Reconfigurable chiral quasi-bound states in the continuum metasurfaces based on an asymmetric interface

Xiaofen Zeng; Kejian Chen; Yang Shen; Qian Wang; Yuke Qin; Yifan Zhu; Zheqi Zhou; Songlin Zhuang

doi:10.1364/PRJ.563471

1. INTRODUCTION

Circular dichroism (CD) is a common chiral optical response used to measure the chirality of an object [1,2]. The CD response is extremely weak in natural materials and is difficult to detect directly. To overcome this drawback, chiral metasurfaces have been proposed, and widely applied in chiral sensing [3,4], information encryption [5,6], chiral light sources [7], chiral imaging [8], and other fields. However, most chiral metasurfaces struggle to achieve both high quality factors (Q-factors) and near-unity CD ( $CD \approx 1$ ), which hinders their applications in biosensing [9,10] and nonlinear optics [11 –13]. In recent years, quasi-bound states in the continuum (Q-BICs) have been introduced to chiral metasurfaces to solve these problems [14 –19]. Chiral Q-BIC metasurfaces can be used in areas such as chiral optoelectronic switches [20,21], nonlinear optics [22,23], near-field imaging [24], chiral sensing [25 –27], and photodetectors [28]. However, most existing chiral Q-BIC metasurfaces depend on the introduction of out-of-plane vertical offsets [29,30], oblique incidence of light sources [31,32], or breaking the symmetry of in-plane geometries in the structure [33 –35], which makes them extremely sensitive to fabrication defects and restricts their application in reconfigurable devices.

Here, a reconfigurable chiral Q-BIC metasurface (RCBM) based on an asymmetric interface is proposed, which can dynamically control the CD response by changing the asymmetric factors (difference of refractive index $Δ n$ , rotation angle $θ$ ). Subsequently, a single biased chiral Q-BIC metasurface (SBCBM) is designed by introducing polyaniline (PANI). By applying a voltage, the CD spectrum of the SBCBM shows “ON” and “OFF” phenomena. Moreover, a double biased chiral Q-BIC metasurface (DBCBM) is proposed for chiral switching applications (the CD is changed from $- 0.9$ to $+ 0.9$ at the same frequency), by reversing the voltage applied to the left and right PANI. The results of this study provide a novel methodology for the excitation and dynamic control of chiral Q-BIC.

2. DESIGN OF THE STRUCTURE

The RCBM consists of two identical ${TiO}_{2}$ rectangular rods placed on indium tin oxide (ITO) and ${SiO}_{2}$ substrates, and the two rectangular rods are embedded in media with different refractive indices (RIs), with one embedded in polymethyl methacrylate (PMMA) and the other placed in the active layer (which is another medium, e.g., air $n_{Air} = 1$ ), as shown in Fig. 1.

Figure 1.RCBM unit (a) top view, (b) periodic unfolding, and (c) enlarged view of the structure.

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The periods of the RCBM in the $x$ and $y$ directions are $P_{x}$ and $P_{y}$ , respectively. The cell geometry parameters are as follows: $l = 450 nm$ , $w = 210 nm$ , $h_{TiO 2} = 140 nm$ , $g = 290 nm$ , $P_{x} = 600 nm$ , $P_{y} = 660 nm$ , $θ = 10 °$ , $h_{ITO} = 50 nm$ , $h_{PMMA} = 200 nm$ , $n_{SiO 2} = 1.45$ , $n_{PMMA} = 1.5$ , $n_{TiO 2} = 2.5$ . Dielectric constants of ITO and ${TiO}_{2}$ are from cited literature [36,37]. The RCBM is simulated and optimized by CST Microwave Studio and finite-difference time-domain (FDTD) simulation software, with periodic boundary conditions in the $x$ and $y$ directions, and open boundary conditions in the $z$ direction. Circularly polarized light is perpendicularly incident to the metasurface, and the simulation results are shown in Fig. 2.

Figure 2.(a) Transmittance for different rotation angles $θ$ (without PMMA). (b) Transmittance for different RIs ( $n_{v}$ ) of active layer ( $θ = 0 °$ , with PMMA, $n_{PMMA} = 1.5$ ). (c) Transmittance spectrum and (d) CD spectrum (simulated in FDTD and CST) of the RCBM ( $θ = 10 °$ ) in air ( $n_{v} = 1$ , $Δ n = - 0.5$ ).

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Figures 2(a) and 2(b) demonstrate the excitation of Q-BIC resonance by altering the rotation angle $θ$ and the active layer’s RI ( $n_{v}$ ) under left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) wave incidence, respectively. Rotating the two parallel rectangular rods (without a PMMA cover), the Q-BIC is excited at $λ = 783.2 nm$ . At this point, in-plane symmetry still exists, so the LCP and RCP transmission spectra completely overlap, as shown in Fig. 2(a). If one of the parallel rectangular rods is embedded in PMMA and the other is placed in an active layer of a different RI ( $n_{v}$ ), when $n_{v} \neq n_{PMMA}$ , the symmetry of the metasurface along the $y$ -axis is broken, which also transforms the BIC into Q-BIC, as shown in Fig. 2(b).

By combining the asymmetry parameters $θ$ and $Δ n (Δ n = n_{v} - n_{PMMA})$ , all specular symmetries within the structural plane are broken, resulting in chirality, as shown in Fig. 2(c). Figure 2(c) shows the transmittance spectrum of RCBM ( $θ = 10 °$ ) in air ( $n_{v} = 1$ , $Δ n = - 0.5$ ); the transmission at $λ = 785.1 nm$ is suppressed for the LCP wave, while it is not suppressed for the RCP wave. Figure 2(d) illustrates the transmission CD spectrum obtained by CST and FDTD simulation software. Here, the chiral response is described using CD, which is defined as $CD = T_{RR} + T_{LR} - T_{LL} - T_{RL}$ , where the transmission $T_{i j} = {| t_{i j} |}^{2}$ ( $i = R$ , L; $j = R$ , L; R denotes RCP; L denotes LCP). At $λ = 785.1 nm$ , the CD is about 0.75. The use of two simulation software tools has enhanced the credibility of the simulation results, and the small differences may mainly stem from the differences in the simulation settings and computational accuracy of the two simulation software tools.

3. RESULTS AND DISCUSSION

In coupled mode theory (CMT), the coupling coefficient between the eigenstates and the perpendicular incident wave along the $z$ -axis can be expressed in terms of $m_{e}$ [38]: $m_{e} \propto \int_{V_{1}, V_{2}} j (r) \cdot e e^{i k z} d V = - i ω (P_{1} \cdot e e^{i k z_{1}} + P_{2} \cdot e e^{i k z_{2}}),$ (1)where $V_{1}, V_{2}$ are the volumes of rectangular rods 1 and 2; $j (r)$ denotes the current density; $P_{1}, P_{2}$ are the dipole moments of the rectangular rods; $z_{1}, z_{2}$ denote the effective $z$ -coordinates of the rectangular rods; $k$ denotes the wave vector; and $e$ denotes the polarization unit vector. When two rectangular rods are placed in parallel and without covered PMMA, the dipole moments $P_{1}$ and $P_{2}$ of the two rectangular rods are equal and antiparallel, and the effective $z$ -coordinates are also equal, that is, $P_{1} = - P_{2}$ and $z_{1} = z_{2}$ , which makes $m_{e} = 0$ . This indicates that the system is in symmetrically protected BIC at this point, exhibiting zero linewidth in the spectrum as well as an infinitely large Q-factor. When the two rectangular rods are rotated by an angle $θ, P_{1}$ and $P_{2}$ are no longer inversely parallel, thus transforming the symmetry-protected BIC into a Q-BIC. When covered with PMMA, the symmetry of the system along the $y$ -axis is disrupted, and all mirror symmetries within the plane are broken, thereby inducing chirality. The simulation results shown in Fig. 2 precisely demonstrate these points.

The multipole decomposition method is used to decompose the metasurface and explain its origin of the chiral Q-BIC. According to the multipole decomposition method, the near field of the metasurface can be decomposed as [39] $\vec{P} = \frac{1}{i ω} \int \vec{j} d^{3} r,$ (2) $\vec{M} = \frac{1}{2 c} \int (\vec{r} \times \vec{j}) d^{3} r,$ (3) $\vec{T} = \frac{1}{10 c} \int [(\vec{r} \cdot \vec{j}) \vec{r} - 2 r^{2} \vec{j}] d^{3} r,$ (4) ${\vec{Q}}_{x, y}^{(e)} = \frac{1}{2 i ω} \int [r_{x} j_{y} + r_{y} j_{x} - \frac{2}{3} (\vec{r} \cdot \vec{j}) δ_{x, y}] d^{3} r,$ (5) ${\vec{Q}}_{x, y}^{(m)} = \frac{1}{3 c} \int [{(\vec{r} \times \vec{j})}_{x} r_{y} + {(\vec{r} \times \vec{j})}_{y} r_{x}] d^{3} r,$ (6)where $\vec{P}, \vec{M}, \vec{T}, {\vec{Q}}_{x, y}^{(e)}$ , and ${\vec{Q}}_{x, y}^{(m)}$ are the scattering powers of the electric dipole (ED), magnetic dipole (MD), toroidal dipole (TD), electric quadrupole (EQ), and magnetic quadrupole (MQ), respectively; $\vec{j}$ denotes the current density, $c$ denotes the speed of light, $\vec{r}$ denotes the position vector, and $ω$ denotes the angular frequency.

The combination of FDTD and MATLAB software is more convenient for the multipole decomposition work than CST software, so the combination of the two is used to carry out the multipole decomposition work. The total scattering intensity of this metasurface can be described as [39] $I = \frac{2 ω^{4}}{3 c^{3}} {| \vec{P} |}^{2} + \frac{2 ω^{4}}{3 c^{3}} {| \vec{M} |}^{2} + \frac{2 ω^{6}}{3 c^{5}} {| \vec{T} |}^{2} + \frac{ω^{6}}{5 c^{5}} \sum {| {\vec{Q}}_{x, y}^{(e)} |}^{2} + \frac{ω^{6}}{40 c^{5}} \sum {| {\vec{Q}}_{x, y}^{(m)} |}^{2} .$ (7)

In Figs. 3(a) and 3(d), the EQ moment exhibits significantly stronger scattering power than ED, MD, TD, and MQ, playing a dominant role in the Q-BIC at $λ = 785.1 nm$ , which suggests that Q-BIC is related to the EQ moment. Under LCP wave incidence, the contribution of the EQ moment is significantly enhanced at $λ = 785.1 nm$ , and the LCP wave couples with Q-BIC, as shown in Fig. 3(a). However, in Fig. 3(d), for RCP wave incidence, the contribution of the EQ moment is weaker compared to the LCP wave, and the RCP wave decouples from Q-BIC. The difference in coupling of Q-BIC to LCP and RCP waves excites the chiral Q-BIC. In Figs. 3(b), 3(c), 3(e), and 3(f), the energies of the electric and magnetic fields of RCBM are much stronger under LCP wave incidence than under RCP wave incidence, which is consistent with the results of the multipole decomposition.

Figure 3.Multipole decomposition of the RCBM ( $θ = 10 °$ , $n_{v} = 1$ , $Δ n = - 0.5$ ) under (a) LCP and (d) RCP wave incidence. $x - y$ plane electric field distribution of the RCBM ( $θ = 10 °$ , $n_{v} = 1$ , $Δ n = - 0.5$ ): (b) LCP and (c) RCP wave incidence. $x - y$ plane magnetic field distribution of the RCBM ( $θ = 10 °$ , $n_{v} = 1$ , $Δ n = - 0.5$ ): (e) LCP and (f) RCP wave incidence at $λ = 785.1 nm$ .

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The rotation angle $θ$ is important for the excitation of the chiral Q-BIC, so the effect of $θ$ on the intrinsic chiral response of RCBM is explored in Fig. 4(a). At $λ = 785.1 nm$ , when $θ = 0 °$ ( $Δ n = - 0.5$ ), the transmission spectra corresponding to the LCP and RCP waves coincide exactly, $CD = 0$ . As the $θ$ increases ( $Δ n = - 0.5$ ), the symmetry of the system is destroyed and the Q-BIC couples to one of the LCP or RCP waves and decouples from the other, resulting in chirality. When $θ$ changes from $- 10 °$ to 10°, CD can be modulated from $- 0.75$ to 0.75 and the resonance wavelength remains constant. This indicates that the RCBM is able to set the CD peak arbitrarily and the resonance wavelength remains constant.

Figure 4.In RCBM (covered PMMA), the effects of (a) different rotation angles $θ$ and (b) different RIs ( $n_{v}$ ) of the active layer on CD, and (c) the dependence of CD and resonance wavelength on $Δ n$ .

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In other researches, once the metasurface is fabricated, it is difficult to dynamically control its CD. However, the RCBM is extremely sensitive to the RI of the medium of the active layer, and its optical response can be dynamically controlled by changing the RI of the media in the active layer. Figure 4(b) shows the effect of different RIs ( $n_{v}$ ) of the active layer on the CD of the RCBM ( $θ = 10 °$ ). In RCBM, when the RI of the medium in the active layer is different from that of PMMA, that is, $n_{v} \neq n_{PMMA}$ , the Q-BIC couples to one of the LCP or RCP waves and decouples from the other, resulting in maximum chirality. As $n_{v}$ increases from 1 to 1.8, the CD spectrum shows ON and OFF phenomena. Figure 4(c) shows the dependence of CD and resonance wavelength on $Δ n$ : when $Δ n < 0$ (materials with RI less than $n_{PMMA} = 1.5$ , e.g., air, water), $CD > 0$ ; when $Δ n = 0$ (materials with the same RI), $CD = 0$ ; when $Δ n > 0$ (materials with RI greater than $n_{PMMA} = 1.5$ , e.g. liquid crystal), $CD < 0$ . With the increase of $Δ n$ , the resonance wavelength is red-shifted, and the CD value first increases and then decreases, achieving the maximum around $Δ n = - 0.3$ ( $n_{v} = 1.2$ ), ${CD}_{MAX} \approx 0.9$ , as in Fig. 4(c).

In order to further simplify dynamic control of the chiral response of RCBM, the PANI material is introduced to play the role of the active layer, whose RI can be transformed from 1.3 (oxidized state) to 1.7 (reduced state) by applying a voltage (from 0.6 V to $- 0.2 V$ ) [37]. Figure 5(a) shows the unit structure of the SBCBM; the thickness of PANI is 200 nm. Figure 5(b) shows the simulation results; when the voltage changes from 0.6 V ( $n_{PANI} = 1.3$ ) to $- 0.2 V$ ( $n_{PANI} = 1.7$ ), the resonant wavelength shifts from 786.3 nm to 788.2 nm, and CD changes from 0.87 to $- 0.87$ . The CD transition does not occur at the same frequency point, but at $λ_{1} = 786.3 nm$ , the CD spectrum shows the phenomenon from ON to OFF; at $λ_{2} = 788.2 nm$ , the CD spectrum shows the phenomenon from OFF to ON. Here, the Q-factor is defined as $λ_{0} / Δ λ$ [40], where $λ_{0}$ is the resonance center wavelength of the chiral Q-BIC mode; $Δ λ$ is full width at half maximum (FWHM) of the chiral Q-BIC mode. When PANI is in the oxidized state, the Q-factor of SBCBM at $λ_{1} = 786.3 nm$ is 3932 ( $CD = 0.87$ ); when PANI is in the reduced state, the Q-factor at $λ_{2} = 788.2 nm$ is 3353 ( $CD = - 0.87$ ). These results indicate that the SBCBM has great applications in the field of chiral electronic control switch. If the PANI part is replaced by a microfluidic channel, it can also be changed into a pneumatic reconfigurable chiral metamaterial device.

Figure 5.(a) Unit structure of SBCBM. (b) CD spectrum of PANI transitioning from oxidized state to reduced state.

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In addition, based on the mechanism of an asymmetric interfacial excited chiral Q-BIC, the RCBM can also be used for substance verification by comparing the RIs of different substances [41]. The two rectangular rods in Fig. 1 are embedded in the measured sample and reference sample of the same thickness. According to whether the CD spectrum is zero, it can be determined whether the measured sample is consistent with the reference sample. The whole process does not require complex measuring equipment, which significantly reduces the time required for substance examination.

4. POTENTIAL APPLICATIONS

Based on the above characteristics of the proposed metasurface and the material PANI, this paper designed a DBCBM for chiral switching applications, as shown in Figs. 6(a) and 6(b). Two rectangular rods are embedded in the PANI and separated by PMMA of width $w_{1}$ ; other parameters are the same as before. Chiral switching is achieved at the same frequency point by applying different voltages to the left and right PANIs to change their RIs ( $n_{1}$ for the left PANI and $n_{2}$ for the right). Figure 6(c) shows the simulation results of CD for PMMA with different $w_{1}$ . The results demonstrate that the width $w_{1}$ of the PMMA does not have much effect on the chirality of the DBCBM, and the width $w_{1}$ can be flexibly adjusted according to practical requirements. Here $w_{1} = 50 nm$ is used for further discussion.

Figure 6.(a) Top view and (b) unit structure of the DBCBM. (c) Effect of different widths $w_{1}$ of the PMMA spacer on CD at $n_{1} = 1.3$ (oxidized state), $n_{2} = 1.7$ (reduced state).

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Figures 7(a)–7(e) show the CD spectra of different $n_{2}$ for $n_{1}$ of 1.3, 1.4, 1.5, 1.6, and 1.7, respectively. With the increase of $n_{1}$ , the tunable range of the resonant wavelength experiences a redshift. When $n_{1} = 1.3$ and $n_{2} = 1.6$ ( $Δ n^{'} = n_{1} - n_{2} = - 0.3$ ), $| {CD}_{MAX} | = 0.94$ ; when $n_{1} = 1.4$ and $n_{2} = 1.7$ ( $Δ n^{'} = - 0.3$ ), $| {CD}_{MAX} | = 0.90$ ; when $n_{1} = 1.5$ and $n_{2} = 1.7$ ( $Δ n^{'} = - 0.2$ ), $| {CD}_{MAX} | = 0.86$ ; when $n_{1} = 1.6$ and $n_{2} = 1.3$ ( $Δ n^{'} = 0.3$ ), $| {CD}_{MAX} | = 0.93$ ; when $n_{1} = 1.7$ and $n_{2} = 1.4$ ( $Δ n^{'} = 0.3$ ), $| {CD}_{MAX} | = 0.90$ . By applying different voltages, different $n_{1}$ and $n_{2}$ can be obtained, so that positive or negative CD can be obtained at an arbitrary wavelength. This indicates that the chirality can be switched simply by reversing the voltages applied to the left and right side PANIs, as shown in Fig. 7(f). At $λ = 787.36 nm$ , CD changes from positive to negative when $n_{1}$ changes from 1.3 (voltage 0.6 V) to 1.7 (voltage $- 0.2 V$ ) and $n_{2}$ changes from 1.7 to 1.3, that is, $Δ n^{'}$ changes from $- 0.4$ to 0.4, $Δ CD = {CD}_{MAX} - {CD}_{MIN} = 1.74$ (where ${CD}_{MAX}$ is the maximum value of CD and ${CD}_{MIN}$ is the minimum value of CD); at $λ = 787.72 nm$ , when $n_{1}$ changes from 1.4 to 1.7 and $n_{2}$ changes from 1.7 to 1.4, that is, $Δ n^{'}$ changes from $- 0.3$ to 0.3, $Δ CD = 1.8$ ; at $λ = 788.03 nm$ , when $n_{1}$ changes from 1.5 to 1.7 and $n_{2}$ changes from 1.7 to 1.5, that is, $Δ n^{'}$ changes from $- 0.2$ to 0.2, $Δ CD = 1.74$ ; at $λ = 788.45 nm$ , when $n_{1}$ changes from 1.6 to 1.7 and $n_{2}$ changes from 1.7 to 1.6, that is, $Δ n^{'}$ changes from $- 0.1$ to 0.1, $Δ CD = 1.26$ . Obviously, the DBCBM has good chiral switching performance.

Figure 7.(a)–(e) CD spectra at different $n_{2}$ for $n_{1}$ of 1.3, 1.4, 1.5, 1.6, and 1.7 in DBCBM, where $| {CD}_{MAX} |$ denotes the maximum absolute value of CD. (f) CD spectra for different $n_{2}$ at $n_{1} = 1.7$ (solid line) and for different $n_{1}$ at $n_{2} = 1.7$ (dashed line) in DBCBM.

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Here, the possible preparation process of DBCBM is briefly described for reference [37,42]. Firstly, ITO and ${TiO}_{2}$ films are sequentially prepared on ${SiO}_{2}$ substrates by a sputtering deposition technique. Secondly, electron beam lithography (EBL) is used for structure patterning. Subsequently, the PANI layer is synthesized using the electrochemical polymerization method. Finally, a PMMA spacer is fabricated after the EBL process.

5. CONCLUSION

In summary, this paper presents a method to excite chiral Q-BIC in geometrically symmetric structured metasurfaces using asymmetric interfaces. The RCBM based on this mechanism can modulate the chiral Q-BIC by the RI ( $n_{v}$ ) of the active layer medium and rotation angle $θ$ , with a maximum chiral CD of $0.9$ . Meanwhile, the proposed SBCBM can dynamically control the CD by means of electronic control of PANI material. The simulation results of CD spectra show a clear ON and OFF phenomenon when PANI changes from the oxidized state to the reduced state. In addition, the DBCBM is designed for chiral switching applications, and by applying voltage its chirality can be switched at any frequency point, with $Δ CD = 1.75$ at $λ = 787.8 nm$ . The study in this paper provides an innovative approach for the excitation of chiral Q-BIC and its dynamic control, and shows great potential for applications in the fields of optical chiral switching and biosensors.

Acknowledgment

Acknowledgment. We gratefully acknowledge B. Y. Shan, Prof. B. Cai, and G. J. Xu from University of Shanghai for Science and Technology for their help.

Category: Surface Optics and Plasmonics

Received: Mar. 26, 2025

Accepted: Jun. 6, 2025

Published Online: Jul. 31, 2025

The Author Email: Kejian Chen (ee.kjchen@gmail.com)

DOI:10.1364/PRJ.563471

CSTR:32188.14.PRJ.563471