Xiao Yan Chew

Two-dimensional metal-halide perovskites have attracted considerable interest in nonlinear optics because of their large third-order nonlinear susceptibilities and strong light–matter interactions. Momentum-mismatch-driven quasi-bound states in the continuum (quasi-BICs), characterized by high Q -factor and strong electromagnetic-field confinement, offer an effective route to enhancing third-harmonic generation (THG). Here, we utilize a momentum-mismatch-driven quasi-BIC grating based on two-dimensional perovskite (BA) ₂ PbI ₄ and establish the magnetic-dipole-dominated nature of its resonance through band structure, Q -factor, polarization, and multipole analyses. Under plane-wave excitation, we investigate THG enhancement by varying the width. Then we develop a Gaussian-beam excitation model using the angular-spectrum expansion method. Compared with plane-wave excitation, Gaussian-beam excitation yields a lower THG efficiency, because its finite angular spectrum excites off-Γ resonances with reduced Q -factors. Increasing the beam waist gradually restores the THG efficiency toward the plane-wave limit, reaching approximately 90% for beam waists above 165 μm. These results clarify the impact of finite-width excitation on quasi-BIC-enhanced THG and provide guidance for experimental implementation.

We study scalarized extremal black holes in the Einstein–Maxwell-scalar (EMS) theory with two different scalar couplings to two U(1) fields. This theory is inspired by the dyonic EMS theory. Two scalarized extremal black holes are found with constant scalar hair. We confirm that these are precisely obtained from the extremal scalarization and entropy function approaches. This may imply that it is not easy to find extremal black holes with primary scalar hair.

Low optical loss and high refractive index semiconductor materials are ideal for light-matter interactions at the nanoscale. In this work, we propose a two-dimensional perovskite core and silicon shell structure to study strong coupling between the perovskite exciton and magnetic Mie mode of the silicon shell. Simulations based on the Lorentz dielectric function of perovskite reveal that Mie resonance peaks can be tuned to maintain the dominant magnetic dipole Mie modes, enabling strong coupling with Rabi splitting of about 100 meV. When the experimental dielectric function of 2D perovskite is performed, the Rabi splitting increases to 115 meV. Further tuning of the 2D perovskite core size, silicon shell thickness, and refractive index indicates that the hybrid modes remain in the strong coupling regime. This work provides a strategy for tunable light-matter interactions in nanophotonic devices.

We explore the role of the affine connection in f ( Q ) gravity, a modified theory where gravity is governed by non-metricity within the symmetric teleparallel framework. Although the connection is constrained to be flat and torsionless, it is not uniquely determined by the metric, allowing for multiple physically distinct formulations. We analyze three such connections compatible with a homogeneous and isotropic universe to show that they yield markedly different cosmological dynamics, even under the same functional form of f ( Q ). Using both analytical and numerical methods, including a Born-Infeld type model of f ( Q ), we demonstrate that specific connections can resolve cosmological singularities like the Big Bang and Big Rip, replacing them with smooth de Sitter phases. Others retain singularities but with notable modifications in their behavior. These findings highlight the physical relevance of connection choice in f ( Q ) gravity and its potential to address fundamental cosmological questions.

We investigate the dynamical collapse of Bronnikov-Ellis (BE) wormhole using the double-null formalism, where its throat is characterized by the coincidence of two curves r _,u = 0 and r _,v = 0. The emission of two ingoing pulses: normal scalar and phantom fields in the wormhole spacetime reveals two distinct instability scenarios: a normal scalar field triggers gravitational collapse into a black hole where the singularity r = 0 hidden by the event horizon (r _,u = 0 and r _,v = 0); while a phantom field drives inflationary expansion of wormhole, decoupling two asymptotic regions with the cosmological horizon (r _,u = 0 and r _,v = 0). The process of two scenarios can be accelerated by increasing the amplitude of pulses but can be delayed by increasing the wormhole's mass. Additionally, the collisions of two identical pulses from ingoing and outgoing null directions in the massless BE wormhole fail to cure the instabilities because the two scenarios can still occur, but the formation of a black hole can be delayed for the collision of normal and phantom fields. Interestingly, the strategic tuning of emission timing for outgoing phantom field to collide with ingoing normal scalar field can temporarily stabilize the wormhole throat by restoring the coincidence of r _,u and r _,v again after their separation. This offers us valuable insights into extending the lifetime of a traversable wormhole.

In this study, we report an angle-independent coupling mechanism between topological interface states and Tamm plasma polaritons within a photonic crystal heterostructure with hyperbolic metamaterials. The heterostructure is formed by two distinct photonic crystals, which have angle-independent photonic bandgaps through phase variation control. By analyzing the Zak phases of the upper and lower bands, we find that these two photonic bandgaps exhibit opposite topological characteristics, leading to the emergence of an angle-independent topological interface state. The metal layer at the bottom of the photonic crystal enables the excitation of Tamm plasmon polaritons; strong coupling with topological interface states is achieved, resulting in a dual-mode, wide-angle, polarization-dependent absorber with two distinct regions. The results reveal a straightforward approach for realizing angle-independent coupling between topological interface states and Tamm plasmonic polaritons, with promising applications in sensors, absorbers, optical filters, and nonlinear optical devices.