arXiv subjects

Yung Kim

Publications and source records attributed to Yung Kim.

5 recordsLinked to original sources

arXiv2026

Sagnac-Loop-Reflector Fabry-Perot Lattices for Modular 1D Topological Photonics

We introduce a modular silicon-photonic Fabry-Perot resonator lattice based on cascaded tunable Sagnac loop reflectors. Each SLR is controlled by a single directional-coupler cross-coupling coefficient, enabling modular control of the effective lattice hoppings. As a representative example, alternating two SLR types maps the lattice onto the Su-Schrieffer-Heeger model in the weak-coupling limit. We derive the Bloch dispersion via a transfer-matrix formulation and obtain an effective tight-binding Hamiltonian in the weak-coupling limit. S-parameter simulations of a 20-site lattice show an isolated midgap resonance with edge-localized power profiles in the topological phase, and disorder tests show robustness against symmetry-preserving hopping perturbations. Our results establish SLR-based FP lattices as a complementary platform for on-chip topological photonics.

physics.optics↗

arXiv2026

Self-organized photonic time quasicrystal from a single imposed clock

A photonic time crystal usually writes a clock into a medium. Here one clock does more than program the medium: it seeds a quasiperiodic temporal order that the nonlinear medium selects for itself. In a guided-wave lattice of nonlinear dipoles, a single-tone pump modulates the polarization sector, while Maxwell--polarization back-action selects two response frequencies whose only resolved low-order relation is the pump-locked sum condition. Their sum phase locks to the pump and the complementary phase winds, producing a photonic discrete time quasicrystal with torus-like phase dynamics and a discrete combination spectrum. Site-resolved measurements show locked-phase coherence across the measured lattice sites over a finite control-parameter window. These results establish a route from externally programmed time-varying media to self-organized temporal order in nonlinear photonic systems.

physics.optics↗

arXiv2025

Analogs of spontaneous emission and lasing in photonic time crystals

We report the first direct mapping of the frequency-resolved local density of states (LDOS) in a photonic time crystal (PTC) implemented as an array of time-periodically modulated LC resonators at microwave frequencies. Broadband white noise probes the system and yields an LDOS lineshape near the momentum gap that can be decomposed into absorptive and dispersive Lorentzian components. The finite LDOS peak at the gap frequency, which grows with modulation strength, implies that the spontaneous emission rate of an emitter coupled to the PTC would be maximized at that frequency. The measured spectra are in good agreement with classical non-Hermitian Floquet theory. As the modulation-induced gain exceeds intrinsic losses, the system undergoes a transition to a narrow-band self-oscillation (lasing) regime. These results open a route to nonequilibrium photonics and bring time-periodic LDOS engineering closer to practical realization.

physics.optics↗

arXiv2025

Boundary-Driven Complex Brillouin Zone in Non-Hermitian Electric Circuits

Complex-valued physical quantities, often non-conserved, represent key phenomena in non-Hermitian systems such as dissipation and localization. Recent advancements in non-Hermitian physics have revealed boundary-condition-sensitive band structures, characterized by a continuous manifold of complex-valued momentum known as the generalized Brillouin zone (GBZ). However, the ability to actively manipulate the GBZ and its associated topological properties has remained largely unexplored. Here, we demonstrate a controllable manipulation of the GBZ by adjusting the boundary Hamiltonian and leveraging the boundary sensitivity in a circuit lattice. Our observations reveal that the GBZ forms multiple separated manifolds containing both decaying and growing wave functions, in contrast to the previously observed non-Hermitian skin effect under open boundary condition (OBC). By continuously deforming the GBZ, we observe the topological phase transitions of innate topological structure of GBZ that are enriched by complex properties of non-Hermitian physical variables. Notably, such topological phase transition is governed by boundary conditions rather than bulk properties, underscoring the extreme boundary sensitivity unique to non-Hermitian systems.

physics.app-ph↗

arXiv2023

Realization of Non-Hermitian Hopf Bundle Matter

Line excitations in topological phases are a subject of particular interest because their mutual linking structures encode robust topological information of matter. It has been recently shown that the linking and winding of complex eigenenergy strings can classify one-dimensional non-Hermitian topological matter. However, in higher dimensions, bundles of linked strings can emerge such that every string is mutually linked with all the other strings. Interestingly, despite being an unconventional topological structure, a non-Hermitian Hopf bundle has not been experimentally clarified. Here, we make the first attempt to explore the non-Hermitian Hopf bundle by visualizing the global linking structure of spinor strings in the momentum space of a two-dimensional electric circuit. By exploiting the flexibility of reconfigurable couplings between circuit nodes, we can study the non-Hermitian topological phase transition and gain insight into the intricate structure of the Hopf bundle. Furthermore, we find that the emergence of a higher-order skin effect in real space is accompanied by the linking of spinor strings in momentum space, revealing a bulk-boundary correspondence between the two domains. The proposed non-Hermitian Hopf bundle platform and visualization methodology pave the way to design new topologically robust non-Hermitian phases of matter.

physics.app-ph↗