arXiv subjects

Ki Tae Nam

Publications and source records attributed to Ki Tae Nam.

11 recordsLinked to original sources

arXiv2026

Broadband Single-Particle Absorption Circular Dichroism Reveals Chiroptical Heterogeneity in Gold Helicoids

Quantitative measurements of absorption circular dichroism (CD) at the single-particle level are essential for understanding how individual chiral nanostructures dissipate optical energy, yet broadband absorptance measurements remain experimentally challenging. Here, we introduce a wavelength-tunable integrating-sphere microscope that determines the absorptance of individual nanoparticles under right- and left-circularly polarized illumination through direct optical energy balance, enabling broadband measurement of the absorption dissymmetry factor, gabs. Measurements of achiral gold nanospheres and strongly anisotropic gold nanorods establish the apparent absorption CD baseline and demonstrate minimal detectable linear-dichroism-to-circular-dichroism (LD-to-CD) leakage under the experimental conditions. Applying this approach to 87 chiral gold helicoids synthesized using L-glutathione (L-helicoids) and 96 synthesized using D-glutathione (D-helicoids) reveals mean particle-level gabs values of opposite sign, with a statistically significant difference between the two populations. Individual particles nevertheless exhibit pronounced heterogeneity in response sign, magnitude, spectral position, and line shape. More than one-third of the particles in each population display opposite-sign absorption CD responses relative to their population-average tendency. Correlative SEM analysis of the L-helicoid population further shows that opposite-sign responses persist among isolated particles exhibiting the characteristic projected helicoid morphology, indicating that aggregation and gross differences in projected morphology are insufficient to explain the observed heterogeneity. These results establish broadband single-particle absorption CD spectroscopy as a direct probe of absorptive chiroptical heterogeneity and reveal particle-specific responses obscured by ensemble averaging.

physics.optics↗

arXiv2026

Helicity-Resolved Spatiotemporal Mapping of Chiral Plexcitons in Helicoids

Plasmon-exciton hybrids, or plexcitons, offer deeply subwavelength light-matter interactions with versatile pathways for energy redistribution. Incorporating chirality into such systems is particularly compelling, enabling spin-sensitive optical functionality that can operate on ultrafast timescales and within ultracompact volumes. Despite recent progress in chiral plexcitonic systems, how structural chirality and plasmon-exciton coupling determine chiroptical spectra and ultrafast energy flow remains elusive. Here we realize chiral plexcitons by functionalizing intrinsically chiral gold helicoid nanoparticles with molecular J-aggregates. Within a non-Hermitian framework, we trace the microscopic origin of the helicoid chiroptical response and its coupling to the excitonic transition, revealing how the helicity of light selectively addresses distinct hybrid responses. At the spatiotemporal extreme, we find that the gap-localized response not only enhances polarization-sensitive contrast but also strengthens the local hybrid interaction, leading to accelerated ultrafast relaxation. Together, these space-, time-, and polarization-resolved measurements provide a physically grounded and experimentally benchmarked picture of chiral plexcitonic coupling, identifying chirality as a practical control parameter for selectively steering nanoscale energy pathways and dynamics.

physics.optics↗

arXiv2026

Simultaneous Measurement of Circular Dichroism and Circular Differential Scattering

Chiroptical spectroscopy provides a non-invasive, label-free approach for resolving microscopic structural details via interactions with circularly polarized light. Despite the widespread application and complementary information provided for chiroptical materials characterization, the simultaneous acquisition of circular dichroism (CD) and circular differential scattering (CDS) spectra has remained challenging. In this work, we develop a dual-channel spectrometer that enables the acquisition of CD and CDS spectra from the same solution. To address the challenge of CDS baseline correction, we introduce a scattering spectral matching method. The performance of the instrument is validated using two representative model systems: a mixture of ammonium d-10 camphor sulfonate and polystyrene nanoparticles (PSNPs), and plasmonic gold helicoid nanoparticles, which exhibit both chiral absorption and scattering. For the former case, the CDS spectra show opposite signs to the CD spectra because the PSNPs are achiral scattering particles and the CDS spectra are affected by the chiral absorption. For the latter case, both CD and CDS spectra exhibit matched resonance wavelengths and stronger responses to the right-handed circularly polarized light, indicating that the chiral absorption and scattering arise from the same plasmonic resonance modes. To the best of our knowledge, this work represents the first experimental demonstration of the concurrent acquisition of ensemble-averaged CD and CDS spectra. The presented technique enables a direct and accurate comparison of CD and CDS spectra acquired under identical conditions.

physics.optics↗

arXiv2025

Broadband ultrafast self-heterodyned chiro-optical spectroscopy

Ultrafast chiro-optical spectroscopy provides unique access to the structural dynamics of molecules, spin-valley relaxation in semiconductors, and the non-equilibrium optical response of chiral nanophotonic systems. Yet, because chiral signals are intrinsically weak and time-resolved spectroscopy probes small photoinduced changes, transient chiro-optical responses are often difficult to isolate from parasitic achiral contributions. Here, we introduce a broadband ultrafast chiro-optical spectroscopy technique that integrates a birefringent common-path interferometer with an optical polarization bridge to sensitively detect photoinduced changes in the polarization state of light. Phase-sensitive self-heterodyned detection enables simultaneous measurement of transient circular dichroism and optical rotatory dispersion across a broad spectral range with ultrafast temporal resolution. Balanced detection suppresses excess laser noise, enabling exceptional sensitivity (<50 $μ$deg) close to shot-noise limit. We demonstrate this approach on an array of gold nano-helicoids, supported by a full-wave time-resolved model of the spatiotemporal dynamics of plasmonic non-equilibrium carriers and their associated optical nonlinearities. The model traces the system's transient chiro-optical response back to photoinduced modulations of the electric-magnetic dipole interaction in the nano-helicoid, elucidating the connection of near- and far-field dynamics in the non-equilibrium regime. We further investigate spin excitation, thermalization, and relaxation in a lead halide perovskite, establishing a novel approach to broadband time-resolved Faraday rotation. The simplicity, sensitivity, and wide applicability of this detection scheme provide a powerful platform for broadband ultrafast chiro-optical spectroscopy, opening new opportunities in biochemistry, solid-state physics, and nanophotonics.

physics.optics↗

arXiv2025

Multi-User SLNR-Based Precoding With Gold Nanoparticles in Vehicular VLC Systems

Visible spectrum is an emerging frontier in wireless communications for enhancing connectivity and safety in vehicular environments. The vehicular visible light communication (VVLC) system is a key feature in leveraging existing infrastructures, but it still has several critical challenges. Especially, VVLC channels are highly correlated due to the small gap between light emitting diodes (LEDs) in each headlight, making it difficult to increase data rates by spatial multiplexing. In this paper, we exploit recently synthesized gold nanoparticles (GNPs) to reduce the correlation between LEDs, i.e., the chiroptical properties of GNPs for differential absorption depending on the azimuth angle of incident light are used to mitigate the LED correlation. In addition, we adopt a signal-to-leakage-plus-noise ratio (SLNR)-based precoder to support multiple users. The ratio of RGB light sources in each LED also needs to be optimized to maximize the sum SLNR satisfying a white light constraint for illumination since the GNPs can vary the color of transmitted light by the differential absorption across wavelength. The nonconvex optimization problems for precoders and RGB ratios can be solved by the generalized Rayleigh quotient with the approximated shot noise and successive convex approximation (SCA). The simulation results show that the SLNR-based precoder with the optimized RGB ratios significantly improves the sum rate in a multi-user vehicular environment and the secrecy rate in a wiretapping scenario. The proposed SLNR-based precoding verifies that the decorrelation between LEDs and the RGB ratio optimization are essential to enhance the VVLC performance.

cs.IT↗

arXiv2025

Chirality-selective optical transport of nanoparticles in the evanescent field of a nanofiber

Optical nanofibers are waveguides known for their unique property to produce intense evanescent fields which have subwavelength transverse confinement easily extendable over thousands of wavelengths along the fiber axis. Moreover, circularly polarized fundamental modes of a nanofiber are chiral, that is, lacking mirror symmetry. Here, we use these two properties to demonstrate chirality-selective optical transport of a waterborne chiral material object - a chemically synthesized gold nanocube with twisted faces. Our experiments, supported by numerical simulations, show that right- and left-handed circularly polarized modes produce clearly distinct velocities of optically trapped nanocubes along the nanofiber axis, whereas non-chiral gold nanospheres of a similar size do not show any such dissymmetry. Furthermore, using a counterpropagating mode configuration, the non-chiral component of the optical force can be effectively zeroed out, yielding selective forward and backward transport of chiral nanocubes. In addition, the chiral optical force was found to be significant even for particle ensembles with natural variations in size and form, showing average behavior in agreement with numerical simulations. This is a clear implementation of optical separation of chiral enantiomers at the scale of 100 nm. Further development towards waveguide-assisted enantio-selective manipulation approaching the molecular scale can be envisaged.

physics.optics↗

arXiv2024

On the Physical Layer Security of Visible Light Communications Empowered by Gold Nanoparticles

Visible light is a proper spectrum for secure wireless communications because of its high directivity and impermeability in indoor scenarios. However, if an eavesdropper is located very close to a legitimate receiver, secure communications become highly risky. In this paper, to further increase the level of security of visible light communication (VLC) and increase its resilience against to malicious attacks, we propose to capitalize on the recently synthesized gold nanoparticles (GNPs) with chiroptical properties for circularly polarized light resulting the phase retardation that interacts with the linear polarizer angle. GNP plates made by judiciously stacking many GNPs perform as physical secret keys. Transmitters send both the intended symbol and artificial noise to exploit the channel variation effect by the GNP plates, which is highly effective when an eavesdropper is closely located to the legitimate receiver. A new VLC channel model is first developed by representing the effect of GNP plates and linear polarizers in the circular polarization domain. Based on the new channel model, the angles of linear polarizers at the transmitters and legitimate receiver are optimized considering the effect of GNP plates to increase the secrecy rate in wiretapping scenarios. Simulations verify that when the transmitters are equipped with GNP plates, even if the eavesdropper is located right next to the legitimate receiver, insightful results on the physical layer security metrics are gained as follows: 1) the secrecy rate is significantly improved and 2) the symbol error rate gap between the legitimate receiver and eavesdropper becomes much larger due to the chiroptical properties of GNP plates.

cs.IT↗

arXiv2022

Strain and Crystallographic Identification of the Helically Concaved Surfaces of Nanoparticles

Identifying the three-dimensional (3D) crystal-plane and strain-field distributions of nanocrystals is essential for optical, catalytic, and electronic applications. Here, we developed a methodology for visualizing the 3D information of chiral gold nanoparticles with concave gap structures by Bragg coherent X-ray diffraction imaging. The distribution of the high-Miller-index planes constituting the concave chiral gap was precisely determined. The highly strained region adjacent to the chiral gaps was resolved, which was correlated to the 432-symmetric morphology of the nanoparticles and its corresponding plasmonic properties were numerically predicted from the atomically defined structures. This approach can serve as a general characterization platform for visualizing the 3D crystallographic and strain distributions of nanoparticles, especially for applications where structural complexity and local heterogeneity are major determinants, as exemplified in plasmonics.

cond-mat.mtrl-sci↗

arXiv2022

Second-harmonic optical circular dichroism of plasmonic chiral helicoid-III nanoparticles

While plasmonic particles can provide optical resonances in a wide spectral range from the lower visible up to the near-infrared, often symmetry effects are utilized to obtain particular optical responses. By breaking certain spatial symmetries, chiral structures arise and provide robust chiroptical responses to these plasmonic resonances. Here, we observe strong chiroptical responses in the linear and nonlinear optical regime for chiral L-handed helicoid III nanoparticles and quantify them by means of an asymmetric factor, the so-called g-factor. We calculate the linear-optical g-factors for two distinct chiroptical resonances to -0.12 and -0.43 and the nonlinear optical g-factors to -1.45 and -1.63. The results demonstrate that the chirality of the helicoid-III nanoparticles is strongly enhanced in the nonlinear regime.

physics.optics↗

arXiv2020

Effects of paramagnetic fluctuations on the thermochemistry of MnO (100) surfaces in the oxygen evolution reaction

We investigated the effects of paramagnetic (PM) fluctuations on the thermochemistry of the MnO(100) surface in the oxygen evolution reaction (OER) using the "noncollinear magnetic sampling method \textit{plus} $U$" (NCMSM$+U$). Various physical properties, such as the electronic structure, free energy, and charge occupation, of the MnO (100) surface in the PM state with several OER intermediates, were reckoned and compared to those in the antiferromagnetic (AFM) state. We found that PM fluctuation enhances charge transfer from a surface Mn ion to each of the intermediates and strengthens the chemical bond between them, while not altering the overall features, such as the rate determining step and resting state, in reaction pathways. The enhanced charge transfer can be attributed to the delocalized nature of valence bands observed in the PM surface. In addition, it was observed that chemical-bond enhancement depends on the intermediates, resulting in significant deviations in reaction energy barriers. Our study suggests that PM fluctuations play a significant role in the thermochemistry of chemical reactions occurring on correlated oxide surfaces.

cond-mat.mtrl-sci↗

arXiv2020

Valley Polarization Enhancement Induced by a Single Chiral Nanoparticle

Valley polarization is amongst the most critical attributes of atomically thin materials. However, achieving a high contrast from monolayer transition metal dichalcogenides (TMDs) has so far been challenging. In this work, a giant valley polarization contrast up to 45% from a monolayer WS2 has been achieved at room temperature by using a single chiral plasmonic nanoparticle. The increased contrast is attributed to the selective enhancement of both the excitation and the emission rate having one particular handedness of the circular polarization. The experimental results were corroborated by the optical simulation using finite-difference time-domain (FDTD) method. Additionally, the single chiral nanoparticle enabled the observation of valley-polarized luminescence with a linear excitation. Our results provide a promising pathway to enhance valley contrast from monolayer TMDs and utilize them for nanophotonic devices.

physics.app-ph↗