arXiv subjects

Yong-Hoon Lee

Publications and source records attributed to Yong-Hoon Lee.

5 recordsLinked to original sources

arXiv2026

Localization in microcavities revealed by phase-space non-Hermitian skin effect

Contrary to the semiclassical expectation for fully chaotic systems, localization of resonances is found to be a common feature in open microcavities. In spiral-shaped dielectric microcavities, a substantial fraction of resonances localize on polygonal patterns in real space, are chiral, and their momentum distributions accumulate near the critical line for total internal reflection. Despite the extensive investigation, the physical mechanism responsible for their remarkable abundance has remained a long-standing question. Addressing this, we reveal a physical correspondence between an inhomogeneous-loss Hatano-Nelson model and the dielectric phase space of a spiral microcavity. We show that the combination of geometry-induced momentum drift and refractive escape yields a generalized non-Hermitian skin effect in the phase space momentum. We identify this mechanism as the origin of the critical-line localization of resonances in open chaotic spiral microcavities, extending the skin-effect concept beyond nonreciprocal lattices to phase space and to open chaotic wave systems.

physics.optics↗

arXiv2025

A Learning Framework for Diverse Legged Robot Locomotion Using Barrier-Based Style Rewards

This work introduces a model-free reinforcement learning framework that enables various modes of motion (quadruped, tripod, or biped) and diverse tasks for legged robot locomotion. We employ a motion-style reward based on a relaxed logarithmic barrier function as a soft constraint, to bias the learning process toward the desired motion style, such as gait, foot clearance, joint position, or body height. The predefined gait cycle is encoded in a flexible manner, facilitating gait adjustments throughout the learning process. Extensive experiments demonstrate that KAIST HOUND, a 45 kg robotic system, can achieve biped, tripod, and quadruped locomotion using the proposed framework; quadrupedal capabilities include traversing uneven terrain, galloping at 4.67 m/s, and overcoming obstacles up to 58 cm (67 cm for HOUND2); bipedal capabilities include running at 3.6 m/s, carrying a 7.5 kg object, and ascending stairs-all performed without exteroceptive input.

cs.RO↗

arXiv2022

Green's function for singular fractional differential equations and applications

In this paper, we study the existence of positive solutions for nonlinear fractional differential equations with a singular weight. We derive Green's function and corresponding integral operator and then examine the compactness of the operator. As an application, we prove an existence-result for positive solutions when a nonlinear term satisfies either superlinear or sublinear conditions. The proof was mainly employed by Krasnoselski's classical fixed point theorem.

math.CA↗

arXiv2022

Regularity of solutions for singular fractional differential equation

In this work, we study the regularity of positive solutions for nonlinear fractional differential equation with a singular weight. We define the new Banach space and use this space to show the regularity. We also give an example with a singular weight which may not be in $L^1.$

math.CA↗

arXiv2019

Universal light-guiding geometry for high-nonlinear resonators having molecular-scale roughness

By providing an effective way to leverage nonlinear phenomena in chip-scale, high-Q optical resonators have induced the recent advances of on-chip photonics represented by micro-combs and ultra-narrow linewidth lasers. These achievements mainly relying on Si, SiO$_{2}$, and Si$_{3}$N$_{4}$ are expected to be further improved by introducing new materials having higher nonlinearity. However, establishing fabrication processes to shape a new material into the resonator geometries having extremely smooth surfaces on a chip has been a challenging task. Here we describe a universal method to implement high-Q resonators with any materials which can be deposited in high vacuum. This approach, by which light-guiding cores having surface roughness in molecular-scale is automatically defined along the prepatterned platform structures during the deposition, is verified with As$_{2}$S$_{3}$, a typical chalcogenide glass of high-nonlinearity. The Q-factor of the developed resonator is 14.4 million approaching the loss of chalcogenide fibers, which is measured in newly proposed tunable waveguide-to-resonator coupling scheme with high ideality. Lasing by stimulated Brillouin process is demonstrated with threshold power of 0.53 mW which is 100 times lower than the previous record based on chalcogenide glasses. This approach paves the way for bringing various materials of distinguished virtues to the on-chip domain while keeping the loss performance comparable to that of bulk form.

physics.optics↗