Editor's Recommendation:

Plasmonic nanostructures have become essential in enhancing light–matter interactions due to their strong field confinement and subwavelength-scale manipulation capabilities. However, their performance is often hindered by the intrinsic loss of metallic materials. A central challenge in the field remains how to achieve high quality factors (Q-factors) while maintaining deep subwavelength field confinement. In recent years, approaches such as surface lattice resonances (SLRs) and bound states in the continuum (BICs) have been proposed to mitigate radiative losses. Yet, realizing and comparing both mechanisms within a single, compact platform—especially one operable at communication wavelengths—remains nontrivial.

Prof. Jinwei Shi's group at Beijing Normal University experimentally demonstrated a reusable plasmonic-photonic hybrid metasurface that supports both SLRs and BICs. By tuning the thickness of the dielectric cladding, waveguide modes are introduced to enhance coupling near the Γ-point, achieving a high Q-factor of 260 in a 200 μm×200 μm footprint. The study provides a systematic comparison between SLRs and BICs, revealing that BICs can surpass SLRs in terms of Q-factor, while SLRs have greater robustness to angular variations. This hybrid platform, combining compact size with high-Q performance, offers a promising strategy for applications in pixel-level displays, sensing, and nonlinear optics.

                                                                                                                                  ——Editor, Photonics Research

Summary:

Surface plasmons (SPs), capable of confining light beyond the diffraction limit, offer a powerful means to enhance light–matter interactions at deep subwavelength scales. However, their practical utility is restricted by multiple loss mechanisms—metallic absorption and fabrication-induced imperfections such as surface roughness and grain boundaries. As these intrinsic losses are hard to eliminate, reducing radiative losses through photonic design has become a promising strategy to increase Q-factors in plasmonic systems.

Among these strategies, bound states in the continuum (BICs) and surface lattice resonances (SLRs) have attracted a lot of attention. BICs rely on topological protection in momentum space, while SLRs arise from the coupling between localized surface plasmons and diffraction channels. In metal-dielectric hybrid systems, the inclusion of photonic modes enables further redistribution of field energy, effectively suppressing intrinsic losses.

In this work, the authors design a metal–dielectric composite structure by tuning the dielectric cladding thickness and optimizing experimental conditions. They observe a series of high-Q SLR and BIC modes experimentally, with Q-factors of quasi-BICs (q-BICs) reaching 260 near the communication band—competitive with leading literature values. A 200 μm×200 μm array of gold nanorods on a glass substrate, coated with water-soluble PVA cladding, enables reusability and precise tuning of mode hybridization. Infrared angle-resolved transmission measurements using fixed-point fiber collection reveal that reduced fiber diameter improves Fourier-plane resolution, enhancing the measured Q-factors.

Figure 1(a) Schematic of the metal–dielectric hybrid structure. (b) Angle-resolved transmission spectra. (c) Transmission spectrum of the quasi-BIC at the Γ point and the corresponding Q-factor fitting.

Notably, all measurements are performed under incoherent light excitation. The authors suggest that further improvements could be achieved with larger arrays and coherent sources. The device's footprint is compatible with pixel-level applications such as metasurface displays and spatial light modulators. Finally, the study directly compares BICs and SLRs under identical experimental conditions, showing BICs can achieve Q-factors up to four times higher than SLRs, though SLRs are less sensitive to angular variation. These findings highlight the complementary advantages of BICs and SLRs for different applications.

Prof. Jinwei Shi commented: "The low Q-factors of conventional plasmonic resonances have long limited their application in strong light–matter interaction regimes. While both SLRs and BICs are promising for achieving high Q-factors, it has been puzzling that radiative SLRs sometimes report higher Q-values than non-radiative BICs in the literature. One key issue is that SLRs and BICs are usually realized in different structures and conditions, making direct comparison difficult. Our work addresses this by enabling mode tuning on the same sample via reconfigurable cladding, reducing both fabrication cost and ambiguity. The findings suggest that while BICs promise higher Q-factors, SLRs are more robust. These insights will help guide the selection of resonant modes for different application scenarios."

In future studies, the team plans to leverage the ability of the hybrid surface to support multiple high-Q resonances to further explore light–matter interactions, including enhanced harmonic generation, on-chip parametric oscillation, angle-sensitive photoluminescence, and spatial filtering—laying the groundwork for next-generation nanophotonic devices.