Jie Ji

Symmetry-protected bound states in the continuum (BICs) exhibit polarization-selective radiation characteristics governed by the symmetry-imposed decoupling from free-space modes. Leveraging this intrinsic property, we demonstrate that controlled symmetry breaking in nonlocal metasurfaces formed by tilted bars converts a symmetry-protected BIC into a quasi-BIC with controlled radiative leakage. The resulting symmetry-engineered coupling pathways simultaneously enable cross-polarization conversion and helicity-dependent scattering. Both experimental and numerical results reveal that strong cross-polarized transmission and circular dichroism (CD) co-emerge and reach their maxima at an optimal tilt angle, reflecting a Q-factor-mediated balance between radiative leakage and field confinement. The cross-polarized channels exhibit a characteristic π phase jump and strong dispersion, indicating significant group delay associated with quasi-BIC-dominated radiation. Optical chiral density analysis further confirms that polarization conversion and chiral response originate from the same symmetry-breaking-induced leakage channel. These findings establish a mechanism linking symmetry breaking, Q-factor modulation, and polarization-selective radiation, providing a versatile strategy for chiral terahertz polarization control.

It is usually believed that symmetry breaking in photonic systems leads to weaker confinement, such as in the case of metasurfaces when bound states in the continuum are replaced by quasibound states with lower quality ( Q ) factors. Here, in contrast, we reveal that symmetry breaking can instead enhance light trapping by strengthening in-plane nonlocal coupling pathways. We consider finite-size arrays of optical resonators supporting Mie resonances (a Mie-tronics platform) and employ diffraction and multiple-scattering analyses to demonstrate that diffractive bands and Mie-tronics supermodes originate from the same underlying Mie resonances but differ fundamentally in their physical nature. Finite arrays exhibit Q -factor enhancement driven by redistributed radiation channels, thus reversing the trends predicted for infinite lattices. We demonstrate that controlled symmetry breaking opens otherwise symmetry-forbidden electromagnetic coupling channels, enabling polarization conversion in nonlocal metasurfaces. These findings establish a unified wave-physics platform linking scattering and diffraction theories. They also outline design principles for multifunctional metasurfaces that exploit nonlocality for advanced light manipulation, computation, and emission.

Dipole–dipole interactions are central to energy-transfer processes. Understanding the interaction dynamics is essential across diverse fields, including solar energy harvesting, organic light-emitting diodes, long-range energy transport, and molecular biosensing. Such interactions are fully described by the photonic Green function, which defines the electromagnetic response of a point dipole source. However, direct experimental access to the individual components of Green’s function at the single-dipole level remains challenging. Here, we employ a double-probe terahertz (THz) near-field microscope to directly map the dipole–dipole interactions in free space and near a resonant dielectric interface formed by a pellet of α-lactose and air. In free space, we observe highly anisotropic energy-transfer dynamics arising from the non-radiative near-field contribution of Green’s function, including the pronounced suppression associated with the magic-angle condition. Near the air/α–lactose interface, we reveal strong modifications of Green’s function along both the in-plane and out-of-plane directions: a reduction in the energy-transfer rate near the vibrational resonance of the medium and pronounced oscillations in intensity and phase due to interference between the direct dipole–dipole interaction and the surface-mediated contribution. Our results provide direct access to the non-radiative components of Green’s function and establish a powerful framework for probing and engineering dipole–dipole interactions in complex and resonant photonic media.

Breaking the fundamental distance limit of dipole-dipole interactions is key to enabling scalable quantum and photonic technologies. Conventional nonradiative energy transfer [Förster resonance energy transfer (FRET)] is intrinsically confined to deep subwavelength distances, limiting its scalability. Here, we demonstrate millimeter-scale, coherent dipole-dipole energy transfer enabled by nonradiative bound states in the continuum (BICs) in a gold metasurface. Using a custom terahertz near-field time-domain microscope with dipolar emitter-detector probes, we observe strongly anisotropic energy transfer enhanced along one axis, where BIC modes establish long-range spatiotemporal coherence, and suppressed along the orthogonal axis. By systematically reducing the array size, we reveal that energy transfer efficiency and spatiotemporal coherence are maximized near the metasurface’s center, where boundary-induced scattering is minimized. These findings establish BICs and quasi-BICs as robust, nonradiative channels for extended, directional, and coherence-preserving dipolar interactions, providing a versatile platform for quantum communication, integrated nanophotonics, and biosensing technologies.

We present a single-shot, ultrabroadband coherent detection scheme capable of resolving millijoule-class, inherently broadband terahertz (THz) waveforms from low-repetition-rate, high-energy laser systems. The technique integrates scanning-free solid-state biased coherent detection with a non-collinear probe–THz geometry and a balanced detection scheme based on orthogonal polarization states of the THz and bias fields. This configuration enables effective suppression of incoherent background and allows extraction of the THz electric field from a single laser pulse. A line-focused overlap between a femtosecond optical probe and a focused THz beam within a 1-μ m-thick silicon nitride detector yields a tunable ∼ ps time window with few-fs time resolution, limited only by the optical probe duration and camera pixel size. We demonstrate the method using millijoule-level THz pulses generated by relativistic fs laser interactions with copper foils, achieving single-shot detection of coherent spectral contents exceeding 15 THz. We also examine the impact of imperfect alignment on measurement accuracy and describe procedures for recovering the coherent THz waveforms under partially imbalanced detection conditions. This technique facilitates field-resolved investigations of extreme THz–matter interactions, where both high field strength and broad spectral coverage are essential.

Bound states in the continuum (BICs) are optical modes that remain decoupled from free-space radiation. Symmetry-protected BICs in metasurfaces offer powerful means to control light-matter interactions. A key property governing these interactions is the partial local density of optical states (PLDOS), which describes the number of electromagnetic modes available for a photon to occupy at a specific position, frequency and polarization. Here, we employ a terahertz near-field microscope with dual local probes to directly excite and detect quasi-BICs in finite metasurfaces that possess inversion symmetry, corresponding to a symmetry-protected BIC in their infinite counterpart. We observe a strong enhancement of the PLDOS associated with these modes. As the metasurface size increases, the quasi-BIC evolves into a true BIC, with the quality factor diverge while the PLDOS saturates. This establishes an upper limit for enhanced light-matter interaction by BICs. Our findings pave the way for on-chip metasurfaces with maximum light-matter interaction strengths.

Bound States in the Continuum (BICs) are exotic optical modes that remain decoupled from free-space radiation. Symmetry-protected BICs in infinite metasurfaces provide unique opportunities to tailor light–matter interactions in extended photonic structures. One of the most important properties of these structures is the partial local density of optical states (PLDOS), which describes the number of electromagnetic modes available for a photon to occupy at a specific position, frequency, and polarization. It quantifies how the surrounding photonic environment modifies the ability of light to couple with matter at a given location. Here, we employ a terahertz near-field microscope with dual local probes to directly excite and detect quasi-BICs in finite metasurfaces with inversion symmetry, which support a symmetry-protected BIC in their infinite counterpart. Our results provide the first direct evidence of a large PLDOS enhancement associated with BICs and quasi-BICs in finite metasurfaces. As the metasurface size increases, the quasi-BIC evolves into the BIC, and the quality factor (Q) diverges while the PLDOS saturates at a finite value. This result reflects an upper limit for enhanced light-matter interaction by BICs. Our findings pave the way for next-generation on-chip metasurfaces with maximum light-matter interaction strengths.

We propose a metasurface-based THz molecular sensor comprising asymmetric metallic rods for simultaneous refractive index and thickness sensing. The asymmetric metasurface supports two distinct modes: a high-quality factor (high-Q) quasi-bound state in the continuum (quasi-BIC) mode and a radiative mode, enabling dual-channel sensing for enhanced detection capability. As a proof-of-concept, we employed terahertz time-domain spectroscopy (THz-TDS) to experimentally characterize the resonances’ shifts of the metasurface integrated into a liquid cell for detecting water concentration in water-ethanol mixtures. Both modes exhibit redshifts with increasing water content. Besides, the experimental characterization of the metasurface, simulations were performed to study the changes in resonant frequencies of the two modes for surface sensing, by adding a thin layer onto the metasurface with variable thickness. Both experimental and simulation results show a higher sensitivity for the quasi-BIC mode (S = −0.15 THz/RIU for bulk refractive index sensing) compared to the radiation mode (S = −0.09 THz/RIU). The simulated field distributions reveal that the quasi-BIC’s strongly localized field enhances the sensitivity. These findings demonstrate that simple metasurfaces can serve as effective THz sensors for absorptive analytes, offering multiple resonance markers for improved accuracy.