Hai Son Nguyen

Bound states in the continuum (BICs) are nonradiative modes embedded within the continuous spectrum of propagating states that remain perfectly localized without leaking into radiation channels. Symmetry-protected BICs (SP-BICs) are induced by structural symmetry at the Γ point, whereas accidental BICs arise under specific structural parameters and incident conditions. By introducing a twist angle between two identical lattices, a moiré metasurface folds multiple resonant modes into the reduced moiré Brillouin zone, enabling several symmetry-protected and accidental BICs to coexist within it. Herein, we numerically demonstrate the existence of multiple BICs in a moiré metasurface formed by twisted bilayer triangular lattices with commensurate angle 2α of 6.01°. We further observe multiple robust flatbands in the band structure, one of which hosts a SP-BIC, so that a single state is localized both in real space by the flatband and in momentum space (k-space) by the BIC. In contrast to magic-angle flatbands, which emerge only within narrow parameter windows, the flatbands in our system persist across a wide range of moiré structural parameters owing to strong intersublattice coupling and arise from interband coupling of folded dispersive bands. In k-space, all BICs coincide with polarization singularities carrying well-defined topological charges, including higher-order charges (q = −2) enabled by the C6 symmetry of the triangular moiré lattice. Notably, the accidental BIC radiates with an anomalous, negative-refraction-like directionality. By combining robust moiré flatbands with symmetry-protected and accidental BICs on a single C6 platform, these results establish twisted triangular lattices as a route toward multiband, high-Q terahertz devices.

Berry monopoles are point degeneracies in parameter space where the Berry curvature emanates radially, analogous to magnetic monopoles. Here, we report the adiabatic merging and splitting of Berry monopoles in a bilayer photonic crystal slab combining one genuine and one synthetic momentum. Two monopoles approach, merge, and split within this hybrid parameter space. The process is described by an effective coupled-mode model and confirmed by full-wave simulations. We further propose a scheme to experimentally probe the Berry monopole merging and splitting by monitoring the evolution of chiral edge states localized at the interface between two topological phases with inverted band structures.

Metal halide perovskites are promising materials for optoelectronic applications owing to their outstanding optical and electronic properties. Among them, all‐inorganic perovskites such as offer superior thermal and chemical stability. However, obtaining high‐quality thin films via solution processing remains challenging due to the low solubility of the precursor, and current additive or solvent engineering strategies are often complex and poorly reproducible. High‐pressure recrystallization has recently emerged as a promising route to improve film quality, yet its impact on film properties remains insufficiently explored. Here, we systematically investigate the morphological, structural, and optical properties of thin films prepared by high‐pressure recrystallization, in comparison with standard non‐recrystallized films. Optimized recrystallization at 300 bar produces smooth, pinhole‐free, single‐phase 3D perovskite layers with sub‐nanometer roughness, while the film thickness is precisely tunable via precursor concentration. The process enhances both grain and crystallite sizes, leading to amplified spontaneous emission with a reduced excitation threshold and improved photostability. Temperature‐dependent X‐ray diffraction further reveals the orthorhombic–tetragonal–cubic phase transition, consistent with single‐crystal behavior. This study provides fundamental insights into pressure‐driven recrystallization and establishes a reproducible, scalable approach for fabricating high‐quality films for optoelectronic devices.

Controlling how an open photonic system exchanges energy with its environment-and in particular how it radiates into the far field-is a cornerstone of non-Hermitian wave physics and a key enabler for directional photonic functionalities. Here, we propose a new route to robust unidirectional emission based on the non-Hermitian hybridization of resonances localized in spatially separated layers of a hetero-bilayer photonic crystal. By tailoring the interlayer coupling, we engineer hybrid photonc bands that exhibit strong unidirectional radiation across a broad spectral and momentum range while maintaining theoretically high quality factors. This asymmetric emission is organized by a topological vortex in a pseudo-polarization field defined from the front/back intensity imbalance, which endows the directionality with robustness against perturbations. We further show that, by tuning the surrounding refractive index, this singularity can be displaced in parameter space, enabling reversible switching of the emission direction and a reconfigurable far-field response. This framework opens perspectives for topological photonic sensing and for directional and switchable light sources, including unidirectional lasing supported by high-quality-factor modes.

Topological lasing leverages concepts from topological physics to achieve single-mode light amplification within topological bandgaps, offering robustness against fabrication imperfections. Recent advances in microelectromechanical systems (MEMSs) and phase-change materials (PCMs) at the subwavelength scale promise new avenues for dynamically reconfigurable topological lasers, enabling robust and tunable nanoscale light sources. Here, we numerically demonstrate a dynamically reconfigurable lasing action at telecom wavelengths in a bilayer photonic crystal through the mechanisms of Thouless pumping. By designing two competing periodic potentialsone slowly translating photonic grating atop another stationary onewe observe a transition between a topological pumping regime and conventional mode oscillation. A carefully engineered heterojunction between these phases supports a robust lasing mode that can be dynamically tuned via MEMSs or reversible PCMs. Our work establishes bilayer photonic crystals as a programmable platform for achieving topological light sources, showcasing a potential pathway for merging topological photonics with reconfigurable photonic devices.

We develop a generalized non‐Hermitian Hamiltonian formalism for guided resonances in photonic crystal slabs, derived directly from Maxwell’s equations through a systematic guided‐mode expansion. By expanding the electromagnetic fields over the complete mode basis of an unpatterned slab and systematically integrating out radiative Fabry–Pérot channels, we obtain the analytical operator structure of the Hamiltonian, which treats guided‐mode coupling and radiation losses on equal footing. The resulting Hamiltonian provides explicit expressions for both dispersive and radiative coupling terms in terms of modal overlap integrals and Fourier components of the permittivity modulation. For specific geometries, the Hamiltonian coefficients can be extracted from full‐wave simulations, enabling accurate modeling without phenomenological assumptions. As a case study, we investigate hexagonal lattices with both preserved and broken C ₆ symmetry, demonstrating predictive agreement for complex band structures, near‐field distributions, and far‐field polarization patterns. In particular, the formalism reproduces symmetry‐protected bound states in the continuum (BICs) at the Γ point, accidental off‐Γ BICs near the Γ point, and the emergence of chiral exceptional points (EPs). It also captures the tunable behavior of eigenmodes near the K point, including Dirac‐point shifts and the emergence of quasi‐BICs or bandgap openings, depending on the nature of C ₆ symmetry breaking. We further demonstrate in the Appendix that the same formalism extends naturally to other symmetry classes, including C ₂ (1D grating) and C ₄ (square lattice) photonic crystal slabs. This approach enables predictive and efficient modeling of complex photonic resonances, revealing their topological and symmetry‐protected characteristics in non‐Hermitian systems.