric/2D heterostructures as a powerful platform for nonvolatile optical dispersion engineering, enabling energy-efficient, reconfigurable photonic and optoelectronic devices.

physics.optics↗

arXiv2026

Photoinduced metastable cation disorder in metal halide double perovskites

Lead-free perovskites have emerged as environmentally benign alternatives to lead-halide counterparts for optoelectronics. Among them, the double perovskite Cs2AgInCl6 family exhibits remarkable white-light emission with proper composition engineering, enabled by strong electron-phonon coupling and the formation of self-trapped excitons (STEs). Despite these advantages, the fundamental photo- and structural dynamics governing their excited-state behavior remain poorly understood. Here, we report a long-lived metastable phase in the Cs2AgInCl6 double perovskite family and unravel this process and the concomitant electronic and structural evolution using a suite of tools including transient optical spectroscopy, time-resolved X-ray diffraction (TR-XRD) and X-ray absorption (TR-XAS). We show that the photoinduced, transient metastable phase is associated with B-site (Ag-In) disorder, which induces a dramatically reduced optical bandgap. Supported by TR-XRD and first-principles calculations, the Ag-In disorder drives the formation of Ag-rich and In-rich domains with millisecond lifetimes, with lifetimes increasing at lower temperatures. TR-XAS further reveals that photogenerated STEs oxidize Ag+ to Ag2+, facilitating this highly temporally asymmetric order-disorder transition. Our findings demonstrate a new mechanism, mediated by hole-localized STE formation, that enables prolongation of transient light-induced states to the multi-millisecond regime in double perovskites, opening possibilities to harvesting the functional properties of metastable phases of these materials.

cond-mat.mtrl-sci↗

arXiv2025

Self-Hybridized Exciton-Polariton Photodetectors From Layered Metal-Organic Chalcogenolates

Exciton-polaritons (EPs) arising from strong light-matter coupling offer new pathways for controlling optoelectronic properties. While typically requiring closed optical cavities for strong coupling, we demonstrate that 2D metal-organic chalcogenolates (MOCs), mithrene (AgSePh), with a high refractive index (~2.5) and strong excitons enable self-hybridized polaritons photodetectors (PDs) without top mirrors, simplifying device architecture. Through thickness-tuned multimode polariton engineering, we achieve photodetection of sub-bandgap photons via lower polariton states, validated through reflectance, photoluminescence (PL), and photocurrent spectroscopy with quantitative theoretical agreement. Trap-assisted two-photon absorption enables sustained strong coupling even under sub-bandgap excitation. The polariton dispersion yields ultrafast group velocities (~65 {\mu}m/ps), extending exciton diffusion lengths from hundreds of nanometers to several micrometers. Strong-coupling devices demonstrate a 2.38-fold enhancement in photo-to-dark current ratio compared to weak-coupling counterparts, establishing a practical route to polariton-enhanced photodetection and light harvesting.

physics.optics↗

arXiv2025

Reconfigurable, non-volatile control of optical anisotropy in ReS2 via ferroelectric gating

Electrically tunable linear dichroism (LD) with non-volatile properties represents a critical yet elusive feature for next-generation integrated photonic elements in practical device architectures. Here, we demonstrate record-breaking, non-volatile control of optical anisotropy in two-dimensional ReS2 via ferroelectric gating with aluminum scandium nitride (AlScN). Our ferroelectric field-effect transistors achieve near-unity (~95%) LD tunability of differential reflectance at room temperature--the highest reported for any electrically controlled 2D optical system. Crucially, the programmed optical states exhibit exceptional retention exceeding 12,000 seconds without applied bias, enabling true non-volatile optical memory. Through combined experimental characterization and ab initio calculations, we reveal that ferroelectric polarization switching induces substantial asymmetric charge transfer to ReS2, selectively populating conduction band states and triggering structural distortions that dramatically enhance optical anisotropy in the "up" polarization state while leaving the "down" state unperturbed. This ferroelectric-semiconductor coupling provides a universal platform for voltage-programmable, energy-efficient photonic devices with dynamic polarization control, addressing critical needs in integrated photonics as well as programmable far-field optics and telecommunications infrastructure.

physics.optics↗

arXiv2025

Natural Hyperbolicity of Hexagonal Boron Nitride in the Deep Ultraviolet

Hyperbolic media enable unique optical phenomena including hyperlensing, negative refraction, enhanced photonic density of states (PDOS), and highly confined polaritons. While most hyperbolic media are artificially engineered metamaterials, certain natural materials with extreme anisotropy can exhibit hyperbolic dispersion. Here, we report the first observation of natural hyperbolic dispersion in hexagonal boron nitride (hBN) in the deep-ultraviolet (DUV) regime, induced by strong, anisotropic exciton resonances. Using imaging spectroscopic ellipsometry (ISE), we characterize the complex dielectric function along in-plane and out-of-plane directions down to 190 nm (6.53 eV), revealing a type-II hyperbolic window in the DUV regime. This hyperbolicity supports hyperbolic exciton polaritons (HEP) with high directionality and slow group velocity. Our findings establish hBN as a promising platform for nanophotonic applications in the technologically significant DUV spectral range.

physics.optics↗

arXiv2025

FedRef: Bayesian Fine-Tuning using a Reference Model to Mitigate Catastrophic Forgetting for Heterogeneous Federated Learning

Federated learning (FL) enables collaborative model training across distributed clients while preserving data privacy. However, data and system heterogeneity often cause catastrophic forgetting and unbounded drift in model updates, leading to degraded predictive performance and increased client-side computation. To address these challenges, we propose FedRef, a Bayesian fine-tuning method that leverages a reference model constructed from previous global models. FedRef integrates a MAP-based regularization term that calibrates global model updates toward a temporally aggregated reference model, thereby mitigating catastrophic forgetting and improving update stability. Unlike prior approaches, FedRef performs all fine-tuning operations on the server side, reducing client-side computational overhead while maintaining effective global optimization. Experiments on image classification (FEMNIST, CINIC-10) and medical image segmentation (FeTS2022) demonstrate that FedRef achieves superior predictive performance and faster convergence under heterogeneous, non-IID settings, while significantly lowering client-side computation compared with existing methods. These results highlight FedRef as an efficient and robust optimization framework for real-world FL scenarios.

cs.LG↗

arXiv2025

High-Temperature-Resilient Hyperbolicity in a Mixed-Dimensional Superlattice

Hyperbolic superlattices are used for sub-wavelength focusing, cloaking, and optical thermal management. Typically, these superlattices are constructed of layers of noble metals and insulators. Despite these systems displaying excellent optical performance, the poor thermal stability of noble metals prevents their application in high-temperature environments. Instead, CMOS-compatible transition-metal nitrides are often substituted for noble metals in plasmonic systems since they have high thermal stability at the expense of optical properties. Here, we fabricate hyperbolic titanium nitride (TiN)/hexagonal boron nitride (hBN) superlattices with 3D-2D interfaces. The mixed-dimensional nature of the interfaces prevents atoms from diffusing across the interface at high temperatures. The hyperbolicity of the superlattice is found to be unaffected by annealing at high temperature (800 oC for 10 hrs), and TiN/hBN is found to have a larger hyperbolic figure of merit than similar superlattices.

physics.optics↗

arXiv2023

Giant Optical Anisotropy in 2D Metal-Organic Chalcogenates

Optical anisotropy is a fundamental attribute of some crystalline materials and is quantified via birefringence. A birefringent crystal not only gives rise to asymmetrical light propagation but also attenuation along two distinct polarizations, a phenomenon called linear dichroism (LD). Two-dimensional (2D) layered materials with high in- and out-of-plane anisotropy have garnered interest in this regard. Mithrene, a 2D metal-organic chalcogenate (MOCHA) compound, exhibits strong excitonic resonances due to its naturally occurring multi-quantum well (MQW) structure and in-plane anisotropic response in the blue wavelength (~400-500 nm) regime. The MQW structure and the large refractive indices of mithrene allow the hybridization of the excitons with photons to form self-hybridized exciton-polaritons in mithrene crystals with appropriate thicknesses. Here, we report the giant birefringence (~1.01) and tunable in-plane anisotropic response of mithrene, which stem from its low symmetry crystal structure and unique excitonic properties. We show that the LD in mithrene can be tuned by leveraging the anisotropic exciton-polariton formation via the cavity coupling effect exhibiting giant in-plane LD (~77.1%) at room temperature. Our results indicate that mithrene is an ideal polaritonic birefringent material for polarization-sensitive nanophotonic applications in the short wavelength regime.

physics.optics↗

arXiv2023

Ultrastrong Light-Matter Coupling in 2D Metal-Chalcogenates

Hybridization of excitons with photons to form hybrid quasiparticles, exciton-polaritons (EPs), has been widely investigated in a range of semiconductor material systems coupled to photonic cavities. Self-hybridization occurs when the semiconductor itself can serve as the photonic cavity medium resulting in strongly-coupled EPs with Rabi splitting energies > 200 meV at room temperatures which recently were observed in layered two-dimensional (2D) excitonic materials. Here, we report an extreme version of this phenomenon, an ultrastrong EP coupling, in a nascent, 2D excitonic system, the metal organic chalcogenate (MOCHA) compound named mithrene. The resulting self-hybridized EPs in mithrene crystals placed on Au substrates show Rabi Splitting in the ultrastrong coupling range (> 600 meV) due to the strong oscillator strength of the excitons concurrent with the large refractive indices of mithrene. We further show bright EP emission at room temperature as well as EP dispersions at low-temperatures. Importantly, we find lower EP emission linewidth narrowing to ~1 nm when mithrene crystals are placed in closed Fabry-Perot cavities. Our results suggest that MOCHA materials are ideal for polaritonics in the deep green-blue part of the spectrum where strong excitonic materials with large optical constants are notably scarce.

physics.optics↗