arXiv subjects

Changsu Kim

Publications and source records attributed to Changsu Kim.

7 recordsLinked to original sources

arXiv2026

Signature of inverse orbital Hall effect in silicon studied using time-resolved terahertz polarimetry

We investigated the anomalous Hall conductivity induced in silicon by circularly polarized light at room temperature using near-infrared (NIR) pump-terahertz (THz) probe spectroscopy. The time-resolved detection scheme eliminates the large nonlinear current generated by the field-induced circular photogalvanic effect, allowing exclusive observation of a long-lived anomalous Hall conductivity of photocarriers that depends on the helicity of NIR light. The magnitude of this conductivity is comparable to that of GaAs despite silicon's much weaker spin-orbit coupling, and its robustness against NIR photon energy rules out a spin-polarization-based origin, which occurs only in the vicinity of the bandgap. These results suggest the emergence of the inverse orbital Hall effect, paving the way for silicon-based orbitronics.

cond-mat.mes-hall↗

arXiv2026

Few-picosecond pulse generation featuring ultrafast spectral dynamics in gain-switched surface-grating DFB lasers via impulsive optical pumping

To investigate the physics of picosecond gain-switching dynamics in single-mode lasers under femtosecond optical pumping at room temperature, we designed and fabricated first-order surface-grating GaAs distributed-feedback (DFB) lasers with five systematically varied grating periods (120-124 nm), corresponding to lasing wavelengths of 825.7-849.5 nm (1.502-1.459 eV). The 124-nm-period device, closest to the quantum-well gain peak among the investigated devices, exhibited the highest output power and spectral bandwidth. Among all devices, the 122-nm-period DFB laser (838.2 nm, 1.480 eV) generated the shortest pulses, despite lasing at a higher photon energy and lower output power than the device closest to the gain peak. All devices exhibited characteristic down-chirp behavior that increased with excitation power. The shortest pulses had a chirped pulse width of 6.6 ps and a chirp rate of 0.13 meV/ps, whereas spectrally resolved measurements revealed a minimum pulse width of 3.8 ps (2.3 ps after deconvolution of the detection time resolution) near the central photon energy of the pulse spectrum. Numerical simulations revealed temporally and spatially resolved dynamics of photons, carriers, gain, and refractive index, reproducing the experimental results qualitatively and quantitatively. Furthermore, a mechanism for generating the shortest pulses at photon energies above the gain peak was identified and attributed to higher differential gain, saturation gain, and a higher transparency carrier density in the high-energy region of the gain spectrum. These experimental and theoretical results elucidate the intrinsic dynamics of picosecond pulse generation in gain-switched DFB lasers and provide design guidance for short-pulse generation and computational tools applicable to both optical and electrical pumping.

physics.optics↗

arXiv2026

FAST: An Efficient Scheduler for All-to-All GPU Communication

All-to-All(v) communication is a critical primitive in modern machine learning workloads, particularly mixture-of-experts (MoE) models. Unfortunately, efficient scheduling is challenging due to workload skew, heterogeneous two-tier fabrics, and incast congestion, compounded by the dynamic nature of MoE workloads, where traffic shifts every few hundred milliseconds. Existing schedulers are hardly scalable, incurring seconds to hours of synthesis time, making them impractical. We present FAST, an efficient All-to-All(v) scheduler. FAST addresses skew through intra-server rebalancing and enforces balanced, one-to-one scale-out transfers that avoid incast. Evaluated extensively on both NVIDIA H200 and AMD MI300X clusters, FAST consistently outperforms state-of-the-art solutions on skewed workloads while reducing synthesis time by orders of magnitude.

cs.DC↗

arXiv2025

Light-induced inverse spin Hall effect and field-induced circular photogalvanic effect in GaAs revealed by two-dimensional terahertz Fourier analysis

The electromotive force transverse to a bias field under irradiation of circularly polarized light, namely the photovoltaic Hall response or light-induced anomalous Hall effect, has attracted considerable attention to investigate the topologically nontrivial states in Floquet engineering and the inverse spin Hall effect of spin-polarized carriers in spintronics. However, taking into account inversion symmetry breaking by the bias field, the circularly polarized light can excite photocarriers with asymmetric momentum distribution, which generates injection current transverse to the bias field. Therefore, the field-induced circular photogalvanic effect (FI-CPGE) should also emerge in the very same experimental configuration for light-induced anomalous Hall effect but has been overlooked in literature. In this work, using terahertz pulses as a bias field for a semiconductor GaAs, we conduct two-dimensional Fourier analysis and demonstrate that FI-CPGE can play a major role in the photovoltaic Hall response. Counterintuitively, FI-CPGE is significantly enhanced when the photocarriers are excited near the bandgap with small density of states and low group velocity, which can be explained by a three-level resonant nonlinear interaction near the band degeneracy point. We also clarified that FI-CPGE would be further largely detected in the contact-type measurement using electrodes because of the absence of a filtering effect inherent to terahertz pulses. This work provides a comprehensive, generalized view of the photovoltaic Hall response in biased materials, paving a new avenue for detecting topological monopoles in momentum space hidden in equilibrium using third-order nonlinear responses.

cond-mat.mtrl-sci↗

arXiv2024

Observation of Terahertz Spin Hall Conductivity Spectrum in GaAs with Optical Spin Injection

We report the first observation of the spin Hall conductivity spectrum in GaAs at room temperature. Our terahertz polarimetry with a precision of several $μ$rads resolves the Faraday rotation of terahertz pulses arising from the inverse spin Hall effect of optically injected spin-polarized electrons. The obtained spin Hall conductivity spectrum exhibits an excellent quantitative agreement with theory, demonstrating a crossover in the dominant origin from impurity scattering in the DC regime to the intrinsic Berry-curvature mechanism in the terahertz regime. Our spectroscopic technique opens a new pathway to analyze anomalous transports related to spin, valley, or orbital degrees of freedom.

cond-mat.mtrl-sci↗

arXiv2020

High-harmonic generation in GaAs beyond the perturbative regime

We experimentally study the field-intensity dependence of high-harmonic generation in bulk gallium arsenide in reflection geometry. We find the oscillatory behavior at high fields where a perturbative scaling law no longer holds. By constructing a theoretical framework based on the Luttinger-Kohn model, we succeed in reproducing the observed oscillatory behavior. The qualitative agreement between the experiment and theory indicates that field-induced dynamic band modification is crucial in the nonperturbative regime. We consider the origin of the oscillatory behavior in terms of dynamical localization based on the Floquet subband picture.

cond-mat.mes-hall↗

arXiv2019

"Visible" 5d orbital states in a pleochroic oxychloride

Transition metal compounds sometimes exhibit beautiful colors. We report here on a new oxychloride Ca3ReO5Cl2 which shows unusually distinct pleochroism; that is, the material exhibits different colors depending on viewing directions. This ple-ochroism is a consequence of the fact that a complex crystal field splitting of the 5d orbitals of the Re6+ ion in a square-pyramidal coordination of low-symmetry occurs accidentally in the energy range of the visible light spectrum. Since the rele-vant d-d transitions possess characteristic polarization dependences according to the optical selection rule, the orbital states are "visible" in Ca3ReO5Cl2.

cond-mat.str-el↗