Classification of fragile topology via matrix homotopy

The moiré flat bands in the twisted bilayer graphene have attracted considerable attention not only because of the emergence of superconductivity but also due to their nontrivial topology. Specifically, they exhibit a new class of topology that can be nullified by the addition of trivial bands, termed fragile topology, which suggests the need for an expansion of existing classification schemes. Here, we show that a position-space framework enables dichotomous classification of fragile topology via matrix homotopy. By providing a fragile invariant that remains valid across diverse physical setups, our method reveals unprecedented properties such as disorder-induced re-entrant fragile topology that contradicts the weak perspective of fragile phases.
(Physical Review Research, accepted, 2025)

Global appearance of unidirectional guided resonance

The unidirectionality of leakage radiation is an extreme effect of light-matter interactions in metasurfaces, crucial for enhancing the efficiency and performance of various photonic devices such as couplers and antennas. So far, this effect has been known to be feasible only under specific frequency-wavevector condition at the local point on a dispersion band because previous approaches fundamentally rely on an accidental interference configuration. Here, we experimentally realize global appearance of predominant single-sided radiation using a glide symmetric bilayer metasurface, not limited to a local point, but existing continuously across nearly the entire dispersion bands.
(Nature Nanotechnology, 2025)

Observation of Dirac domain-wall in optical metasurfaces

The Jackiw-Rebbi soliton, a zero-energy solution in the domain-wall of Dirac equation, is of significant importance in topological physics as a fundamental description for boundary states. Despite being implemented across various platforms from electronic to photonic systems, the Jackiw-Rebbi states under the non-Hermitian characteristics of metasurfaces, which induce a strong out-of-plane interaction, remain largely unexplored. Here, we report clear experimental results for a fruitful connection between Dirac physics and optical metasurfaces.  
(Light: Science & Applications, 2025)

Dirac bilayer metasurfaces as an inverse Gires-Tournois etalon 

Pure transmission phase modulation is a highly desired means for the efficient implementation of compact optical phased-array elements such as a metalens and LiDAR. To date, such functionality has been shown to be limited at reflection-type operation (Gires-Tournois etalon) or certain local points of dispersion bands. Here, we show global manifestation of the pure-phase modulation in transmission along the entire Dirac band in bilayer metasurfaces. We name this phenomenon as the inverse Gires-Tournois etalon, based on the fact that the ideal transmission pure-phase resonance phenomenon of this device appears as a completely inverted form compared to the conventional reflective type.
(Submitted, 2023)

Topological beaming of light

Nanophotonic light emitters are key components in numerous application areas because of their compactness and versatility. Here, we propose an unprecedented beaming mechanism of leaky Jackiw-Rebbi state bound to the interface of  a  topological junction metasurface that takes advantage of submicrometer footprint size, small divergence angle, high efficiency, and adaptable beamshaping capability. This effect may trigger the realization of a new class of nanophotonic antennas and offer a practical guidance for designing topological resonances in flat optical systems.
(Science Advances, 2022)

Synthetic photonic topological nodal phase

The notion of synthetic dimensions in artificial photonic systems has received considerable attention as it provides novel methods for exploring hypothetical topological phenomena as well as potential device applications. We present nanophotonic manifestation of a 2D topological nodal phase in bilayer gratings by an analogy between a topological semimetal and glide symmetric photonic lattices
(Physical Review Letters, 2022)

Topological guided-mode resonances

We show that robust guided-mode-resonance states exist in metasurface domain-wall structures. Using the non-Hermitian photonic analogy of the Dirac equation, we derive essential conditions for photonic Jackiw-Rebbi-state resonances taking advantage of unique spatial confinement and spot-like spectral features which are remarkably robust against random parametric errors.
(Nanophotonics, 2021)

PN junction metasurfaces for efficient spatial light modulators

We propose a free-space electro-optic transmission modulator based on multiple p-n-junction semiconductor subwavelength gratings. The proposed device operates with a high-Q guided-mode resonance undergoing electro-optic resonance shift due to direct electrical control. Using rigorous electrical and optical modeling methods, we theoretically demonstrate a modulation depth of 84%, on-state efficiency 85%, and on-off extinction ratio of 19 dB at 1,550 nm wavelength under electrical control signals within a favorably low bias voltage range from −4 V to +1 V.
(Scientific Reports, 2017)