Chirality, or the inability of objects to align with their mirror counterparts, is prevalent at all scales, from the quantum [1] to the macroscopic [2]. In biology, many essential biomolecules, such as DNA and amino acids, exist exclusively as right-handed or left-handed enantiomers. Alternations in the chirality of these biomolecules can lead to serious diseases. Consequently, the study of chirality has garnered significant attention for its applications in various fields, including the food industry [3], drug development [4,5], medical diagnostics [6,7], materials science [8], and lateral optical forces [9,10]. Chiral molecules often exhibit optical activity, such as circular dichroism (CD), which reflects the differential absorption of right-handed and left-handed circularly polarized (RCP and LCP) light. Based on this principle, CD spectroscopy is considered to be a valuable tool for chiroptical analysis. However, due to the scale mismatch between chiral molecules and wavelength, CD signals are typically significantly weaker (∼10−3−10−6) than the absorbance signals [11], limiting the sensitivity of chiral detection.

Pioneering works [12,13] have indicated that CD signals are proportional to the optical chirality C, defined as C=−ω2c2Im(E*·H), where ω and c denote the angular frequency and speed of light, and E and H are the complex electric and magnetic field vectors, respectively. Therefore, improving chiral detection sensitivity, or the magnitude of the CD signal, lies in generating enhanced optical chirality [14–16], particularly in the light field interacting with chiral molecules. Recently, metasurfaces [17–22] have been widely used to amplify CD signals by manipulating the electromagnetic field to generate enhanced optical chirality. Plasmonic metasurfaces [23–25] were initially proposed to improve chiral sensing through the localized surface plasmon resonance of metals. However, concerns arose in the photothermal heating effects potentially damaging biological molecules. Subsequently, dielectric Mie metasurfaces [26–30] were proposed to create strong chiral hotspots, offering a promising alternative to plasmonic metasurfaces. Nonetheless, the enhancements of optical chirality achieved through these fundamental multipoles are typically limited to an order of magnitude and primarily occur within high-index nanostructures, hindering the full interaction between chiral molecules and chiral fields, thereby reducing detection sensitivity.

Here, we introduce a new paradigm for ultrasensitive chiral detection by proposing a toroidal dipole Fabry–Perot bound state in the continuum (TD FP-BIC) metasurface, aimed at accessing significant optical chirality enhancements outside the nanostructures, as illustrated in Fig. 1(a). The polarization-insensitive TD resonance is induced through dual symmetry breaking, creating strong chiral hotspots outside the clustered metasurface. By controlling the coupling between the TD resonance and its multilayer reflector-induced perfect mirror image, we achieve a metasurface operating at FP-BIC [31,32]. Unlike metallic mirrors that introduce significant absorption losses, the dielectric reflector provides high reflectivity with minimal dissipation, enabling low-loss interference. When the spacer thickness satisfies the FP-BIC phase condition, destructive interference between the TD mode and its mirror image suppresses radiation leakage, giving rise to a high-Q resonance with strongly enhanced electromagnetic fields and optical chirality. Under these conditions, the local maximum enhancement and average optical chirality enhancement (Cmax and Cavg), normalized to circularly polarized light in air, reach approximately 6×104-fold and 2×103-fold, respectively, within the near-infrared (NIR) spectral regime. Notably, the choice of the NIR regime is primarily motivated by its superior compatibility with practical sensing requirements, including higher detector quantum efficiency, lower material absorption losses, and more mature nanofabrication techniques. This marks significant progress compared to state-of-the-art dielectric metasurfaces [11,26–28,30,33–37], as shown in Fig. 1(b). As a proof of concept, the TD FP-BIC metasurface enables an 866-fold enhancement of CD signals compared to the chiral molecules alone without nanostructures. More detailed comparisons between this work and previously reported studies on chiral sensing are provided in Appendix B. Our results present a novel strategy for manipulating the chiral electromagnetic field, revealing new possibilities in ultrasensitive sensing and chiral photonics.

TD [38–41], the third family of electromagnetic dipoles alongside the electric dipole (ED) and magnetic dipole (MD), has garnered considerable attention due to its significant field enhancements and unique exposed field characteristic. A straightforward method to induce TD resonance [42] involves creating the symmetry breaking within the dielectric cluster, as shown in Figs. 2(a) and 2(b). The unit cell of the metasurface is immersed in water and composed of four Si nanofins on a SiO2 substrate. The metasurface has a period (p) of 600 nm in both x and y directions, with an intra-cluster distance (d) of 220 nm and height (h) of 500 nm. The square nanofins have lengths of a=180nm and b=120nm. For a symmetric metasurface with four identical nanofins [Fig. 2(a)], only one resonance appears in the transmission spectra, but not the TD resonance. Analysis of multipole decomposition and electromagnetic field distributions (see Fig. 5 in Appendix C) suggests this resonance arises from a collective response of four longitudinal MDs. Due to the confinement of field enhancements within nanostructures, symmetric metasurfaces with strong MD responses are not suitable for chiral sensing applications. Introducing symmetry breaking results in the splitting of the previous single resonance in transmission spectra [Fig. 2(b)], revealing additional resonances. Multipole decomposition and electromagnetic field distributions (see Fig. 6 in Appendix C) show a pronounced TD response at approximately 1090 nm exclusively under the TM excitation. However, polarization-dependent TD responses in the metasurface hinder the generation of enhanced optical chirality necessary for high-sensitivity chiral detection.

In response to this challenge, we propose the concept of dual symmetry breaking to obtain a polarization-independent TD response. Although the metasurface exhibits asymmetries in both x and y directions, as shown in Fig. 2(c), identical additional resonances are observed in the transmission spectra for both TM and TE polarization excitations. Multipole decomposition and electromagnetic field distributions for these excitations are detailed in Fig. 7 (Appendix C), confirming the polarization-independent TD responses. This design evolution, as detailed in Appendix C, enables the stepwise activation of high-Q TD resonances under orthogonal polarizations. As a result, the final dual-symmetry-breaking configuration achieves polarization-insensitive high-Q responses, extending previous single-axis designs toward more versatile bidirectional control [42]. Multipole decomposition under RCP excitation, illustrated in Fig. 2(d), highlights the predominance of the TD resonance. To assess chiral detection enhancement, we calculate the Cavg under RCP and LCP excitations, as illustrated in Fig. 2(e). Here, the simulation domain for calculating Cavg is complementary to the nanostructures, as indicated by the light orange region in the inset of Fig. 2(e). It corresponds to the non-structured portion of a 600nm×600nm×500nm volume along the x, y, and z directions, respectively, and is normalized to the incident circularly polarized light. Remarkably, the Cavg reaches nearly a 100-fold enhancement. This achievement is attributed to the significant electromagnetic field enhancements and unique field configuration of TD resonance.

For an intuitive understanding of the significant enhancements in optical chirality, Figs. 2(f)–2(h) depict the electromagnetic field distributions at z=h/2 under the RCP illumination. The z=h/2 plane corresponds to the midpoint of the nanofin’s height. Within the unit cell, torus-like electromagnetic field lines and ring-like electric fields closely resemble the theoretical patterns of TD responses [42–44], affirming their presence and significant contribution. Therefore, although both MD and TD modes are present in the multipole decomposition, the dominant toroidal features support the identification of this structure as a TD metasurface. The electric and magnetic fields exhibit maximum enhancements of approximately 35 times and 45 times, respectively, while Cmax reaches up to 650 times outside the nanostructures, providing a favorable condition for detection. Similarly, Figs. 2(i)–2(k) illustrate the electromagnetic field distributions at z=h/2 under the LCP illumination. It is noteworthy that the TD metasurface exhibits identical electromagnetic field distributions but opposite enhancements in optical chirality under RCP and LCP excitation, respectively. These results confirm the intrinsically achiral nature of the metasurface and ensure that the subsequently observed CD signals originate solely from the chiral molecules, allowing the metasurface to function as a high-efficiency and background-free CD signal amplifier.

To further enhance optical chirality, we create multilayer reflector-induced TD resonance to couple with the TD resonance metasurface. This controlled coupling between the TD resonance and its multilayer reflector-induced counterpart gives rise to TD FP-BIC phenomena. Figure 3(a) illustrates that the TD FP-BIC metasurface consists of the previously designed TD metasurface and an all-dielectric multilayer reflector separated by a SiO2 spacer, with additional details provided in Appendix E. To determine the BIC condition, the dielectric spacer is meticulously designed to control the coupling between the TD resonance and its mirror image. By varying the thickness of the spacer (t), we can observe clear transitions in the transmission spectra from quasi-TD FP-BIC to TD FP-BIC and back to quasi-TD FP-BIC in Fig. 3(a). Moreover, the Cavg exhibits a similar variation pattern to the transmission spectra, as shown in Fig. 3(b), suggesting that the significant enhancement in optical chirality stems from the BIC phenomena. Opting for a spacer thickness of 843 nm, as indicated by the green dashed rectangular frame, allows us to excite quasi-TD FP-BIC and achieve significant optical chirality enhancement. We speculate that the low transmission of this quasi-TD FP-BIC resonance arises from the reflectance mismatch between the upper metasurface and the underlying multilayer reflector, with reflectance of 95% and 100%, respectively. Further analysis and evidence can be found in Appendix D.

Figure 3(c) illustrates the multipole decomposition of the TD FP-BIC metasurface, indicating that the TD responses remain dominant. The difference is that, in this case, the MTD dominates instead of the ETD. Under these conditions, the Cavg in Fig. 3(d) shows a clear sign reversal from negative to positive around the FP-BIC resonance, achieving a remarkable enhancement of 2200-fold. To gain a better insight into the enhancement of optical chirality, we demonstrate the optical chirality enhancement distributions at z=h/2 for three different wavelengths, as shown in Figs. 3(e)–3(g). Here, the blue, green, and yellow pentagons are consistent with those in Fig. 3(d), representing wavelengths of 1152.44 nm, 1152.45 nm, and 1152.46 nm, respectively. Strikingly, the local maximum enhancement of optical chirality reaches about 6×104-fold for a wavelength of 1152.45 nm. To the best of our knowledge, this is the highest value within non-structured regions reported to date. Strikingly, the local maximum enhancement of optical chirality reaches approximately 6×104-fold at a wavelength of 1152.45 nm. To the best of our knowledge, this represents one of the highest values reported to date, even when accounting for the slight material absorption due to fabrication imperfections. Further details on the impact of fabrication-induced variations and material absorption on the metasurface performance are provided in Fig. 9 of Appendix F, highlighting the robustness of our design strategy.

Additionally, cut planes demonstrating optical chirality enhancements at various z heights under RCP illumination are illustrated in Appendix G. Remarkably, these enhancements maintain consistently high levels across different heights, ranging from z=0 to z=h. These results underscore the significant potential of the proposed TD FP-BIC metasurface to enhance CD signals for chiral molecules across a wide volume range, emphasizing its utility in chiral sensing.

To explore the practical applications of the TD FP-BIC metasurface in ultrasensitive chiral detection, we consider the scenario where the metasurface is uniformly immersed in a homogenous chiral molecule layer with a thickness of 500 nm. The chiral molecules exhibit a refractive index of n=1.33+10−5i and a fixed Pasteur parameter of κ=10−5i, consistent with the parameters used in previous works [45] and the realistic value for chiral biomolecules [46,47], such as aqueous solutions of monosaccharides [48,49].

In conclusion, we have introduced an ultrasensitive chiral detection platform utilizing a TD FP-BIC metasurface. The TD FP-BIC metasurface leverages controlled coupling between TD resonance and its mirrored counterpart, achieving record-high Cmax and Cavg enhancements of 6×104-fold and 2×103-fold, respectively, in the non-structured region. With the TD FP-BIC metasurface, we realize a 2-order-of-magnitude enhancement in the CD signal. To our knowledge, this marks a significant advancement over prior studies. Additionally, our proposed paradigm is compatible with resonance-gradient metasurfaces [53,55,56] for broad spectral range chiral detection. Compared with the actively tunable-material-based metasurfaces [57–59], this approach avoids additional fabrication complexities, reduces costs, and eliminates response time delays, while maintaining high-Q resonances and strong optical chirality. This strategy offers a practical, scalable solution for real-world chiroptical sensing, as shown in Fig. 12 of the Appendix I. Our results pave a new avenue toward high-performance enantiomeric detection and separation, providing a novel design strategy to manipulate the near-field electromagnetic fields, which may inspire more innovative photonic devices for enhanced light–matter interaction in the future.