Ji-Hun Kang

Fresnel scattering at a sharp interface is a fundamental process in optics, yet the scattering of hyperbolic phonon polaritons presents a unique challenge because of their inherently multimodal nature. Finite slabs of hyperbolic polar crystals support a dense spectrum of polariton modes whose coherent superposition reconstructs ray-like propagation, enabling multimodal energy transport in deeply subwavelength structures. When the substrate changes abruptly, such as at a dielectric-to-metal transition beneath the slab, the regions on either side support different eigenmode sets, which couple at the boundary. The intermodal coupling accumulated at each discontinuity shapes the overall polaritonic response, but predictive models that capture this cumulative process remain limited. Here, we introduce Fresnel multimodal scattering coefficient matrices at substrate boundaries of uniaxial hyperbolic slabs whose optic axis is normal to the slab. Using coupled-mode theory, we nonperturbatively derive mode-resolved reflection and transmission matrices that explicitly retain the radiation continuum and satisfy reciprocity, without any fitting to full-wave simulations. These interface coefficients serve as fundamental building blocks whose systematic composition efficiently yields the response of complex substrate-defined heterostructures. The theory also remains accurate for lossy media, as its weak-loss extension agrees with full-wave simulations to within a few percent for realistic material parameters. The framework accommodates large modal bases at a computational cost that can be substantially lower than that of full-wave simulations, providing a practical tool for the analysis and design of multimodal polaritonic nanodevices.

We propose two‐dimensional (2D) in‐plane heterostructures, composed of a 2D crystal adjoining a perfect electric conductor (PEC) plane, that enable ultranarrow polaritonic resonant cavities. Specifically, we theoretically investigate the interaction of 2D surface polaritons (2DSPs) with the junction between the 2D crystal and a PEC plane. We reveal that when 2DSPs are strongly confined, the reflected 2DSPs experience a phase shift of 3 π /4, which exhibits π /2 deviation from the so‐called edge reflection value. This non‐trivial phase shift is shown to play a crucial role in enabling resonant cavities whose size can be far smaller than the wavelength of the 2DSPs. Furthermore, we demonstrate that the spatial dimensionality of our heterostructure allows a direct mapping to metasurface‐based heterostructures, where the 2D crystal is replaced by a metasurface supporting spoof surface polaritons (SSPs). This correspondence extends the feasibility of our concept to SSP‐based resonators and broadens the accessible frequency range into the terahertz and microwave regimes. Our work provides not only deeper insight into low‐dimensional polariton optics but also a design strategy for ultracompact polaritonic metaresonators.

We theoretically investigate the tunneling of two‐dimensional surface polaritons (2DSPs) through plasmonic nanoplates on atomically thin crystals. By developing an analytic model based on coupled‐mode theory, we examine the tunneling efficiency as a function of the plate’s width and height. Our analysis reveals that the tunneling is mediated by evanescent waves induced near the plate edges due to interactions between 2DSPs and the plate, enabling the consistent recoupling of 2DSPs across plates of finite width, regardless of their height. Additionally, we clarify that these evanescent waves not only facilitate tunneling but also induce an anomalous phase shift in the 2DSPs reflected from the plate. This phase shift is shown to play a critical role in the resonant tunneling of 2DSPs through two plates, effectively creating a polariton‐assisted resonator in the deep subwavelength regime.

Owning to their unusual optical properties, such as electrical tunability and strong spatial confinement, two‐dimensional surface polaritons (2DSPs) hold great promise for deep sub‐wavelength manipulation of light in a reduced low‐dimensional space. Control of 2DSPs is possible by using their interaction with a boundary between two media, similar to how light behaves in three‐dimensional (3D) space. The understanding of the interaction in the 2D case is still in its early stages, unlike the 3D case, as in‐depth investigations are only available in a few cases including the interaction of 2DSPs with structured 2D crystals. Here, we extend the scope of our understanding to the interaction of 2DSPs with metallic nano‐plates on 2D crystals, focusing on the reflection of 2DSPs. Through our rigorous model, we reveal that, for strongly confined 2DSPs having much larger momentum than free space photons, the interaction results in almost total internal reflection of 2DSPs as the radiative coupling of the 2DSPs to free space is negligible. We also find that the reflection involves an anomalous phase shift dependent on the thickness of the nano‐plate, due to the temporary storing of electromagnetic energy in the evanescent waves induced near the edge of the nano‐plate. Our theory predicts that the phase shift saturates to an anomalous value, 0.885 π , as the nano‐plate becomes thicker. Our work provides a detailed understanding of how to manipulate the 2DSPs by using one of the simplest nanostructures, essential for the further development of nanostructure‐integrated low‐dimensional devices for polariton optics.

We theoretically investigate a metal-to-insulator transition in artificial two-dimensional (2D) crystals (i.e., metasurfaces) of tightly coupled closed-ring resonators. Strong interaction between unit resonators in the metasurfaces yields the effective permittivity highly dependent on the lattice spacing of unit resonators. Through our rigorous theory, we provide a closed form of effective permittivity of the metasurface and reveal that the permittivity possesses a Lorentzian-type resonant behavior, implying that the transition of the effective permittivity can arise when the lattice spacing passes a critical value.

Polaritons in two-dimensional (2D) materials have shown their unique capabilities to concentrate light into deep sub-wavelength scales. Precise control of the excitation and propagation of 2D polaritons has remained a central challenge for future on-chip nanophotonic devices and circuits. To solve this issue, we exploit Cherenkov radiation - a classic physical phenomenon that occurs when a charged particle moves at a velocity greater than the phase velocity of light in that medium - in low dimensional material heterostructures. Here, we report experimental observation of Cherenkov phonon polariton wakes emitted by superluminal one-dimensional plasmon polaritons in a silver nanowire and hexagonal boron nitride heterostructure using near-field infrared nanoscopy. The observed Cherenkov radiation direction and radiation rate exhibit large tunability through varying the excitation frequency. Such tunable Cherenkov phonon polaritons provide opportunities for novel deep sub-wavelength-scale manipulation of light and nanoscale control of energy flow in low-dimensional material heterostructures.