Planar surface nanostructures engineered to exhibit specific optical functionalities, such as optical metasurfaces, have revolutionized the field of optical devices, owing to their compact sizes and versatile functionalities across a diverse range of applications [1–9]. In particular, plasmonic nanostructures, renowned as an exceptional platform for manipulating light–matter interactions at the nanoscale, have captured significant attention in the scientific community [10] primarily by virtue of plasmonic resonances. In general, the strength of resonant light–matter interactions can be characterized by the quality (Q) factor, a crucial characteristic that is determined by both radiative and dissipative losses, with the latter being inherent to metallic materials [11–13]. Recent advancements have spotlighted the bound states in the continuum (BICs) as a novel approach to dramatically enhance Q factors by blocking radiation channels [14] especially in all-dielectric nanostructures where dissipative losses do not exist. When the condition of BIC occurrence is slightly perturbed, typically by breaking the configuration symmetry, BICs evolve into quasi-BICs featuring finitely large Q factors, which are accessible with the far-field excitation [15–18]. For these reasons, BICs and quasi-BICs have been studied across a wide spectrum range, from microwave to visible, and found numerous applications, ranging from detection and sensing to absorption enhancement [9,15,19–36], highlighting their versatility. Despite the overall impressive progress, formidable challenges remain in realizing plasmonic BICs, e.g., in the fabrication of three-dimensional (3D) plasmonic structures, such as gold cones, that suffer from processing inaccuracies and compatibility issues with other optical components [10,26]. Planar metasurface alternatives, such as arrays of split ring resonators, are often more amenable to simpler, more effective solutions, while facilitating strong electromagnetic confinement and resonances.

In this work, we introduce a novel approach to achieve chiral quasi-BICs through the strategic manipulation of structural symmetry by rotating one of the paired plasmonic bricks. By fine-tuning the rotation angle, we enable the accurate modulation of the radiation loss in quasi-BICs, facilitating a transformative capability: transition from a perfect half-wave plate to an effective absorber for left-handed circularly polarized (LCP) light. This advancement not only demonstrates moderately high circular dichroism (CD) of ∼0.35, consistent with both simulation and experimental observations, but also marks a significant leap in controlling the propagation and confinement of chiral light at sub-wavelength scales. Our findings provide deep insights into the impact of symmetry breaking on the emergence of chiral quasi-BICs and herald new possibilities for the design of ultra-compact chiral photonic devices tailored for enhanced performance in applications such as chiral biosensors, CD spectroscopy, and advanced light manipulation platforms for information processing.

We utilize a well-established metal–insulator–metal (MIM) structure to design the reflective platform [37–40]. The schematic of the plasmonic unit cell is shown in Fig. 1(a), where an optically thick gold (Au) mirror and a pair of Au bricks with rounded corners are separated by a dielectric silicon dioxide (SiO2) spacer. The thickness of the middle SiO2 spacer layer is h=95nm, and the thickness of the Au substrate is 130 nm. The paired Au bricks have a length of l=310nm, a width of w=120nm, a height of t=50nm, and a corner radius of 20 nm, which are separated by a distance of s=350nm and arranged in a rectangular lattice with periodicities along the x and y axes of 700 and 350 nm, respectively. By judiciously rotating one of the paired plasmonic bricks, structural symmetry is perturbed, enabling the transformation from BICs to chiral quasi-BICs. To elucidate the rotation-mediated chiral quasi-BICs, we conducted numerical simulations to explore the interplay between rotation angles and radiative losses (Appendix A) [41]. Since plasmonic metasurfaces have inherent dissipative losses and can never fulfill ideal BIC conditions with infinite Q factors, the total Q factor Qtot is governed by the relationship Qtot−1=Qdis−1+Qrad−1, where Qdis and Qrad denote dissipative and radiative Q factors, respectively. In the simulation, we first calculated plasmonic unit cells with different orientations to retrieve Qtot. We then set Au as lossless and recalculated the structures to get Qrad at the eigenwavelength, as indicated by red stars in Fig. 1(b). The trend in the radiative Q factor aligns with the expected evolution behavior from BICs to quasi-BICs under symmetry perturbation. Specifically, a BIC with an ultra-high Qrad>108 occurs at the eigenwavelength of 946.49 nm when θ=0°, where the plasmonic metasurface is symmetric and achiral with respect to the circularly polarized (CP) excitation at normal incidence. As such, the plasmonic metasurface functions as a highly efficient half-wave plate (HWP), which is completely decoupled from the cross-polarized far-field radiation and reverses the spin direction of incident LCP or right-handed circularly polarized (RCP) light (Appendix B), a polarization transformation that occurs due to the near-equal reflection amplitudes and approximately ∼π phase retardation under two orthogonal linear polarization excitations (Appendix B). In this instance, the implemented plasmonic HWP adheres to the BIC condition of being effectively decoupled from the radiation continuum, which is evidenced by the cross-polarized reflection spectrum without any discernible resonances. When the rotation is introduced to perturb the BIC condition, the radiative Q factor Qrad decreases dramatically. Intriguingly, the radiative Q factor and structural asymmetry, represented by sin(θ), follow an inverse quadratic correlation, indicated by the solid green line in Fig. 1(c). Despite significant changes in radiative losses, dissipative losses remain essentially stable, as confirmed by the black line in Fig. 1(b). Therefore, the total Q factor, traced by a blue line in Fig. 1(b), exhibits a diminishing trend in our model, indicating the nuanced interplay between structural parameters and resonant behavior.

To further show the chiral nature of evolved quasi-BICs, we simulated cross-polarized reflection spectra under both LCP and RCP excitations. The results in Fig. 1(d) indicate that modifying the rotation angle significantly boosts the interaction between LCP incident light and the plasmonic metasurface with a quasi-BIC resonance occurring in the narrow spectrum range from 920 to 950 nm. Notably, at a rotation angle of θ=50°, the metasurface suppresses circular polarization conversion with RRL approaching 0. If the incident light switches to RCP, this quasi-BIC resonance is seen at θ=−50° [Fig. 1(e)], which is completely symmetrical to the LCP counterpart. Moreover, the plasmonic metasurface exhibits a pronounced chiral response in cross-polarized reflectance when one of the paired plasmonic bricks is rotated, ascribed to the rotation-induced chirality that induces distinct coupling strengths between LCP and RCP beams, which could be seen in the CD spectrum [Fig. 1(f)]. By varying the structural parameters, such as the lateral dimensions of bricks and the spacer thickness, the CD can be adjusted (Appendix C).

Therefore, the chirality can be continuously modulated by varying the rotation angle to engineer spin-selective radiative losses. To fully investigate the impact of rotation angle θ on chiral quasi-BICs, we analyzed the normalized electric field distributions and current flows for the metasurfaces with rotation angles of θ=10° and 50°, respectively. As depicted in Fig. 2(a), distinct electric field localizations are formed at θ=10° and 50°, with significant variations in distribution between the paired Au bricks at θ=50° [42,43]. In particular, the electric field at θ=10° is predominantly centralized between the two Au bricks as the symmetry is slightly perturbated, whereas it shifts markedly at θ=50°, localizing at the edges of the right Au brick and the center of the left. Meanwhile, a pair of antiparallel currents is formed between the top and bottom Au layers, resulting in a pair of magnetic dipoles [43]. Further simulations indicate that at θ=10°, the electric field distribution under RCP excitation approximately mirrors that of LCP incidence, as shown in Fig. 2(b). However, at θ=50° under RCP incidence, the field is predominantly confined to the right Au brick, with the left brick capturing minimal energy. This asymmetry suggests a chiral optical response, leading to a flipped electric field distribution in the spacer at different polarizations, consistent with the simulated cross-polarized reflectivities. Our simulations demonstrate enhanced electric field confinement in the spacer layer under RCP incidence post-rotation [Fig. 2(b)], underscoring the spin-selective tunable confinement capabilities. Due to the asymmetric field distributions, pronounced magnetic dipoles are not excited for RCP incitation.

After design, we fabricated several metasurface samples with different rotation angles using the standard fabrication technique (Appendix D). Figure 3(a) and Appendix E showcase the scanning electron microscope (SEM) images of fabricated samples, highlighting the good uniformity and efficiency of the sample processing. Each sample comprises 70×140Au antennas with a total area of 49μm×49μm, ensuring the laser spot fully encompasses the sample area in the measurement (Appendix F). Figure 3(b) shows the measured reflection spectra under LCP incidence, respectively, where a notable reduction in cross-polarized reflectivities with increasing rotation angle is observed over a broadband spectral range spanning from 860 to 980 nm, manifesting the emergence of chiral BICs that exhibit enhanced field localization and light–matter interaction due to the broken symmetry. This trend generally aligns with our simulations in Fig. 3(c), indicating a diminishing efficacy in flipping the spin of the incident light, a pivotal characteristic influenced by the chirality of the structure induced by the rotation. Nevertheless, a gradual red shift in the spectra is evident in the experiment as rotation angles increase from 30° to 60°, different from simulation results, which may be ascribed to the slight difference in the dimensions of fabricated bricks with rotations. Complementing this, we provided simulated and measured cross-polarized reflection spectra RLR under RCP incidence in Figs. 3(d) and 3(e), where experimental results generally match the simulation outcomes, although there is a blue shift and reduced reflectivity in the overall cross-polarized reflection spectra under RCP incidence. Compared to RRL,RLR exhibits a larger discrepancy, which can be ascribed to the slightly reduced degree of circular polarization (DoCP) for the RCP incident light. This reduction in DoCP affects the consistency of the polarization state, leading to the observed variance in the reflection coefficients. Notably, the absence of structural symmetry has a more pronounced effect on the polarization selection of LCP compared to RCP, illustrating the chiral nature of the quasi-BICs.

Finally, we analyzed the absorption spectrum under LCP incidence by incorporating co- and cross-polarized reflectance (AL=1−RRL−RLL). Figure 4 shows the simulated and experimental absorption spectra of fabricated quasi-BIC metasurfaces with various angles. The close resemblance between the simulated and experimental results indicates the effectiveness of our metasurface design in facilitating tunable CP light absorption through the introduction of rotation-induced radiative losses. The absorption spectrum at RCP incidence is shown in Appendix G. Additionally, we observed a moderately high CD of approximately 0.35 at a wavelength of 920 nm for the metasurface with a rotation angle of 60° in both simulation and experiment, as marked by black dots (Appendix H). Higher CD values could be achieved by introducing structural asymmetry (Appendix C) or by using more complicated unit cell designs [43,44].

We have demonstrated a plasmonic chiral quasi-BIC metasurface by properly rotating one of the paired plasmonic bricks to perturb structural symmetry and thus to modulate spin-selective radiative losses for CP light. The fabricated metasurface with a precisely controlled rotation angle enables a critical transition from an HWP to an efficient absorber for LCP light. Specifically, the rotation-induced chiral quasi-BICs achieve a moderately high CD of approximately 0.35, consistently observed in both simulation and experimental frameworks. This CD value is comparable to those of other plasmonic quasi-BIC metasurfaces (see Table 1 in Appendix I). This achievement marks a significant step forward in controlling the propagation and confinement of chiral light at sub-wavelength dimensions, thereby paving the way for the development of ultra-compact chiral photonic devices without any complicated 3D nanostructures. These innovations also promise to revolutionize various applications, from chiral sensing, enantiomer sorting, and enhanced CD spectroscopy to robust platforms for light manipulation in information processing.