Chirality is a structural property of objects, where an object with chirality cannot be superimposed onto its mirror image by translation or rotation without undergoing a flip [1]. From an optical perspective, some chiral materials exhibit different responses to left-circularly polarized (LCP) light and right-circularly polarized (RCP) light. Circular dichroism is a spectroscopic technique that is utilized to describe the differential response of LCP and RCP to chiral structures. However, the chiral response of natural materials is very weak and requires optical path accumulation. Metasurfaces, artificially designed subwavelength meta-atom array with a strong response of light–matter interaction, were widely researched [2–5]. The metasurfaces have a chiral response that can greatly increase the CD value. The displacement/conduction current is excited in the chiral dielectric/metal metasurfaces, the net electric dipole moment (p), and the net magnetic dipole moment (m) perpendicular to incident wave vector (k) that can be coupled (p_⊥ • m_⊥ ≠ 0) [6].

Bound states in the continuum (BICs) were proposed by von Neumann and Wigner in the field of mathematics and have attracted extensive attention in electromagnetic, acoustic, and water waves. BICs are localized states with infinite lifetimes and reside in the radiation continuum [7–11]. Due to their infinitely high Q-factors and ability to greatly enhance light–matter interactions, metasurfaces based on BICs have been widely utilized in various fields [12,13]. Due to the BICs’ resonant mode that is completely decoupled from the environment, the BICs’ mode cannot be found on the spectrum. In numerous researches, the perturbation of the structure is introduced, and the BICs collapse into a Fano resonance with a high Q-factor which we call as quasi-BICs. The quasi-BICs have a chiroptical response, called as chiral BICs [14–16]. Meanwhile, chiral BICs show a wide range of applications in chiral light sources, circularly polarized light detectors, and chiral sensing [5,17].

In present research, it is difficult to actively tune the chiral response of metasurfaces since change in the manufactured shape of the metasurfaces is hard. Graphene [18], vanadium dioxide (VO₂) [19], and Ge₂Sb₂Te₅ (GST) [20–22] have been widely utilized to construct chiral metasurfaces that can actively tune and switch the CD spectrum due to the adjustable optical properties of these materials by external environment. The tunable chiral metasurfaces that can simultaneously enable fast modulation speed, simultaneously keep CD value unattenuated, and migrate stably are more desirable. In general, the eigenvalues of leaky modes in all-dielectric metasurface are sensitive to the refractive index of the metasurface itself [23]. Indium tin oxide (ITO) and hydrogen-doped indium oxide (IHO) have been utilized to control the thermo-optical effect of optical element [24,25]. Nevertheless, the quasi-BIC resonant mode is sensitive to the imaginary part of the material. Once ITO or IHO is introduced into the system, the Q-factor will decay rapidly [26].

Herein, we theoretically propose an actively chiral metasurface based on the high-Q chiral quasi-BIC. Our metasurface consists of a set of nanocube particles etched off a diagonal. This metasurface can support chiral BICs, when the two corners are etched to the different sizes. Although the silicon has a weak thermo-optical effect (coefficient ∼1.86 × 10⁻⁴K^-1) [27], the CD spectra of this chiral metasurface can be redshifted by a wavelength of full width at half maximum when the environmental temperature increases by only 0.8 K.

A schematic diagram of the chiral metasurface is illustrated in Figs. 1(a)–1(c). This chiral metasurface consists of a Si meta-atom array of cuboids with two corners etched. The optical and electrical constants used in the simulation can be obtained from Table 1. The period of this chiral metasurface is P_x = P_y = 800 nm, the length/width of this meta-atom is w = 400 nm, the thickness is h = 350 nm, the width of the etched corners is a₁= a₂ = 100 nm, and the length of the etched corners is set to b₁ = 160 nm, b₂ = b₁ + δ (δ is the perturbation of this structure), respectively. To analyze the physical mechanism of this metasurface, it is essential to research the response of this system in the momentum space. When this chiral metasurface has C₂-symmetry (δ = 0), the band structure is presented in Fig. 1(d). In this case, the TE mode band near the wavelength 1394.1 nm is to be considered, which supports a symmetry-protected BICs at the Г point of the Brillouin zone. Meanwhile, this BIC is characterized by a magnetic dipole mode (Fig. 1(e)). Far-field polarization map and Q-factor distribution are utilized to demonstrate this BIC. Figure 1(f) exhibits the polarization states of radiative modes, and the polarization singularity (V point) is surrounded by elliptic polarization vector at the Г point. V point is forbidden to radiate into the far field due to its special condition of ill-defined polarization [28].

 

Fig. 1. (a) Illustration of the designed metasurface. (b) A unit cell of the proposed chiral metasurface. (c) Top view of the unit cell. (d) Band structure of this metasurface. (e) Magnetic field of H_z distribution of x–y plane. The arrows indicate the electric field vector. (f) The eigen-polarization profile of the BIC mode: red and blue present the left-circularly polarized state and the right-circularly polarized state, respectively.

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Table 1. Optical and Electrical Parameters of Materials Utilized in this Structure

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Figure 2(a) shows the transmission and CD spectra of the chiral metasurface with δ = 20 nm under the LCP and RCP incidence. When the incident wavelength is 1374.882 nm, T_rl can reach 0.82, and T_lr is close to 0, while T_ll and T_rr are both approximately equal to 0.08. This results in the peak of CD reaching 0.82, and the FWHM of this peak is 0.023 nm.

 

Fig. 2. (a) The simulated transmission and CD spectra of this metasurface with δ = 20 nm under normal incidence. T_ij (i = r,l; j = r,l; r represents RCP, l represents LCP) represents the transmittance of output i-polarized light from j-polarized incidence. CD is defined as CD = (T_ll+ T_rl)–(T_rr+ T_lr). (b) The evolution of CD spectra by continuous varying structural perturbation (δ). (c) Numerically calculated the Q-factor as the function of the perturbation (δ) of this structure. (d) The net electric dipole moment (p) and the net magnetic dipole moment (m) distribution in the unit cell under LCP incidence.

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The CD spectra as a function of δ are presented in Fig. 2(b). When the structure has a rotational symmetry (δ = 0), the BIC resonant mode is excited and does not radiate into a free space at the wavelength of 1394.1 nm. When structural perturbation is applied (δ > 0), a symmetry-protected BIC transforms into quasi-BIC and is presented in the CD spectra. With the increase of δ, the peak of CD has a redshift, and the Q-factor decreases rapidly with the increase of FWHM (Fig. 2(c)). According to the classic chiroptical theory, the optical chirality can be decided by the coupling of m and p perpendicular to k. In this case, p is parallel to the x–y plane, and m is not parallel to k. There is a little angle between m_⊥ and k_⊥, resulting in the coupling of p_⊥ and m_⊥ and optical chirality of this metasurface (Fig. 2(d)).

In the current study, the dynamical control of the chiroptical response in metasurfaces has extensively stimulated the interest of researchers, due to the fact that tunable chiral metasurfaces can offer more new opportunities for optical polarization engineering. Phase change materials (PCM) have been widely studied in the field of controllable chiral optics, but the metasurfaces manufactured by PCM have the disadvantages of slow response time and single function. Many reports have indicated that the eigenvalue of leaky mode resonance is strongly dependent on the refractive index of the structure itself and the environment [29]. Silicon-based metasurfaces coated with ITO or IHO can quickly and stably control the resonance mode by adding voltage to generate electromagnetic heat. Since IHO and ITO have relatively high imaginary parts of the refractive index in the near-infrared band, this method can only be used for resonance modes with low Q-factors. We utilize extremely low imaginary part and excellent electromagnetic thermal effect of graphene in the near-infrared band to achieve a fast and stable tunable chiroptical response.

The schematic diagram of the structure is illustrated in Fig. 3(a). The chiral metasurface with δ = 20 nm under normal incidence is chosen as the research platform. The formula for calculating the dielectric constant of graphene is shown in Supplement 1 [33]. At room temperature, the dielectric constant of graphene is shown in Fig. 3(b). When the incident wavelength is greater than 1400 nm, we can see that the imaginary part of the dielectric constant of graphene is extremely small. Although the imaginary part of dielectric coefficient of graphene is small enough, it still causes the Q-factor of the quasi-BIC resonance mode to drop to 8260. The decline of the Q-factor also leads to the drop of the CD value. Figure 3(c) presents the CD spectra of this structure at temperatures of 293 K, 350 K, 400 K, 450 K, and 500 K, and we observe that the peak value of CD can remain constant with the increase of temperature. Figure 3(d) presents the wavelength corresponding to the CD peak as the function of temperature rises from 293 K to 500 K. It can be seen from the fitted curve that the wavelength and temperature are linearly correlated. We define the sensitivity of temperature as S=△λ/△T, and the sensitivity of temperature is 0.06 nm/K for this metasurface. The thermo-optical coefficient of Si is theorized to be 1.86 × 10⁻⁴K^-1. When the temperature changes by 0.8 K, the peak of CD shifts by a wavelength of FWHM, which can clearly distinguish the resonant peak movement in the spectrum (Fig. 3(e)). At the same time, we can see a steady migration of CD peaks. Next, we investigate the electromagnetic–thermal response of graphene.

 

Fig. 3. (a) Schematic of the tunable chiral metasurface by the means of electromagnetic heat. Chiral metasurface placed on the silica substrate is encased in a silica coating with a thickness of 450 nm, while a layer of graphene is covered on top of the coating. (b) Real (black) and imaginary parts of dielectric coefficient of graphene under the Fermi energy of 0.45 eV. (c) Simulation CD spectra of this metasurface when the temperature rises from 293 K to 500 K. (d) Wavelength position of the CD peak as the function of temperature rises from 293 K to 500 K. (e) CD spectra of this chiral metasurface when the temperature is at 293.0 K, 293.8 K, and 294.6 K.

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Figure 4(a) illustrated the results of the applied voltage and temperature changes. The average temperature of the metasurface gradually increased over time, and it only took 10 µs to reach 550 K, which is much higher than the current devices of the same type, when we applied a voltage of 2 V. The heat comes from the ohmic loss of graphene. The relationship between input voltage, time, and temperature is shown in Supplement 1. Considering that the application of the bias voltage affects the Fermi energy of graphene, we can observe the optical response after the heating is finished. When the input voltage is 2 V and the time is 10 µs, the temperature distribution of a metasurface of 21 × 21 Si blocks array is shown in Fig. 4(b). In order to more clearly observe the temperature distribution in the metasurface, the temperature distribution on the y axis is shown in Fig. 4(c). The lowest temperature on the metasurface is 552 K, the highest temperature is 554 K, and the temperature is concentrated in the range of 2 K. In addition to time, the average temperature of the metasurface is also related to the input voltage. As the bias voltage increases, the rate of temperature gradually increases, because the heat generation is proportional to the square of the excitation electric field (Fig. 4(d)) [34]. In order to explore the feasibility of the experiment, we put the detailed fabrication process in Supplement 1.

 

Fig. 4. (a) Change of input voltage with time and change of metasurface mean temperature. (b) Temperature distribution of metasurface (21 × 21 Si block array). (c) Spatial temperature distribution of this structure along the y axis at t = 10 µs. (d) Average temperature of this metasurface as a function of different input voltage: the time is set to 10 µs.

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In summary, we both theoretically and numerically demonstrate an electromagnetic heating-assisted metasurface for stably tunable and fast-responding chiroptics via a graphene–dielectric metasurface platform. The metasurface supports a chiral BIC. Silicon is utilized for this metasurface due to its advantages of compatibility with CMOS technology and its intrinsic thermo-optical response. Based on the high sensitivity of the leaky modes to the surrounding’s refractive index, we demonstrate theoretically that the eigenfrequency of the chiral BICs can be quantitatively tuned by adjusting the index of this silicon-based metasurface. The resonant positions for the CD peak are observed to be linearly scaled by the temperature with the spectral sensitivity of up to 0.06 nm/K. In addition, we further demonstrate such thermal adjusting platform by using the intentional electromagnetic thermal effect of graphene. It achieves ultra-fast and efficient modulation effects with the time scale down to microseconds and the temperature of up to 550 K by a rather low voltage of 2 V. These findings open a new way for efficient, fast, quantitatively tunable chiroptical devices.