Polarization is a fundamental property of light that plays a pivotal role in investigating photon–matter interactions and exploring the electronic, structural, and dynamic properties of materials at the nanoscale. The ability to actively and rapidly control the polarization state of light is critical for various free-space and integrated photonic applications^[1,2]. Ultrafast polarization modulation, in particular, has the potential to drive significant advancements in interdisciplinary fields such as nanophotonics, optomechanics, and nonlinear optics^[3–7]. Traditional polarization modulation techniques, such as mechanically rotating wave plates, electro-optic modulators, and liquid crystal devices, are limited by their response time or integration challenges^[8,9]. In contrast, metasurfaces offer exceptional capabilities for manipulating electromagnetic waves, enabling compact and efficient polarization control^[10–18]. Resonant metasurfaces, in particular, enhance light–matter interactions, providing a platform for high-Q-factor resonances and far-field polarization manipulation^[19–23].

Chiral bound state in the continuum (BIC) metasurfaces have emerged as a promising approach for polarization control due to their unique far-field polarization characteristics^[24–26]. These metasurfaces support high-Q resonances and enable the generation of circularly polarized light^[27–30]. Moreover, all-optical control of polarization states—where the polarization of a signal light pulse can be modulated by another pulse—can significantly enhance the rate of optical polarization modulation^[31–33]. However, achieving high-purity chiral quasi-BIC modes typically requires breaking the out-of-plane symmetry of the metasurface, which reduces the Q-factor due to enhanced radiative losses. For all-optical modulation, this lower Q-factor necessitates larger refractive index changes to achieve comparable polarization modulation amplitude, thereby increasing the required pump energy density. Addressing the trade-off between symmetry breaking and resonance quality remains a key challenge.

In this study, we utilize the intrinsic modes of a silicon-based chiral BIC metasurface to achieve all-optical ultrafast control of light polarization. By optimizing the structural asymmetry, we achieve high-purity chiral resonances (S3∼−0.92) and rapid polarization switching (∼0.4ps) with minimal energy consumption (0.088GW/cm2). To address the reduction in the Q factor due to decreased structural symmetry, we implemented a refractive index matching layer similar to that of the substrate, and then induced an inclination angle in the vertical direction of the unit cell. The PMMA matching layer’s refractive index minimizes scattering losses at the metasurface interface, preserving the Q-factor despite symmetry breaking. The measured approximate sub-picosecond polarization switching is comparable to modulation time in plasmonic materials^[34,35]; while our demonstration uses α-Si, and the mechanism could extend to direct-bandgap semiconductors^[36] where faster carrier recombination may enable faster modulation.

The transmission/reflection spectrum, Q-factors, and corresponding field pattern are calculated with a finite-element method using commercial software (COMSOL Multiphysics). A perfect matching layer is set at the top and bottom of the model to absorb the waves radiated to the outside of the simulation area. Periodic boundary conditions are set in the x and y directions. Only zero-order diffracted light can be deduced from the grating formula. The plane perpendicular to the zero order is selected for calculating electric field components Px in the x direction and Py in the y direction. The details can be seen in the Supplementary Information.

The chiral metasurface is prepared by the standard nanofabrication process combined with slanted etching (The details can be seen in the Supplementary Information). First, the 202 nm α-Si film is deposited on the SiO2 substrate by plasma enhanced chemical vapor deposition (PECVD). Second, an 80 nm PMMA film is spin-coated and baked at 180°C for an hour. Next, the PMMA resist is exposed to the electron beam (Raith E-line, 30 kV) and developed in MIBK/IPA solution for 30 s at 0°C to form the PMMA nanostructures. After the resist development, the sample is immersed in the remover PG solution at room temperature for 12 h to lift off. Then, the Cr nanostructure is utilized as a mask for slanted etching of samples. In the end, the remaining Cr film is removed by immersing the sample in the chromium etchant for 10 min.

A detailed experimental setup for characterizing chiroptical response and ultrafast polarization control has been shown in the Supplementary Information. The light source is the supercontinuum light emitted by NKT super K, and its spectral range is 450–2400 nm. The supercontinuum light changes from linearly polarized light to circularly polarized light through the polarizer and quarter-wave plate. The angle of the quarter-wave plate can be adjusted to enable the outgoing light to be left-handed circularly polarized (LCP) light and right-handed circularly polarized (RCP) light. In order to find the relative position of the sample and supercontinuum laser in the focal plane of the lens, an illumination and imaging system is added to the experimental setup.

The ultrafast polarization switching effect is measured using a homemade pump–probe setup. The femtosecond light with a center wavelength of 800 nm is split into two beams. One femtosecond laser beam is converted into 400 nm pump light through a BBO crystal. The half-wave plate behind the BBO crystal can be used to change the linear polarization angle of the incident pump light, ensuring that the polarization state of the pumped linearly polarized light does not change after passing through the quarter-wave plate. Another femtosecond laser passes through a delay line, generating a supercontinuum laser for use as a probe light. During the measurement process, the supercontinuum laser and pump light are focused together on the sample. The reflected supercontinuum laser is collected into the spectrometer through a quarter-wave plate and a polarizer. The LCP and RCP components of the reflected light can be measured by rotating the polarizer at different angles. By adjusting the position of the delay line, the LCP and RCP components of the reflected light with different time delays between the pump light and probe light can be measured.

The schematic diagram illustrating ultrafast polarization control is presented in Fig. 1(a). When an LCP light is incident upon a chiral quasi-BIC metasurface, only LCP light is reflected due to the absence of orthogonal polarization conversion. Upon the incidence of another laser pulse on the chiral dielectric metasurface, free carriers are generated, leading to changes in the refractive index. The contribution of free carrier generation to the real part (Δε1) and imaginary part (Δε2) of the dielectric constant change is known as^[37]Δε1=−Ne2m*ε0(ω2+τd−2),(1)Δε2=−Δε1ωτd,(2)where τd is the Drude damping time (0.5 fs for α-Si), N is the photoinduced density of the electron-hole plasma, e is the electron charge, and m*=0.09m0 is the reduced carrier mass. Here, we take the free carrier density as 9×1019cm−3, and then the refractive index induced by free carriers is about Δn=−0.04 and Δk=0.03. A negative change in the real component of the refractive index causes a blue shift of chiral resonance, while a positive variation in the imaginary component enhances light absorption and reduces reflected light intensity. This refractive index change modifies the chiral resonance, resulting in reflected light containing both LCP and RCP components. Consequently, the polarization of the reflected light transforms into elliptical polarization, as shown in Fig. 1(b). When the refractive index changes, the reflected polarization state on the Poincaré sphere transitions from the pole toward the equator.

The chiral quasi-BIC metasurface was designed by employing tilted silicon elliptical cylinders to simultaneously break both in-plane and out-of-plane symmetries, as illustrated in the inset of Fig. 1(a). Here, we select 700 nm as the operational wavelength because it optimizes both the nonlinear response (Δn/Δk) and fabrication feasibility for our slanted α-Si metasurface. It offers stronger free carrier modulation compared to NIR alternatives while maintaining sub-wavelength feature sizes for high-Q quasi-BIC formation. The unit cell comprises two slanted amorphous silicon (α-Si) elliptical cylinders with the following geometric parameters: p=400nm, h=205nm, l=200nm, s=100nm, d=180nm, θ=10°, and β=23°. We choose the parameters to approach the most optimized dimensions, and elevated Q-values for chiral resonance coincide with enhanced CD and polarization ellipticity (see the Supplementary Information, Note 1). The fabrication of the chiral quasi-BIC metasurface was carried out using a standard nanofabrication process, with an additional silicon slanted etching technique employed to achieve the desired structure (detailed fabrication procedures are provided in the Supplementary Information, Note 2). Furthermore, a PMMA layer was deposited on the sample surface to function as a refractive index matching layer. Figure 1(c) presents both top-view and cross-sectional scanning electron microscope (SEM) images of the fabricated sample, demonstrating that the silicon elliptical cylinders exhibit parallel and smooth sidewalls with a measured tilt angle of 10°±0.5°.

In an ideal scenario, the BIC mode is non-radiative and cannot couple with any incident light due to its infinite quality factor. In momentum space, the far-field polarization characteristics of a BIC are characterized by polarization singularities. To realize chiral quasi-BICs, it is crucial to break the symmetry not only within but also beyond the structural plane^[25]. Figure 2 illustrates the transformation from an ideal BIC structure to a chiral quasi-BIC structure, accompanied by the corresponding eigenpolarization in momentum space. The left panel of Fig. 2(a) depicts the initial metasurface design, which exhibits C4 symmetry and supports the BIC mode. This design consists of two vertically aligned elliptical columns of identical dimensions. To analyze the polarization characteristics of the system, the polarization state of the Bloch mode is represented using polarization ellipses, where right-handed and left-handed states are denoted by red and blue, respectively. As demonstrated in Fig. 2(c), the polarizations at non-Γ points are predominantly linear, while the BIC mode, characterized by an infinite Q-factor, appears as a polarization singularity at the Γ point.

The unit cell depicted in the middle panel of Fig. 2(a) is constructed by rotating the two elliptical cylinders clockwise and counterclockwise by an angle β, respectively. This rotational transformation breaks the in-plane symmetry of the unit cell, reducing its symmetry from C4 to C2. As a result, the BIC mode, originally characterized by an infinite Q-factor, transitions into a quasi-BIC mode with a finite but significantly large Q-factor. From the perspective of conserved topological charges, the original polarization singularity, which possesses a topological charge of +1, splits into two chiral points, each carrying a topological charge of +1/2. These chiral points are symmetrically distributed about the axis kx=0. In Fig. 2(d), the blue solid circle represents a chiral point with LCP eigenpolarization, while the red solid circle denotes a chiral point with RCP eigenpolarization. Following the breaking of in-plane symmetry, the far-field polarization states at most positions in momentum space become elliptically polarized. However, the eigenpolarization at the Γ point retains its linear polarization characteristics.

To relocate the chiral point to the Γ point, the out-of-plane mirror symmetry is broken by introducing a tilt to the silicon elliptical cylinders, as depicted in the right panel of Fig. 2(a). When the elliptical cylinders are tilted, the overall polarization state shifts leftward and undergoes a rotational transformation, as illustrated in Fig. 2(e). By optimizing the tilt angle θ to 10° and the in-plane rotation angle β to 23°, the blue solid circle, which represents the LCP eigenpolarization, is successfully shifted to the Γ point in momentum space. Notably, Fig. 2(b) illustrates the electric field distribution in the vicinity of the unit cell structure shown in Fig. 2(a). It is evident that the ideal standing-wave-type BIC resonant modes are no longer maintained as the structural asymmetry increases. To mitigate the reduction in the quality factor resulting from the decreased structural symmetry, a 500-nm-thick PMMA matching layer, with a refractive index closely matched to that of the substrate, was deposited on the sample surface. Since the introduction of a vertical inclination angle, the conditions necessary for achieving a chiral response and the corresponding breaking of out-of-plane symmetry in the metasurface structure remain fully satisfied.

The eigenpolarization characteristics of the intrinsic chiral metasurface were then experimentally validated through angle-resolved transmission and reflection spectroscopy. Figures 3(a) and 3(b) present the simulated angle-resolved transmission spectra for incident LCP light and RCP light on the metasurface, respectively. Two distinct chiral points are identified, corresponding to incident angles of 0° and 6°. At an incident angle of 0°, the quasi-BIC resonance peak is observed at approximately 700 nm. Under normal incidence, the transmission of the structure approaches near-zero values for LCP light, while reaching near-unity values for RCP light. In contrast, at an incident angle of 6°, the transmission for LCP light approaches unity, whereas for RCP light, it approaches zero, demonstrating the chiral selectivity of the metasurface.

Figures 3(d) and 3(e) display the experimentally measured angle-resolved transmission spectra for incident LCP light and RCP light on the metasurface, respectively. The experimental results exhibit excellent agreement with the simulated data, robustly confirming the intrinsic chirality of the metasurface under normal incidence. The measured spectrum reveals a Q-factor of approximately 118, which is nearly twice as high as that of chiral metasurfaces based on split-ring resonators^[31]. This significant enhancement underscores the efficacy of the refractive index matching layer in maintaining the resonance quality and minimizing optical losses, thereby preserving the mode resonance characteristics.

Additionally, the reflection spectrum for normally incident LCP light was measured, as shown in Figs. 3(c) and 3(f). In this analysis, the reflected light ratio (RLR) is defined as the ratio of the power of reflected RCP light to the power of incident LCP light. Owing to the intrinsic chirality of the metasurface, the co-polarized reflection channel remains active, while the cross-polarized reflection channel is significantly suppressed. The ratio of LCP to RCP components (indicated by the green dashed line) in the reflected light reaches 52.3, and then the Stokes parameter [S3=(IRCP−ILCP)/(IRCP+ILCP)] can be calculated. We get a high value of −0.96 at the resonance wavelength in simulation. These results further corroborate the strong chiral response of the metasurface, demonstrating its exceptional polarization selectivity.

We subsequently explored the ultrafast control of polarization using a homemade pump–probe setup, as depicted in Fig. 1(a). The femtosecond pulses at 400 nm were employed as the pump light to optically excite carriers within the silicon-based metasurface. The supercontinuum pulsed white light is set as the probe beam, which is generated by focusing a femtosecond laser beam on the water. The delay time between the pump and probe beams was defined as Δt=tprobe−tpump, and we then measured the instantaneous spectrum changes in the reflected light. Notably, the experiments revealed that the change in reflected light polarization at resonance was significantly larger than the change in transmitted light polarization. This enhancement arises from the closure of the circular polarization conversion channel due to the introduction of a nonlinear refractive index (see Note 1 in the Supplementary Information).

As previously established, the excitation of amorphous silicon induces variations in the free carrier density, which subsequently modifies the refractive index of the metasurface. This alteration consequently affects the intrinsic chiral response of the metasurface. Figure 4(a) provides a direct comparison of the reflection polarization at delay time of −0.8 and 0 ps, illustrating the temporal evolution of this phenomenon. At a delay time of −0.8ps, minimal pump–probe temporal overlap preserves unperturbed resonance characteristics, maintaining chiral polarization at the Γ point with predominantly LCP reflected light. Under these conditions, the ratio of LCP light to RCP light is calculated to be 23.6 (at 700 nm, indicated by the green dashed line), and the corresponding Stokes parameter S3 reaches a high value of −0.92. At a delay time of 0 ps, the refractive index change induced by the pump beam is maximized, leading to a blue shift in the resonant wavelength because of the negative refractive index change induced by free carriers. As a result, the reflected light contains both LCP and RCP components, resulting in a transition from circular to elliptical polarization. From the measured spectrum, the ratio of LCP light to RCP light was calculated to be 4.23, and the corresponding S3 value decreased to −0.62. These experimental results demonstrate that ultrafast polarization modulation can be achieved on the sub-picosecond timescale.

To further investigate this phenomenon, we measured the detailed reflection spectra for LCP light and RCP light as a function of the delay time (Δt), with LCP light incident on the metasurface. The pump pulse density is fixed at 0.088GW/cm2. The results reveal a significant increase in the reflection of the RCP component and a marked decrease in the reflection of the LCP component when Δt>0. At the resonant wavelength (∼700nm), the maximum increase in reflected RCP light reaches 232%, while the change at non-resonant wavelengths is comparatively smaller. In contrast, the reflected LCP light at the resonant wavelength exhibits a decrease of approximately 40%. This pronounced variation in the reflected RCP component can be attributed to the gradual alteration of the refractive index caused by free carrier recombination in amorphous silicon. Notably, refractive index changes induced by thermal effects typically exhibit relaxation time on the order of tens of picoseconds. Furthermore, the Stokes parameter S3 of the reflected light shows significant dynamic variation. Figure 4(d) illustrates the evolution of ΔS3 with delay time near the resonant wavelength. Specifically, S3 reaches its maximum value within approximately 1 ps and then decreases rapidly. The rise time is approximately 0.4 ps, while the time for ΔS3 to decay from its maximum to zero is around 1.3 ps.

The above results highlight the ultrafast nature of polarization modulation and the strong correlation between the dynamic refractive index changes and the metasurface’s chiral optical response. We emphasize that the pump intensity used in this measurement was only half of that used in Ref. [31], while ΔS3 (∼0.3) was larger than that (∼0.19) in Ref. [31] at the resonance wavelength. This is because we obtained a higher Q value chiral resonance in the spectrum by designing chiral points located at the Γ point. By carefully engineering the relative spectral position between the pump light wavelength and the chiral resonance, polarization states can be modulated more rapidly by leveraging the Kerr effect in silicon^[6]. Moreover, modulation speed could be further enhanced by utilizing direct-bandgap semiconductors, such as gallium nitride^[36], which exhibit faster carrier dynamics. This presents a promising pathway to achieve even faster and more efficient polarization control.

In summary, we have successfully demonstrated ultrafast control of light polarization on a sub-picosecond timescale utilizing a silicon-based quasi-BIC metasurface. The intrinsic chiral modes of the metasurface facilitate the generation of high-purity circularly polarized light. Through systematic pump–probe experiments, we have verified that the incorporation of nonlinear refractive index change significantly enhances the dynamic modulation of both LCP light and RCP light. The pronounced variations observed in the Stokes parameter S3 provide compelling evidence for the efficacy of our approach in achieving high-precision control over polarization states. These findings not only advance our fundamental understanding of the mechanisms governing ultrafast polarization manipulation in chiral metasurfaces but also establish a foundation for the development of next-generation chiral optical devices, such as high-speed optical switches and modulators.