Topological spin textures, particularly Meron half-Skyrmions, have attracted great interest due to their nontrivial topological properties and potential applications in optical storage and high-speed communication. However, existing implementations are mostly limited to real-space operations at optical frequencies, and momentum-space Meron structures based on bound states in the continuum (BICs) suffer from narrow operational bandwidth, limited tunability, and insufficient understanding of the inversion mechanism between Meron and anti-Meron configurations. To overcome these challenges, this work proposes and demonstrates a magneto-optical control scheme for terahertz (THz) topological spin textures using an InSb-based metasurface. The objective is to achieve dynamic, wideband, and reconfigurable generation of Meron and anti-Meron structures in momentum space, with full coverage of the Poincaré sphere and multi-state topological switching at a fixed frequency.

The metasurface consists of a periodic array of InSb substrates with embedded SiO₂ square holes, designed with C_4v symmetry. The structural parameters (period a = 83.4 μm, hole width w = 55.66 μm, thickness h = 20.75 μm) were optimized through a two-stage process: first using a constant permittivity approximation and then validating with a full frequency- and magnetic-field-dependent dielectric tensor of InSb based on the Drude model. Numerical simulations were performed using the COMSOL Multiphysics wave optics module, employing eigenfrequency and frequency-domain solvers with periodic boundary conditions and perfectly matched layers. The band structures, quality factors (Q-factors), electric field distributions, and far-field Stokes parameters were calculated to characterize the BICs and their topological properties. By applying external magnetic fields along the z-direction, x-direction, y-direction, and oblique directions, the evolution of the far-field polarization, chiral transmission, and Meron configurations were systematically analyzed. The topological charge and Skyrmion number were derived from the polarization vortex and the spin texture via the Stokes parameter S₃ and the amplitude ratio between cross-polarized and co-polarized components. The robustness of the Meron generation against magnetic field misalignment and fabrication-induced symmetry breaking was also evaluated.

Under zero magnetic field, the C_4v metasurface supports symmetry-protected BICs at the Γ point in the TE1 (q = –1) and TE4 (q = +1) bands, generating second-order anti-Meron and Meron configurations, respectively. The polarity is determined by the incident circular polarization, while the Skyrmion number is governed by the BIC topological charge. Meron formation occurs only at the BIC frequencies (1.823 THz and 2.19 THz), where the amplitude matching condition between cross- and co-polarized components is satisfied.

With a z-directed magnetic field, the InSb becomes gyrotropic, inducing band blue shifts and transforming the far-field polarization from linear to elliptical vortices. This leads to chiral transmission, enabling selective excitation or suppression of Meron states depending on the incident handedness. Meanwhile, the ultrahigh Q factor (~10⁸) remains nearly unchanged. Reversing the magnetic field flips the chirality, realizing switchable control of Meron generation.

Under oblique magnetic fields, the Meron configuration remains robust up to ~15° misalignment. In contrast, transverse magnetic fields break in-plane symmetry, splitting the Γ-point V singularity into two C points with half-integer charges (q = ±1/2) and reducing the Q factor to ~10⁴–10⁵. This symmetry breaking enables continuous evolution of polarization states in momentum space, achieving full Poincaré sphere coverage without structural reconfiguration.

Furthermore, by exploiting magnetic-field-induced band shifts, four distinct Meron states (different polarity and Skyrmion number) are realized at a single frequency, forming a four-state topological switch. Compared with previous studies, the proposed metasurface achieves a significantly broader relative bandwidth (~25%) and enables flexible magnetic control without structural redesign.

We propose and numerically demonstrate an InSb-based magneto-optical metasurface that enables dynamic and broadband control of momentum-space Meron spin textures at THz frequencies. By exploiting the interplay between symmetry-protected BICs and the gyrotropic response of InSb, reconfigurable Meron and anti-Meron states with tunable Skyrmion numbers and chiral responses are realized. In the absence of a magnetic field, second-order Meron and anti-Meron configurations emerge at dual BIC bands, while the application of a magnetic field induces band shifts and polarization ellipticity, allowing selective activation or suppression of Meron states over a ~25% bandwidth. Transverse magnetic fields further break the in-plane symmetry, enabling continuous polarization evolution and full Poincaré sphere coverage, whereas magnetic-field reversal provides switchable chirality. By combining band alignment under different magnetic conditions, four distinct Meron states are achieved at a single frequency. The metasurface also exhibits strong robustness against magnetic-field misalignment (~15°) and fabrication imperfections, while maintaining ultrahigh Q factors up to 1.8×10⁸ under z-directed fields. These results provide a versatile platform for reconfigurable THz topological photonic devices, including polarization control, information encoding, and magnetic sensing.